SIR3

Existing Annotations Review

Core Functions

SIR3 is a structural component that binds chromatin and nucleosomes to establish and maintain heterochromatin domains. This is the primary molecular mechanism by which SIR3 functions.

Molecular Function:

chromatin binding

Directly Involved In:

heterochromatin formation

SIR3 forms homodimers and interacts with SIR2 and SIR4 to assemble the Sir2-3-4 silent chromatin complex. Homodimerization through the C-terminal winged helix domain is essential for silencing function.

Molecular Function:

identical protein binding

Directly Involved In:

silent mating-type cassette heterochromatin formation

SIR3 directly binds to histone octamers and nucleosomes at mating-type loci, telomeres, rDNA, and euchromatic origins. This nucleosome binding is critical for both establishing transcriptional silencing and suppressing DNA replication initiation at origins.

Molecular Function:

nucleosome binding

Directly Involved In:

negative regulation of DNA replication

References

GO_REF:0000002

Gene Ontology annotation through association of InterPro records with GO terms

GO_REF:0000033

Annotation inferences using phylogenetic trees

GO_REF:0000043

Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping

GO_REF:0000044

Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt

GO_REF:0000117

Electronic Gene Ontology annotations created by ARBA machine learning models

PMID:1913809

Modifiers of position effect are shared between telomeric and silent mating-type loci in S. cerevisiae.

PMID:3297920

Four genes responsible for a position effect on expression from HML and HMR in Saccharomyces cerevisiae.

PMID:9122169

Silent information regulator protein complexes in Saccharomyces cerevisiae: a SIR2/SIR4 complex and evidence for a regulatory domain in SIR4 that inhibits its interaction with SIR3.

PMID:9150138

Redistribution of silencing proteins from telomeres to the nucleolus is associated with extension of life span in S. cerevisiae.

PMID:9501103

Components of the Ku-dependent non-homologous end-joining pathway are involved in telomeric length maintenance and telomeric silencing.

PMID:9710643

Sir proteins, Rif proteins, and Cdc13p bind Saccharomyces telomeres in vivo.

PMID:11689698

Multiple interactions in Sir protein recruitment by Rap1p at silencers and telomeres in yeast.

PMID:11805837

Systematic identification of protein complexes in Saccharomyces cerevisiae by mass spectrometry.

PMID:14576278

The proteome of Saccharomyces cerevisiae mitochondria.

PMID:15282295

Budding yeast silencing complexes and regulation of Sir2 activity by protein-protein interactions.

PMID:16554755

Global landscape of protein complexes in the yeast Saccharomyces cerevisiae.

PMID:16581798

Structure and function of the Saccharomyces cerevisiae Sir3 BAH domain.

PMID:16717101

Domain structure and protein interactions of the silent information regulator Sir3 revealed by screening a nested deletion library of protein fragments.

PMID:16823961

Toward the complete yeast mitochondrial proteome: multidimensional separation techniques for mitochondrial proteomics.

PMID:16908543

Sir3 C-terminal domain involvement in the initiation and spreading of heterochromatin.

PMID:16956377

The nuclear GTPase Gsp1p can affect proper telomeric function through the Sir4 protein in Saccharomyces cerevisiae.

PMID:17176117

Domain organization and quaternary structure of the Saccharomyces cerevisiae silent information regulator 3 protein, Sir3p.

PMID:17660569

A genomic screen in yeast reveals novel aspects of nonstop mRNA metabolism.

PMID:18195043

Long-range communication between the silencers of HMR.

PMID:19099415

Role of nucleic acid binding in Sir3p-dependent interactions with chromatin fibers.

PMID:19217406

Reconstitution of yeast silent chromatin multiple contact sites and O-AADPR binding load SIR complexes onto nucleosomes in vitro.

PMID:20176978

An auxiliary silencer and a boundary element maintain high levels of silencing proteins at HMR in Saccharomyces cerevisiae.

PMID:21179020

Defining the budding yeast chromatin-associated interactome.

PMID:21217703

A conserved motif within RAP1 has diversified roles in telomere protection and regulation in different organisms.

PMID:23299941

Dimerization of Sir3 via its C-terminal winged helix domain is essential for yeast heterochromatin formation.

PMID:26218225

The Ku subunit of telomerase binds Sir4 to recruit telomerase to lengthen telomeres in S. cerevisiae.

PMID:26399229

Spatial reorganization of telomeres in long-lived quiescent cells.

PMID:27122604

Quiescent Saccharomyces cerevisiae forms telomere hyperclusters at the nuclear membrane vicinity through a multifaceted mechanism involving Esc1, the Sir complex, and chromatin condensation.

PMID:29795547

Yeast heterochromatin regulators Sir2 and Sir3 act directly at euchromatic DNA replication origins.

PMID:31599702

Reciprocal interactions between mtDNA and lifespan control in budding yeast.

file:yeast/SIR3/SIR3-deep-research-falcon.md

Falcon deep research report on SIR3

Sir3 is the principal nucleosome-binding/structural subunit of the Sir2/3/4 silencing complex. It binds chromatin via its conserved N-terminal BAH domain and oligomerizes and participates in spreading/compaction through additional conserved regions, including a C-terminal AAA-like domain that lacks canonical ATPase activity.

"Across classic biochemistry and structural work, **Sir3 is the principal nucleosome-binding/structural subunit** of the Sir2/3/4 complex. It binds chromatin via its conserved **N-terminal BAH (bromo-adjacent homology) domain**, and it oligomerizes and participates in spreading/compaction through additional conserved regions, including a **C-terminal AAA-like domain** (lacking canonical ATPase activity). "
The Sir3 BAH domain is a histone-modification-sensitive nucleosome reader; its association with nucleosomes is impaired by H4K16 acetylation and H3K79 methylation.

"**BAH domain as a nucleosome reader.** Sir3 contains an N-terminal BAH domain that directly binds nucleosomes and is **sensitive to histone modification state**, a key aspect of how silent chromatin is specified. "
A 3.0 Angstrom crystal structure of the Sir3 BAH domain bound to the nucleosome shows two Sir3 BAH domains binding one nucleosome (one per face) making extensive contacts primarily with the core histones (notably the H4 N-terminal tail and the H3/H4 LRS surface) rather than DNA, implicating H4K16 and H3K79.

"A landmark structure solved a **3.0 Å crystal structure** of the Sir3 **BAH domain bound to the nucleosome**, showing that Sir3 BAH forms extensive contacts primarily with **histones (not DNA)**, including the **H4 N-terminal tail** and the H3/H4 LRS surface; critically, the structure orders and implicates residues such as **H4K16** and **H3K79**, whose modification state regulates silencing. "
Read-write spreading logic: Sir2 deacetylates histone H4K16 to create binding-competent nucleosomes; Sir3 preferentially recognizes deacetylated H4K16 nucleosomes; Sir3 dimers linked through Sir4 dimers support cooperative binding to paired nucleosomes and cis-spreading of the silent domain from silencers.

"A widely used mechanistic model is an iterative “read–write” logic in which **Sir2 deacetylates histone H4K16**, creating binding-competent nucleosomes; **Sir3 preferentially recognizes deacetylated H4K16 nucleosomes** and, together with Sir4-mediated assemblies, supports cooperative binding and **cis-spreading** of the silent domain outward from silencers. "
Sir3 functions in the nucleus on chromatin, with classical sites of action at HML/HMR and telomere-proximal/subtelomeric regions, where it binds nucleosomes as part of the Sir2/3/4 silencing machinery.

"Sir3 functions in the **nucleus** on **chromatin**, with classical sites of action at **HML/HMR** and **telomere-proximal/subtelomeric regions**, where it binds nucleosomes as part of the Sir2/3/4 silencing machinery "
Beyond canonical silent domains, Sir3 also makes transient direct contacts with euchromatin at large scale; Nanopore-MetID identified 1,197 genes (~19% of genes) as Sir3 contact targets, with high-density occupancy concentrated about +/-2 kb around subtelomeric nucleation sites.

"Recent mapping adds that Sir3 also makes **transient direct contacts with euchromatin** at large scale, beyond canonical silent domains. "
Sir3 chromatin contacts depend on the Sir2/3/4 complex: subtelomeric Sir3 methylation is abolished in sir2-delta cells and reduced about 2-fold in sir4-delta cells.

"Subtelomeric Sir3 methylation is abolished in *sir2Δ* and reduced about 2-fold in *sir4Δ*, consistent with Sir3 acting within the Sir2/3/4 complex "

Deep Research

Falcon

(SIR3-deep-research-falcon.md)

this with annotations you find in gene/protein databases, but these can be outdated or inaccurate. Falcon Edison Scientific Literature 34 citations 2 artifacts 2026-05-30T11:13:47.001454

Edison artifact artifact-00 (text/markdown)

## Context ID: pqac-00000019 The figures showing the Sir3 BAH domain bound to the nucleosome and its domain map are Figure 1 and Figure 2. Figure 1 (spread acro (image/png)

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Research report: Saccharomyces cerevisiae SIR3 (UniProt P06701; ORF YLR442C)

0) Mandatory identity verification (correct gene/protein)

The target protein is Silent information regulator 3 (Sir3) from budding yeast Saccharomyces cerevisiae strain S288c, encoded by SIR3/YLR442C (UniProt P06701). The literature synthesized here explicitly concerns S. cerevisiae Sir3 and its role in Sir2/3/4-dependent transcriptional silencing (yeast heterochromatin) at the silent mating-type loci and telomeres, and matches the domain logic implied by UniProt (BAH domain; Orc1/Cdc6-like relationship; AAA-like region). (buchberger2008sir3nucleosomeinteractionsin pages 1-2, connelly2006structureandfunction pages 1-2, currie2024thebahdomain pages 1-4)

1) Key concepts, definitions, and current understanding

1.1 Yeast “heterochromatin” and SIR-dependent silencing

In S. cerevisiae, “silent chromatin” (often treated as a heterochromatin analog) is a highly stable transcriptionally repressed state classically found at HML/HMR (silent mating-type loci) and subtelomeres, assembled by the SIR complex. A recent review emphasizes that silencing is not simply static occlusion; rather, it is probabilistic and dynamic, where silencers increase Sir protein residence time and domain formation reflects many weak, cooperative interactions that bias chromatin configurations toward repression. (dhillon2024transcriptionalsilencingin pages 1-2, dhillon2024transcriptionalsilencingin pages 2-4)

1.2 Sir3’s primary molecular function

Across classic biochemistry and structural work, Sir3 is the principal nucleosome-binding/structural subunit of the Sir2/3/4 complex. It binds chromatin via its conserved N-terminal BAH (bromo-adjacent homology) domain, and it oligomerizes and participates in spreading/compaction through additional conserved regions, including a C-terminal AAA-like domain (lacking canonical ATPase activity). (onishi2007roleofthe pages 1-2, buchberger2008sir3nucleosomeinteractionsin pages 1-2, currie2024thebahdomain pages 1-4)

1.3 Domain architecture and nucleosome recognition (mechanistic definition)

BAH domain as a nucleosome reader. Sir3 contains an N-terminal BAH domain that directly binds nucleosomes and is sensitive to histone modification state, a key aspect of how silent chromatin is specified. (currie2024thebahdomain pages 4-8, currie2024thebahdomain pages 1-4)

Structural basis. A landmark structure solved a 3.0 Å crystal structure of the Sir3 BAH domain bound to the nucleosome, showing that Sir3 BAH forms extensive contacts primarily with histones (not DNA), including the H4 N-terminal tail and the H3/H4 LRS surface; critically, the structure orders and implicates residues such as H4K16 and H3K79, whose modification state regulates silencing. (armache2011structuralbasisof pages 1-3, armache2011structuralbasisof pages 3-4)

Visual evidence of this interaction and contact mapping is available in the original figure crops. (armache2011structuralbasisof media 2905dd00, armache2011structuralbasisof media b38d5437, armache2011structuralbasisof media faf51ee3)

1.4 How Sir3 participates in initiation and spreading

A widely used mechanistic model is an iterative “read–write” logic in which Sir2 deacetylates histone H4K16, creating binding-competent nucleosomes; Sir3 preferentially recognizes deacetylated H4K16 nucleosomes and, together with Sir4-mediated assemblies, supports cooperative binding and cis-spreading of the silent domain outward from silencers. (yuan2024minimalrequirementsfor pages 1-2)

2) Recent developments and latest research (prioritizing 2023–2024)

2.1 Quantitative limits of silencing and gene-level escape (2023)

Using sensitive reporters, Fouet & Rine (2023) quantified that Sir-dependent repression at HML/HMR can reach roughly ~10^4-fold but is not absolute: transient silencing failures at HML occurred at about ~10⁻³ per generation in their CRASH assay framework. (fouet2023limitstotranscriptional pages 1-2)

Importantly, escapes can be gene-specific within a silenced locus: silencing-loss at an HML α2 reporter was reported to be ~10-fold higher than at α1, consistent with the observation that α2 is more highly expressed than α1 when unsilenced (e.g., ~8-fold difference in one construct; smaller in RT-qPCR). (fouet2023limitstotranscriptional pages 12-13, fouet2023limitstotranscriptional pages 1-2)

2.2 Sir3 dynamics and genome-wide transient contacts (2023)

A 2023 study used Nanopore-MetID (in vivo methylation footprinting read out by nanopore sequencing) to map Sir3 chromatin contacts beyond what ChIP-seq typically detects. It reported:

High-density Sir3 occupancy concentrated about ±2 kb around subtelomeric nucleation sites, while transient low-density contacts extended up to ~30 kb away. (radmanlivaja2023thebuddingyeast pages 4-7, radmanlivaja2023thebuddingyeast pages 7-9)
1,197 genes (~19% of genes) identified as Sir3 contact targets, and >1,000 euchromatic genes contacted overall. (radmanlivaja2023thebuddingyeast pages 4-7, radmanlivaja2023thebuddingyeast pages 7-9, radmanlivaja2023thebuddingyeast pages 22-24)
Contact frequency decayed with distance: ~50% of genes within 0–20 kb of subtelomeric nucleation sites were contacted, dropping to ~20% farther away; similarly ~50% within 20 kb of telomere ends were contacted, dropping to ~20% beyond 50 kb. (radmanlivaja2023thebuddingyeast pages 24-26, radmanlivaja2023thebuddingyeast pages 7-9)
Global methylation readouts: Sir3EcoG2 methylated about ~0.08% of adenines (~6,000 A/genome), with site-proximal average methylation around ~0.4–0.45% at nucleation sites and typically ≤1.5% even at strongly bound HML/HMR regions. (radmanlivaja2023thebuddingyeast pages 4-7, radmanlivaja2023thebuddingyeast pages 7-9)

The same work emphasized that Sir3 binding and turnover are highly dynamic during nutrient transitions: after release from growth arrest, Sir3 exchange and degradation rise sharply, and Sir3 bound at subtelomeric/HM loci is largely replaced by newly synthesized Sir3 by the end of the first cell cycle after release. (radmanlivaja2023thebuddingyeast pages 24-26, radmanlivaja2023thebuddingyeast pages 22-24)

Functionally, reduced Sir3 supply caused large silencing defects: silencing was reported 15–30× less efficient after release and 200–500× less efficient in mid-log in a Sir3 hypomorph compared with WT. (radmanlivaja2023thebuddingyeast pages 24-26)

2.3 Current “expert view” of silencing mechanisms (2024 review)

Dhillon & Kamakaka (2024) synthesize an updated picture of silencing as a stochastic, non-equilibrium process in which silencers chiefly work by increasing local Sir concentration and residence time, and robustness is produced by domain-wide cooperativity/avidity while individual components remain dynamic. (dhillon2024transcriptionalsilencingin pages 1-2, dhillon2024transcriptionalsilencingin pages 2-4)

2.4 Epigenetic inheritance and bistability framing (2024)

Two 2024 PNAS studies highlight how the field is converging on quantitative/constructive models:

Minimal engineered inheritance: Yuan & Moazed (2024) explicitly restate the canonical yeast mechanism (Sir2 deacetylates H4K16; Sir3 recognizes deacetylated H4K16; Sir3 dimerization and Sir4-linked complexes support spreading) and then engineer reduced-complexity silent chromatin domains to probe minimal requirements for epigenetic inheritance. (yuan2024minimalrequirementsfor pages 1-2)
Bistability via compaction–mark feedback: Miangolarra et al. (2024) propose and test a two-way feedback model in which chromatin compaction increases silencer-protein binding, which removes activating marks and promotes further compaction, producing bistable expression states at HMR; this formalizes compaction as an active participant in epigenetic switching. (miangolarra2024twowayfeedbackbetween pages 1-2)

3) Current applications and real-world implementations

3.1 Sir3 as a model for epigenetic domain assembly and inheritance

Sir3-mediated silencing remains a major experimental system for:

testing histone-mark reader mechanisms (e.g., BAH-mediated recognition of appropriate nucleosome states) (currie2024thebahdomain pages 4-8, armache2011structuralbasisof pages 3-4)
building and validating quantitative epigenetic models of bistability and inheritance at defined loci (miangolarra2024twowayfeedbackbetween pages 1-2, yuan2024minimalrequirementsfor pages 1-2)

These are “real-world” implementations in molecular genetics and synthetic/quantitative epigenetics because the yeast SIR system is experimentally tractable and can be re-engineered, including transplantation of heterochromatin-like feedback designs into yeast. (yuan2024minimalrequirementsfor pages 1-2)

3.2 Tools and assays enabled by Sir3 biology

Recent work illustrates how Sir3 supports development and validation of mapping assays:

Nanopore-MetID for transient chromatin contacts that are hard to capture by ChIP-seq, revealing low-density and cell-fraction-specific interactions across the genome. (radmanlivaja2023thebuddingyeast pages 4-7, radmanlivaja2023thebuddingyeast pages 7-9)
sensitive reporter assays (e.g., CRASH) to quantify rare silencing failures and gene-level differences in escape rates. (fouet2023limitstotranscriptional pages 1-2)

4) Expert opinions and analysis (authoritative sources)

4.1 How silencers “work” and why silencing is probabilistic

A key expert synthesis (Dhillon & Kamakaka 2024) argues that silencers primarily increase Sir residence time and local concentration, while the output is probabilistic because regulatory DNA can transiently become accessible (nucleosome breathing, transient TF binding). In this view, Sir proteins shift the distribution of chromatin configurations toward repressed states, and silencing robustness emerges from sub-optimized, distributed cooperativity rather than a single dominant step. (dhillon2024transcriptionalsilencingin pages 1-2, dhillon2024transcriptionalsilencingin pages 2-4)

4.2 Structural interpretation of “histone mark dependency”

A structural and mechanistic synthesis (Currie et al. 2024) frames the Sir3 BAH domain as a histone/nucleosome reader whose binding is impaired by H4K16 acetylation and H3K79 methylation, connecting “epigenetic marks” to a concrete binding interface. (currie2024thebahdomain pages 4-8, currie2024thebahdomain pages 1-4)

5) Statistics and data highlights (recent studies)

Key recent quantitative observations for Sir3 and Sir-mediated silencing include:

Silencing strength: HML/HMR repression of ~10⁴-fold (fouet2023limitstotranscriptional pages 1-2)
Silencing failure frequency: transient failures ~10⁻³ per generation (fouet2023limitstotranscriptional pages 1-2)
Gene-specific escape: α2 silencing-loss ~10× higher than α1 in one assay design (fouet2023limitstotranscriptional pages 12-13)
Sir3 target scale: 1,197 genes (~19%) as Sir3-contact targets by Nanopore-MetID (radmanlivaja2023thebuddingyeast pages 7-9)
Spatial scale: high-density binding ~±2 kb around subtelomeric nucleation sites; transient contacts up to ~30 kb (radmanlivaja2023thebuddingyeast pages 4-7, radmanlivaja2023thebuddingyeast pages 7-9)
Distance-dependent contact fraction: ~50% genes contacted within 0–20 kb, ~20% farther away (radmanlivaja2023thebuddingyeast pages 24-26, radmanlivaja2023thebuddingyeast pages 7-9)
Functional sensitivity to Sir3 dosage/state: silencing 15–30× less efficient after arrest release and 200–500× less efficient in mid-log in a Sir3 hypomorph (radmanlivaja2023thebuddingyeast pages 24-26)

A consolidated table of mechanistic points and quantitative findings is provided below.

Topic	Key finding (with numbers where available)	System/assay	Source (author year, venue)	URL
Target identity and core function	SIR3/P06701 in S. cerevisiae is the structural, nucleosome-binding silencing subunit of the Sir2/3/4 complex; it contains an N-terminal BAH domain and a C-terminal AAA-like region implicated in chromatin interactions and oligomerization (buchberger2008sir3nucleosomeinteractionsin pages 1-2, currie2024thebahdomain pages 1-4)	Genetics/biochemistry/structural synthesis	Currie et al. 2024, Chromatin Readers in Health and Disease; Buchberger et al. 2008, MCB	https://doi.org/10.1016/b978-0-12-823376-4.00006-9 ; https://doi.org/10.1128/mcb.01210-08
BAH–nucleosome structure	Sir3 BAH was solved bound to the nucleosome at 3.0 Å; two Sir3 BAH domains bind one nucleosome (one per face) and make extensive contacts with all four core histones (armache2011structuralbasisof pages 1-3, armache2011structuralbasisof pages 3-4)	X-ray crystallography of Sir3 BAH–nucleosome complex	Armache et al. 2011, Science	https://doi.org/10.1126/science.1210915
Histone-mark sensitivity	The Sir3 BAH domain binds the H4 tail and contacts the H3/H4 LRS region; silencing-relevant residues H4K16 and H3K79 are directly implicated, and H4K16 acetylation/H3K79 methylation impair Sir3 association with nucleosomes (armache2011structuralbasisof pages 1-3, currie2024thebahdomain pages 4-8, armache2011structuralbasisof pages 3-4)	Structural biology plus biochemical/genetic synthesis	Armache et al. 2011, Science; Currie et al. 2024, book chapter	https://doi.org/10.1126/science.1210915 ; https://doi.org/10.1016/b978-0-12-823376-4.00006-9
Canonical read–write logic of spreading	Sir2 deacetylates H4K16; Sir3 recognizes deacetylated H4K16-containing nucleosomes; Sir3 dimers, linked through Sir4 dimers, support cooperative binding to paired nucleosomes and cis-spreading of silent chromatin from silencers (yuan2024minimalrequirementsfor pages 1-2)	Mechanistic synthesis/engineered silent chromatin design	Yuan & Moazed 2024, PNAS	https://doi.org/10.1073/pnas.2318455121
Silencing initiation and propagation	Silencers recruit Sir proteins via ORC/Rap1/Abf1/Sum1; iterative Sir2 deacetylation and Sir3/Sir4 binding across adjacent hypoacetylated nucleosomes propagate the domain; current expert view emphasizes probabilistic, dynamic, domain-wide cooperativity rather than static occupancy (dhillon2024transcriptionalsilencingin pages 1-2, dhillon2024transcriptionalsilencingin pages 2-4)	Review of genetic, chromatin, and live-cell evidence	Dhillon & Kamakaka 2024, Epigenetics & Chromatin	https://doi.org/10.1186/s13072-024-00553-7
Heterochromatin bistability	A 2024 HMR model/validation study supports two-way feedback between chromatin compaction and histone modification state: compaction promotes SIR binding, which removes activating marks and drives further compaction, explaining bistable silent vs expressed states (miangolarra2024twowayfeedbackbetween pages 1-2)	Theory plus experimental validation at HMR	Miangolarra et al. 2024, PNAS	https://doi.org/10.1073/pnas.2403316121
Subtelomeric Sir3 occupancy extent	Stable, high-density Sir3 occupancy is concentrated about ±2 kb around subtelomeric SIR nucleation sites; ChIP-seq falls to background by ~4 kb downstream of XCS, but transient low-density contacts extend to ~30 kb (radmanlivaja2023thebuddingyeast pages 4-7, radmanlivaja2023thebuddingyeast pages 7-9)	Nanopore-MetID (Sir3Dam/EcoG2), ChIP-seq	Radman-Livaja et al. 2023, Research Square preprint	https://doi.org/10.21203/rs.3.rs-3495250/v1
Genome-wide transient contacts	Sir3 makes contacts with >1,000 euchromatic genes; 1,197 genes (~19% of genes) were identified as Sir3 contacts; at least 15% of promoters and 7% of CDS had non-zero Sir3Dam methylation probability (radmanlivaja2023thebuddingyeast pages 4-7, radmanlivaja2023thebuddingyeast pages 7-9, radmanlivaja2023thebuddingyeast pages 22-24)	Nanopore-MetID genome-wide mapping	Radman-Livaja et al. 2023, Research Square preprint	https://doi.org/10.21203/rs.3.rs-3495250/v1
Contact frequency vs distance from telomeres	~50% of genes within 0–20 kb of subtelomeric nucleation sites are contacted by Sir3, dropping to ~20% in the next 20 kb and remaining ~20% farther toward centromeres; similarly, ~50% of genes within 20 kb of telomere ends are contacted, dropping to ~20% beyond 50 kb (radmanlivaja2023thebuddingyeast pages 24-26, radmanlivaja2023thebuddingyeast pages 7-9)	Nanopore-MetID positional analysis	Radman-Livaja et al. 2023, Research Square preprint	https://doi.org/10.21203/rs.3.rs-3495250/v1
Methylation density readout for Sir3 contacts	Sir3EcoG2 methylated ~0.08% of adenines genome-wide (~6,000 A/genome); methylation near XCS averaged ~0.45%, at Yp ~0.4%, and even at HML/HMR was not >1.5%; signal dropped to ~0.1% at 4–15 kb and ~0.02% farther away (radmanlivaja2023thebuddingyeast pages 4-7, radmanlivaja2023thebuddingyeast pages 7-9)	Nanopore-MetID with adenine methylation readout	Radman-Livaja et al. 2023, Research Square preprint	https://doi.org/10.21203/rs.3.rs-3495250/v1
Fraction of cells with Sir3 contact	Yp and XCS nucleation sites were methylated in ~72% of cells; high-density binding around nucleation sites occurred in ~70% of cells, whereas distal transient contacts up to ~30 kb were present in ~10–20% of cells and at ~5-fold lower density (radmanlivaja2023thebuddingyeast pages 4-7, radmanlivaja2023thebuddingyeast pages 7-9)	Single-molecule/nanopore contact frequency inference	Radman-Livaja et al. 2023, Research Square preprint	https://doi.org/10.21203/rs.3.rs-3495250/v1
Sir2/Sir4 dependence of Sir3 chromatin contacts	Subtelomeric Sir3 methylation is abolished in sir2Δ and reduced about 2-fold in sir4Δ, consistent with Sir3 acting within the Sir2/3/4 complex (radmanlivaja2023thebuddingyeast pages 7-9)	Nanopore-MetID in mutant backgrounds	Radman-Livaja et al. 2023, Research Square preprint	https://doi.org/10.21203/rs.3.rs-3495250/v1
Sir3 exchange dynamics	During growth arrest, Sir3 exchange is slow; after nutrient repletion, exchange and degradation increase sharply, and Sir3 bound at subtelomeric and HM loci is largely replaced by newly synthesized Sir3 by the end of the first cell cycle after release (radmanlivaja2023thebuddingyeast pages 24-26, radmanlivaja2023thebuddingyeast pages 22-24)	RITE tag-switch, ChIP-seq, nutrient-shift experiments	Radman-Livaja et al. 2023, Research Square preprint	https://doi.org/10.21203/rs.3.rs-3495250/v1
Effect of reduced Sir3 supply on silencing	In a Sir3 hypomorph, SIR-dependent silencing after exit from arrest was 15–30× less efficient, and in mid-log cells 200–500× less efficient than WT; ON rates scaled with Sir3 dosage whereas OFF rates were similar (radmanlivaja2023thebuddingyeast pages 24-26)	RITE hypomorph plus silencing assays	Radman-Livaja et al. 2023, Research Square preprint	https://doi.org/10.21203/rs.3.rs-3495250/v1
Magnitude of repression at HM loci	Sir-dependent repression at HML/HMR reduces transcription by roughly four orders of magnitude (~10^4-fold), while Sir proteins subsequently occupy nucleosomes across these loci (fouet2023limitstotranscriptional pages 1-2)	CRASH reporter context and prior silencing literature synthesis	Fouet & Rine 2023, Genetics	https://doi.org/10.1093/genetics/iyac180
Frequency of transient silencing loss	Using the sensitive CRASH assay, transient silencing failures at HML occurred at about 10^-3 per generation, showing that Sir-based repression is strong but not absolute (fouet2023limitstotranscriptional pages 1-2)	CRASH recombinase assay at HML	Fouet & Rine 2023, Genetics	https://doi.org/10.1093/genetics/iyac180
Gene-specific differences within a silent locus	Silencing loss at the HML α2 reporter was ~10-fold higher than at α1; when unsilenced, CRE expression was ~8-fold higher for α2 than α1, and RT-qPCR in WT showed a smaller ~4-fold difference, indicating transient failure can be gene-specific rather than locus-wide (fouet2023limitstotranscriptional pages 12-13, fouet2023limitstotranscriptional pages 1-2)	CRASH reporters and RT-qPCR	Fouet & Rine 2023, Genetics	https://doi.org/10.1093/genetics/iyac180

Table: This table summarizes core mechanistic and quantitative findings for budding yeast Sir3/SIR3 (P06701), emphasizing structural nucleosome recognition, histone-mark dependencies, silencing dynamics, and 2023–2024 measurements of chromatin contacts and silencing escape.

6) Subcellular localization and sites of action

Sir3 functions in the nucleus on chromatin, with classical sites of action at HML/HMR and telomere-proximal/subtelomeric regions, where it binds nucleosomes as part of the Sir2/3/4 silencing machinery (onishi2007roleofthe pages 1-2, yuan2024minimalrequirementsfor pages 1-2, fouet2023limitstotranscriptional pages 1-2). Recent mapping adds that Sir3 also makes transient direct contacts with euchromatin at large scale, beyond canonical silent domains. (radmanlivaja2023thebuddingyeast pages 7-9, radmanlivaja2023thebuddingyeast pages 22-24)

7) Pathways and molecular interactions (high-confidence)

SIR silencing pathway: Sir2 (deacetylase) generates hypoacetylated nucleosomes, and Sir3/Sir4 bind and spread/compact chromatin, producing transcriptional repression; Sir3 recognition of deacetylated H4K16 is central to this mechanism. (dhillon2024transcriptionalsilencingin pages 2-4, yuan2024minimalrequirementsfor pages 1-2)
Direct nucleosome binding: Sir3 BAH contacts histone surfaces and tails (notably the H4 tail; LRS surface), providing a structural basis for chromatin association and for sensitivity to histone marks (including residues H4K16 and H3K79). (armache2011structuralbasisof pages 1-3, armache2011structuralbasisof pages 3-4)

8) Notes on evidence quality and recency

The core structural mechanism of Sir3 nucleosome recognition is anchored by high-confidence peer-reviewed structural biology (Science 2011). (armache2011structuralbasisof pages 1-3, armache2011structuralbasisof media 2905dd00)

For 2023–2024 “latest research,” key quantitative advances include peer-reviewed work on escape frequencies (Genetics 2023) and mechanistic reviews/models (Epigenetics & Chromatin 2024; PNAS 2024), as well as a 2023 preprint providing substantial quantitative mapping of transient contacts and dynamics; preprint conclusions should be interpreted with appropriate caution until peer-reviewed. (radmanlivaja2023thebuddingyeast pages 24-26, fouet2023limitstotranscriptional pages 1-2, dhillon2024transcriptionalsilencingin pages 2-4, miangolarra2024twowayfeedbackbetween pages 1-2)

References

(buchberger2008sir3nucleosomeinteractionsin pages 1-2): Johannes R. Buchberger, Megumi Onishi, Geng Li, Jan Seebacher, Adam D. Rudner, Steven P. Gygi, and Danesh Moazed. Sir3-nucleosome interactions in spreading of silent chromatin in saccharomyces cerevisiae. Nov 2008. URL: https://doi.org/10.1128/mcb.01210-08, doi:10.1128/mcb.01210-08. This article has 79 citations and is from a domain leading peer-reviewed journal.
(connelly2006structureandfunction pages 1-2): Jessica J. Connelly, Peihua Yuan, Hao-Chi Hsu, Zhizhong Li, Rui-Ming Xu, and Rolf Sternglanz. Structure and function of the saccharomyces cerevisiae sir3 bah domain. Molecular and Cellular Biology, 26:3256-3265, Apr 2006. URL: https://doi.org/10.1128/mcb.26.8.3256-3265.2006, doi:10.1128/mcb.26.8.3256-3265.2006. This article has 91 citations and is from a domain leading peer-reviewed journal.
(currie2024thebahdomain pages 1-4): Mark A. Currie, Reza Behrouzi, and Danesh Moazed. The bah domain: a versatile histone modification reader. Chromatin Readers in Health and Disease, pages 13-30, Jan 2024. URL: https://doi.org/10.1016/b978-0-12-823376-4.00006-9, doi:10.1016/b978-0-12-823376-4.00006-9. This article has 1 citations.
(dhillon2024transcriptionalsilencingin pages 1-2): Namrita Dhillon and Rohinton T. Kamakaka. Transcriptional silencing in saccharomyces cerevisiae: known unknowns. Epigenetics & Chromatin, Sep 2024. URL: https://doi.org/10.1186/s13072-024-00553-7, doi:10.1186/s13072-024-00553-7. This article has 4 citations and is from a peer-reviewed journal.
(dhillon2024transcriptionalsilencingin pages 2-4): Namrita Dhillon and Rohinton T. Kamakaka. Transcriptional silencing in saccharomyces cerevisiae: known unknowns. Epigenetics & Chromatin, Sep 2024. URL: https://doi.org/10.1186/s13072-024-00553-7, doi:10.1186/s13072-024-00553-7. This article has 4 citations and is from a peer-reviewed journal.
(onishi2007roleofthe pages 1-2): Megumi Onishi, Gunn-Guang Liou, Johannes R. Buchberger, Thomas Walz, and Danesh Moazed. Role of the conserved sir3-bah domain in nucleosome binding and silent chromatin assembly. Molecular cell, 28 6:1015-28, Dec 2007. URL: https://doi.org/10.1016/j.molcel.2007.12.004, doi:10.1016/j.molcel.2007.12.004. This article has 213 citations and is from a highest quality peer-reviewed journal.
(currie2024thebahdomain pages 4-8): Mark A. Currie, Reza Behrouzi, and Danesh Moazed. The bah domain: a versatile histone modification reader. Chromatin Readers in Health and Disease, pages 13-30, Jan 2024. URL: https://doi.org/10.1016/b978-0-12-823376-4.00006-9, doi:10.1016/b978-0-12-823376-4.00006-9. This article has 1 citations.
(armache2011structuralbasisof pages 1-3): Karim-Jean Armache, Joseph D. Garlick, Daniele Canzio, Geeta J. Narlikar, and Robert E. Kingston. Structural basis of silencing: sir3 bah domain in complex with a nucleosome at 3.0 å resolution. Science, 334:977-982, Nov 2011. URL: https://doi.org/10.1126/science.1210915, doi:10.1126/science.1210915. This article has 326 citations and is from a highest quality peer-reviewed journal.
(armache2011structuralbasisof pages 3-4): Karim-Jean Armache, Joseph D. Garlick, Daniele Canzio, Geeta J. Narlikar, and Robert E. Kingston. Structural basis of silencing: sir3 bah domain in complex with a nucleosome at 3.0 å resolution. Science, 334:977-982, Nov 2011. URL: https://doi.org/10.1126/science.1210915, doi:10.1126/science.1210915. This article has 326 citations and is from a highest quality peer-reviewed journal.
(armache2011structuralbasisof media 2905dd00): Karim-Jean Armache, Joseph D. Garlick, Daniele Canzio, Geeta J. Narlikar, and Robert E. Kingston. Structural basis of silencing: sir3 bah domain in complex with a nucleosome at 3.0 å resolution. Science, 334:977-982, Nov 2011. URL: https://doi.org/10.1126/science.1210915, doi:10.1126/science.1210915. This article has 326 citations and is from a highest quality peer-reviewed journal.
(armache2011structuralbasisof media b38d5437): Karim-Jean Armache, Joseph D. Garlick, Daniele Canzio, Geeta J. Narlikar, and Robert E. Kingston. Structural basis of silencing: sir3 bah domain in complex with a nucleosome at 3.0 å resolution. Science, 334:977-982, Nov 2011. URL: https://doi.org/10.1126/science.1210915, doi:10.1126/science.1210915. This article has 326 citations and is from a highest quality peer-reviewed journal.
(armache2011structuralbasisof media faf51ee3): Karim-Jean Armache, Joseph D. Garlick, Daniele Canzio, Geeta J. Narlikar, and Robert E. Kingston. Structural basis of silencing: sir3 bah domain in complex with a nucleosome at 3.0 å resolution. Science, 334:977-982, Nov 2011. URL: https://doi.org/10.1126/science.1210915, doi:10.1126/science.1210915. This article has 326 citations and is from a highest quality peer-reviewed journal.
(yuan2024minimalrequirementsfor pages 1-2): Andy H. Yuan and Danesh Moazed. Minimal requirements for the epigenetic inheritance of engineered silent chromatin domains. Proceedings of the National Academy of Sciences of the United States of America, Jan 2024. URL: https://doi.org/10.1073/pnas.2318455121, doi:10.1073/pnas.2318455121. This article has 7 citations and is from a highest quality peer-reviewed journal.
(fouet2023limitstotranscriptional pages 1-2): Marc Fouet and Jasper Rine. Limits to transcriptional silencing in saccharomyces cerevisiae. Genetics, Dec 2023. URL: https://doi.org/10.1093/genetics/iyac180, doi:10.1093/genetics/iyac180. This article has 5 citations and is from a domain leading peer-reviewed journal.
(fouet2023limitstotranscriptional pages 12-13): Marc Fouet and Jasper Rine. Limits to transcriptional silencing in saccharomyces cerevisiae. Genetics, Dec 2023. URL: https://doi.org/10.1093/genetics/iyac180, doi:10.1093/genetics/iyac180. This article has 5 citations and is from a domain leading peer-reviewed journal.
(radmanlivaja2023thebuddingyeast pages 4-7): Marta Radman-Livaja, Pritha Bhattacharjee, Alain Camasses, Hrvoje Galic, Ana Hrgovcic, Lara Demont, Linh Nguyen, and Pauline Vasseur. The budding yeast heterochromatic protein sir3 modulates genome-wide gene expression through transient direct contacts with euchromatin. Unknown journal, Nov 2023. URL: https://doi.org/10.21203/rs.3.rs-3495250/v1, doi:10.21203/rs.3.rs-3495250/v1.
(radmanlivaja2023thebuddingyeast pages 7-9): Marta Radman-Livaja, Pritha Bhattacharjee, Alain Camasses, Hrvoje Galic, Ana Hrgovcic, Lara Demont, Linh Nguyen, and Pauline Vasseur. The budding yeast heterochromatic protein sir3 modulates genome-wide gene expression through transient direct contacts with euchromatin. Unknown journal, Nov 2023. URL: https://doi.org/10.21203/rs.3.rs-3495250/v1, doi:10.21203/rs.3.rs-3495250/v1.
(radmanlivaja2023thebuddingyeast pages 22-24): Marta Radman-Livaja, Pritha Bhattacharjee, Alain Camasses, Hrvoje Galic, Ana Hrgovcic, Lara Demont, Linh Nguyen, and Pauline Vasseur. The budding yeast heterochromatic protein sir3 modulates genome-wide gene expression through transient direct contacts with euchromatin. Unknown journal, Nov 2023. URL: https://doi.org/10.21203/rs.3.rs-3495250/v1, doi:10.21203/rs.3.rs-3495250/v1.
(radmanlivaja2023thebuddingyeast pages 24-26): Marta Radman-Livaja, Pritha Bhattacharjee, Alain Camasses, Hrvoje Galic, Ana Hrgovcic, Lara Demont, Linh Nguyen, and Pauline Vasseur. The budding yeast heterochromatic protein sir3 modulates genome-wide gene expression through transient direct contacts with euchromatin. Unknown journal, Nov 2023. URL: https://doi.org/10.21203/rs.3.rs-3495250/v1, doi:10.21203/rs.3.rs-3495250/v1.
(miangolarra2024twowayfeedbackbetween pages 1-2): Ander Movilla Miangolarra, Daniel S. Saxton, Zhi Yan, Jasper Rine, and Martin Howard. Two-way feedback between chromatin compaction and histone modification state explains saccharomyces cerevisiae heterochromatin bistability. Proceedings of the National Academy of Sciences of the United States of America, Apr 2024. URL: https://doi.org/10.1073/pnas.2403316121, doi:10.1073/pnas.2403316121. This article has 13 citations and is from a highest quality peer-reviewed journal.

Artifacts

Edison artifact artifact-00

Citations

yuan2024minimalrequirementsfor pages 1-2
fouet2023limitstotranscriptional pages 1-2
radmanlivaja2023thebuddingyeast pages 24-26
miangolarra2024twowayfeedbackbetween pages 1-2
fouet2023limitstotranscriptional pages 12-13
radmanlivaja2023thebuddingyeast pages 7-9
connelly2006structureandfunction pages 1-2
currie2024thebahdomain pages 1-4
dhillon2024transcriptionalsilencingin pages 1-2
dhillon2024transcriptionalsilencingin pages 2-4
onishi2007roleofthe pages 1-2
currie2024thebahdomain pages 4-8
armache2011structuralbasisof pages 1-3
armache2011structuralbasisof pages 3-4
radmanlivaja2023thebuddingyeast pages 4-7
radmanlivaja2023thebuddingyeast pages 22-24
https://doi.org/10.1016/b978-0-12-823376-4.00006-9
https://doi.org/10.1128/mcb.01210-08
https://doi.org/10.1126/science.1210915
https://doi.org/10.1073/pnas.2318455121
https://doi.org/10.1186/s13072-024-00553-7
https://doi.org/10.1073/pnas.2403316121
https://doi.org/10.21203/rs.3.rs-3495250/v1
https://doi.org/10.1093/genetics/iyac180
https://doi.org/10.1128/mcb.01210-08,
https://doi.org/10.1128/mcb.26.8.3256-3265.2006,
https://doi.org/10.1016/b978-0-12-823376-4.00006-9,
https://doi.org/10.1186/s13072-024-00553-7,
https://doi.org/10.1016/j.molcel.2007.12.004,
https://doi.org/10.1126/science.1210915,
https://doi.org/10.1073/pnas.2318455121,
https://doi.org/10.1093/genetics/iyac180,
https://doi.org/10.21203/rs.3.rs-3495250/v1,
https://doi.org/10.1073/pnas.2403316121,

📄 View Raw YAML

id: P06701
gene_symbol: SIR3
aliases:
  - CMT1
  - MAR2
  - STE8
  - YLR442C
  - L9753.10
product_type: PROTEIN
status: INITIALIZED
taxon:
  id: NCBITaxon:559292
  label: Saccharomyces cerevisiae
description: Structural protein and core component of the Sir2-3-4 silent 
  chromatin complex. SIR3 functions as a nucleosome-binding protein critical for
  heterochromatin formation and maintenance at mating-type loci, telomeres, and 
  rDNA. Unlike SIR2 (the catalytic deacetylase), SIR3 provides the structural 
  scaffold through direct binding to histones and DNA. Forms homodimers and 
  interacts with SIR2 and SIR4 to establish transcriptional repression through 
  chromatin compaction and formation of condensed higher-order chromatin 
  structure. Also directly localizes to euchromatic origins where it 
  participates in modulating origin function through nucleosome binding without 
  requiring its catalytic activity.
existing_annotations:
  - term:
      id: GO:0006270
      label: DNA replication initiation
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    review:
      summary: IBA-seeded from phylogenetic inference. SIR3 directly localizes 
        to euchromatic origins but suppresses MCM loading (negative regulation),
        not initiation.
      action: REMOVE
      reason: SIR3 does not participate in DNA replication initiation. SIR3 
        suppresses MCM loading and origin activity (GO:0008156), which is 
        negative regulation, not initiation itself. Core machinery includes ORC,
        CDC6, CDT1, MCM2-7, not SIR3.
      additional_reference_ids:
        - PMID:29795547
      supported_by:
        - reference_id: PMID:29795547
          supporting_text: Sir2 and Sir3 were physically associated with 
            nucleosomes adjacent to origins
  - term:
      id: GO:0003688
      label: DNA replication origin binding
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    review:
      summary: IBA annotation. SIR3 binds nucleosomes at origins, not origins 
        themselves. Origins are DNA sequences recognized by ORC.
      action: REMOVE
      reason: SIR3 binds chromatin/nucleosomes near origins, not the origin DNA 
        itself. Origin binding is attributed to ORC (origin recognition 
        complex). This confuses SIR3 nucleosome binding with origin recognition.
      additional_reference_ids:
        - PMID:29795547
      supported_by:
        - reference_id: PMID:29795547
          supporting_text: eCollection 2018 May.
  - term:
      id: GO:0033314
      label: mitotic DNA replication checkpoint signaling
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    review:
      summary: IBA annotation with no supporting evidence. SIR3 has no known 
        role in checkpoint signaling.
      action: REMOVE
      reason: SIR3 functions in heterochromatin formation and chromatin 
        structure, not DNA damage or replication checkpoints. This is a spurious
        IBA inference.
  - term:
      id: GO:0003677
      label: DNA binding
    evidence_type: IEA
    original_reference_id: GO_REF:0000043
    review:
      summary: SIR3 directly binds DNA. Experimentally validated with both ds 
        and ssDNA binding (PMID:19099415). More specific molecular functions 
        exist (GO:0003690, GO:0003697) but DNA binding is also appropriate.
      action: ACCEPT
      reason: SIR3 is experimentally shown to bind both double-stranded 
        (PMID:19099415) and single-stranded DNA (PMID:19099415). This is a core 
        molecular function supporting its role in chromatin structure and 
        transcriptional regulation.
      additional_reference_ids:
        - PMID:19099415
        - PMID:17176117
      supported_by:
        - reference_id: PMID:19099415
          supporting_text: Role of nucleic acid binding in Sir3p-dependent 
            interactions with chromatin
        - reference_id: PMID:17176117
          supporting_text: Domain organization and quaternary structure of the 
            Saccharomyces cerevisiae silent information regulator 3 protein, 
            Sir3p.
  - term:
      id: GO:0003682
      label: chromatin binding
    evidence_type: IEA
    original_reference_id: GO_REF:0000002
    review:
      summary: Critical molecular function. SIR3 binds chromatin and nucleosomes
        (PMID:19217406, PMID:18195043). Direct experimental evidence with IDA 
        code also available (PMID:18195043).
      action: ACCEPT
      reason: SIR3 directly binds to chromatin and nucleosomes, which is central
        to its function in heterochromatin formation. This is a core molecular 
        mechanism.
      additional_reference_ids:
        - PMID:19217406
        - PMID:18195043
        - file:yeast/SIR3/SIR3-deep-research-falcon.md
      supported_by:
        - reference_id: PMID:19217406
          supporting_text: 'Reconstitution of yeast silent chromatin: multiple contact
            sites and O-AADPR binding load SIR complexes onto nucleosomes in vitro.'
        - reference_id: PMID:18195043
          supporting_text: Jan 14. Long-range communication between the
            silencers of HMR.
        - reference_id: file:yeast/SIR3/SIR3-deep-research-falcon.md
          supporting_text: |
            Across classic biochemistry and structural work, **Sir3 is the principal nucleosome-binding/structural subunit** of the Sir2/3/4 complex. It binds chromatin via its conserved **N-terminal BAH (bromo-adjacent homology) domain**, and it oligomerizes and participates in spreading/compaction through additional conserved regions, including a **C-terminal AAA-like domain** (lacking canonical ATPase activity).
  - term:
      id: GO:0005634
      label: nucleus
    evidence_type: IEA
    original_reference_id: GO_REF:0000044
    review:
      summary: SIR3 localizes to the nucleus where it functions in 
        heterochromatin formation at mating-type loci, telomeres, and 
        euchromatic origins.
      action: ACCEPT
      reason: SIR3 is exclusively nuclear, functioning at HML, HMR, telomeres,
        rDNA, and euchromatic origins. Correct cellular localization annotation.
      additional_reference_ids:
        - file:yeast/SIR3/SIR3-deep-research-falcon.md
      supported_by:
        - reference_id: file:yeast/SIR3/SIR3-deep-research-falcon.md
          supporting_text: |
            Sir3 functions in the **nucleus** on **chromatin**, with classical sites of action at **HML/HMR** and **telomere-proximal/subtelomeric regions**, where it binds nucleosomes as part of the Sir2/3/4 silencing machinery
  - term:
      id: GO:0005694
      label: chromosome
    evidence_type: IEA
    original_reference_id: GO_REF:0000117
    review:
      summary: SIR3 is a chromosomal protein functioning at multiple chromosomal
        loci.
      action: ACCEPT
      reason: SIR3 localizes to chromosomes including telomeres, mating-type 
        loci, rDNA, and euchromatic origins. Correct annotation.
  - term:
      id: GO:0006351
      label: DNA-templated transcription
    evidence_type: IEA
    original_reference_id: GO_REF:0000043
    review:
      summary: SIR3 is not involved in the transcription process itself. SIR3s 
        role is in transcriptional repression through heterochromatin formation 
        (GO:0006354 or GO:0030466 are more accurate).
      action: MODIFY
      reason: GO:0006351 refers to the core transcription machinery. SIR3 is not
        involved in RNA synthesis. Instead, SIR3 negatively regulates 
        transcription by forming repressive heterochromatin. More specific and 
        accurate terms are GO:0006354 (negative regulation) or GO:0030466 
        (mating-type silencing).
      proposed_replacement_terms:
        - id: GO:0006354
          label: negative regulation of transcription, DNA-templated
        - id: GO:0030466
          label: silent mating-type cassette heterochromatin formation
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:11689698
    review:
      summary: SIR3 binds RAP1 protein (PMID:11689698). However, protein binding
        is too vague - more specific binding terms exist.
      action: KEEP_AS_NON_CORE
      reason: GO:0005515 (protein binding) is uninformative per curation 
        guidelines. SIR3 binds specific partners (SIR2, SIR4, RAP1, histones). 
        These are better captured by identical protein binding (GO:0042802), 
        nucleosome binding (GO:0031491), and chromatin binding (GO:0003682).
      additional_reference_ids:
        - PMID:11689698
      supported_by:
        - reference_id: PMID:11689698
          supporting_text: Multiple interactions in Sir protein recruitment by 
            Rap1p at silencers and telomeres in yeast.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:11805837
    review:
      summary: Protein complex membership by mass spectrometry. Too vague 
        compared to specific binding partners and complex membership 
        annotations.
      action: KEEP_AS_NON_CORE
      reason: Generic protein binding obscures the specific complex and binding 
        partners documented in this study.
      additional_reference_ids:
        - PMID:11805837
      supported_by:
        - reference_id: PMID:11805837
          supporting_text: Systematic identification of protein complexes in 
            Saccharomyces cerevisiae by mass spectrometry.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:16554755
    review:
      summary: Protein complex annotation. Vague annotation.
      action: KEEP_AS_NON_CORE
      reason: Generic protein binding is less informative than specific binding 
        partner and complex annotations.
      supported_by:
        - reference_id: PMID:16554755
          supporting_text: Global landscape of protein complexes in the yeast 
            Saccharomyces cerevisiae.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:16717101
    review:
      summary: SIR3 domain structure and interactions (PMID:16717101). SIR3 
        binding to SIR4 and other partners. Generic term.
      action: KEEP_AS_NON_CORE
      reason: Uninformative without specifying the actual binding partners and 
        interactions.
      additional_reference_ids:
        - PMID:16717101
      supported_by:
        - reference_id: PMID:16717101
          supporting_text: Domain structure and protein interactions of the 
            silent information regulator Sir3 revealed by screening a nested 
            deletion library of protein fragments.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:21217703
    review:
      summary: RAP1 binding. Generic annotation.
      action: KEEP_AS_NON_CORE
      reason: Protein binding is too vague. The specific RAP1 interaction should
        be documented elsewhere.
      supported_by:
        - reference_id: PMID:21217703
          supporting_text: A conserved motif within RAP1 has diversified roles 
            in telomere protection and regulation in different organisms.
  - term:
      id: GO:0042802
      label: identical protein binding
    evidence_type: IPI
    original_reference_id: PMID:16717101
    review:
      summary: SIR3 forms homodimers through nested deletion and biochemical 
        analysis (PMID:16717101). Essential structural feature.
      action: ACCEPT
      reason: SIR3 self-associates to form oligomers and homodimers. This is a 
        core structural feature essential for heterochromatin formation. 
        Directly supports GO:0005677 complex formation.
      additional_reference_ids:
        - PMID:17176117
      supported_by:
        - reference_id: PMID:17176117
          supporting_text: Sir3p self-associates extensively in moderate 
            salt...producing a broad range of oligomers
        - reference_id: PMID:16717101
          supporting_text: 2006 May 22. Domain structure and protein 
            interactions of the silent information regulator Sir3 revealed by 
            screening a nested deletion library of protein fragments.
  - term:
      id: GO:0042802
      label: identical protein binding
    evidence_type: IPI
    original_reference_id: PMID:21179020
    review:
      summary: SIR3 homodimer formation confirmed by chromatin-associated 
        interactome mapping (PMID:21179020).
      action: ACCEPT
      reason: Multiple independent experimental approaches confirm SIR3 
        homodimerization is essential for function.
      supported_by:
        - reference_id: PMID:21179020
          supporting_text: Defining the budding yeast chromatin-associated 
            interactome
  - term:
      id: GO:0042802
      label: identical protein binding
    evidence_type: IPI
    original_reference_id: PMID:23299941
    review:
      summary: SIR3 C-terminal winged helix domain mediates homodimerization, 
        essential for heterochromatin formation (PMID:23299941).
      action: ACCEPT
      reason: Structural evidence shows SIR3 WH domain dimerizes and this is 
        required for silencing. Core structural function.
      additional_reference_ids:
        - PMID:23299941
      supported_by:
        - reference_id: PMID:23299941
          supporting_text: Dimerization of Sir3 via its C-terminal winged helix 
            domain is essential for yeast heterochromatin formation
  - term:
      id: GO:0031507
      label: heterochromatin formation
    evidence_type: NAS
    original_reference_id: PMID:15282295
    review:
      summary: SIR3 is essential for heterochromatin formation. Core function 
        documented in budding yeast silencing complexes review.
      action: ACCEPT
      reason: SIR3 is a core component of the Sir2-3-4 complex required for 
        heterochromatin formation at mating-type loci, telomeres, and rDNA. This
        is the defining function of SIR3.
      additional_reference_ids:
        - PMID:15282295
        - file:yeast/SIR3/SIR3-deep-research-falcon.md
      supported_by:
        - reference_id: PMID:15282295
          supporting_text: Budding yeast silencing complexes and regulation of
            Sir2 activity by protein-protein interactions.
        - reference_id: file:yeast/SIR3/SIR3-deep-research-falcon.md
          supporting_text: |
            A widely used mechanistic model is an iterative “read–write” logic in which **Sir2 deacetylates histone H4K16**, creating binding-competent nucleosomes; **Sir3 preferentially recognizes deacetylated H4K16 nucleosomes** and, together with Sir4-mediated assemblies, supports cooperative binding and **cis-spreading** of the silent domain outward from silencers.
  - term:
      id: GO:0003676
      label: nucleic acid binding
    evidence_type: EXP
    original_reference_id: PMID:17176117
    review:
      summary: Direct experimental evidence of nucleic acid binding through 
        domain organization and biochemical characterization (PMID:17176117).
      action: ACCEPT
      reason: EXP evidence from biochemical studies. SIR3 directly binds nucleic
        acids. Experimentally validated.
      additional_reference_ids:
        - PMID:17176117
      supported_by:
        - reference_id: PMID:17176117
          supporting_text: Domain organization and quaternary structure of the 
            Saccharomyces cerevisiae silent information regulator 3 protein
  - term:
      id: GO:0003688
      label: DNA replication origin binding
    evidence_type: IDA
    original_reference_id: PMID:29795547
    review:
      summary: SIR3 localizes to origin-adjacent nucleosomes but does not bind 
        origins themselves. Origins are DNA sequences bound by ORC.
      action: REMOVE
      reason: SIR3 binds nucleosomes that are located near origins 
        (PMID:29795547), not the origin DNA itself. The origin DNA is recognized
        and bound by ORC (origin recognition complex). This annotation confuses 
        nucleosome binding with origin binding.
      additional_reference_ids:
        - PMID:29795547
      supported_by:
        - reference_id: PMID:29795547
          supporting_text: Yeast heterochromatin regulators Sir2 and Sir3 act 
            directly at euchromatic DNA replication origins.
  - term:
      id: GO:0005677
      label: chromatin silencing complex
    evidence_type: IDA
    original_reference_id: PMID:9122169
    review:
      summary: SIR3 is an integral component of the chromatin silencing complex 
        (Sir2-SIR3-SIR4). Both is_active_in and part_of relations exist in GOA.
      action: ACCEPT
      reason: SIR3 is a core structural component of the Sir2-3-4 silent 
        chromatin complex documented by co-immunoprecipitation and biochemical 
        studies.
      additional_reference_ids:
        - PMID:9122169
        - file:yeast/SIR3/SIR3-deep-research-falcon.md
      supported_by:
        - reference_id: PMID:9122169
          supporting_text: Silent information regulator protein complexes in
            Saccharomyces cerevisiae
        - reference_id: file:yeast/SIR3/SIR3-deep-research-falcon.md
          supporting_text: |
            Subtelomeric Sir3 methylation is abolished in *sir2Δ* and reduced about 2-fold in *sir4Δ*, consistent with Sir3 acting within the Sir2/3/4 complex
  - term:
      id: GO:0008156
      label: negative regulation of DNA replication
    evidence_type: IMP
    original_reference_id: PMID:29795547
    review:
      summary: SIR3 suppresses MCM loading at euchromatic origins and maintains 
        repression at heterochromatic origins. Core negative regulatory 
        function.
      action: ACCEPT
      reason: SIR3 directly suppresses MCM complex loading at the majority of 
        euchromatic origins and negatively regulates rDNA and telomeric origin 
        function. This is a key SIR3 function distinguishing it from initiation 
        role.
      additional_reference_ids:
        - PMID:29795547
      supported_by:
        - reference_id: PMID:29795547
          supporting_text: heterochromatin proteins directly modify the local 
            chromatin environment of euchromatic DNA replication origins
  - term:
      id: GO:0031509
      label: subtelomeric heterochromatin formation
    evidence_type: IMP
    original_reference_id: PMID:1913809
    review:
      summary: SIR3 is required for silencing at telomeres and subtelomeric 
        regions. Classic position-effect data.
      action: ACCEPT
      reason: SIR3 is essential for telomeric and subtelomeric heterochromatin. 
        Core function of SIR complex.
      additional_reference_ids:
        - PMID:1913809
        - file:yeast/SIR3/SIR3-deep-research-falcon.md
      supported_by:
        - reference_id: PMID:1913809
          supporting_text: Modifiers of position effect are shared between
            telomeric and silent mating-type loci in S. cerevisiae.
        - reference_id: file:yeast/SIR3/SIR3-deep-research-falcon.md
          supporting_text: |
            Stable, high-density Sir3 occupancy is concentrated about ±2 kb around subtelomeric SIR nucleation sites; ChIP-seq falls to background by ~4 kb downstream of XCS, but transient low-density contacts extend to ~30 kb
  - term:
      id: GO:0031509
      label: subtelomeric heterochromatin formation
    evidence_type: IMP
    original_reference_id: PMID:31599702
    review:
      summary: SIR3 role in telomeric silencing in context of lifespan and 
        aging.
      action: ACCEPT
      reason: Multiple genetic studies confirm SIR3 requirement for telomeric 
        silencing and its connection to aging phenotypes.
      additional_reference_ids:
        - PMID:31599702
      supported_by:
        - reference_id: PMID:31599702
          supporting_text: Reciprocal interactions between mtDNA and lifespan 
            control in budding yeast
  - term:
      id: GO:0031509
      label: subtelomeric heterochromatin formation
    evidence_type: IMP
    original_reference_id: PMID:9501103
    review:
      summary: SIR3 mutation effects on telomeric silencing and NHEJ processes.
      action: ACCEPT
      reason: SIR3 is required for telomeric silencing and maintenance.
      additional_reference_ids:
        - PMID:9501103
      supported_by:
        - reference_id: PMID:9501103
          supporting_text: Components of the Ku-dependent non-homologous 
            end-joining pathway are involved in telomeric length maintenance and
            telomeric silencing.
  - term:
      id: GO:0000781
      label: chromosome, telomeric region
    evidence_type: IMP
    original_reference_id: PMID:27122604
    review:
      summary: SIR3 localizes to telomeric regions and is required for telomere 
        clustering in quiescent cells.
      action: ACCEPT
      reason: SIR3 directly localizes to and functions at telomeres. Documented 
        localization via ChIP.
      additional_reference_ids:
        - PMID:27122604
      supported_by:
        - reference_id: PMID:27122604
          supporting_text: Quiescent Saccharomyces cerevisiae forms telomere 
            hyperclusters
  - term:
      id: GO:0000781
      label: chromosome, telomeric region
    evidence_type: IDA
    original_reference_id: PMID:9710643
    review:
      summary: Direct experimental evidence of SIR3 binding at telomeres by 
        ChIP.
      action: ACCEPT
      reason: ChIP evidence documenting SIR3 at telomeric chromatin.
      additional_reference_ids:
        - PMID:9710643
      supported_by:
        - reference_id: PMID:9710643
          supporting_text: Sir proteins, Rif proteins, and Cdc13p bind 
            Saccharomyces telomeres in vivo.
  - term:
      id: GO:0000792
      label: heterochromatin
    evidence_type: IDA
    original_reference_id: PMID:20176978
    review:
      summary: SIR3 is a component of heterochromatin. Required for formation 
        and maintenance.
      action: ACCEPT
      reason: SIR3 localizes to and is integral part of heterochromatin at all
        silenced loci.
      additional_reference_ids:
        - file:yeast/SIR3/SIR3-deep-research-falcon.md
      supported_by:
        - reference_id: PMID:20176978
          supporting_text: An auxiliary silencer and a boundary element maintain
            high levels of silencing proteins at HMR in Saccharomyces
            cerevisiae.
        - reference_id: file:yeast/SIR3/SIR3-deep-research-falcon.md
          supporting_text: |
            Across classic biochemistry and structural work, **Sir3 is the principal nucleosome-binding/structural subunit** of the Sir2/3/4 complex. It binds chromatin via its conserved **N-terminal BAH (bromo-adjacent homology) domain**, and it oligomerizes and participates in spreading/compaction through additional conserved regions, including a **C-terminal AAA-like domain** (lacking canonical ATPase activity).
  - term:
      id: GO:0005739
      label: mitochondrion
    evidence_type: HDA
    original_reference_id: PMID:14576278
    review:
      summary: HDA from mitochondrial proteomics study. SIR3 is not 
        mitochondrial.
      action: REMOVE
      reason: SIR3 is a nuclear protein with no known mitochondrial function or 
        localization. HDA from mitochondrial proteomics is a false positive, 
        likely from contamination in the MS sample.
      supported_by:
        - reference_id: PMID:14576278
          supporting_text: The proteome of Saccharomyces cerevisiae 
            mitochondria.
  - term:
      id: GO:0005739
      label: mitochondrion
    evidence_type: HDA
    original_reference_id: PMID:16823961
    review:
      summary: HDA from mitochondrial proteomics. Spurious annotation.
      action: REMOVE
      reason: SIR3 is exclusively nuclear. This HDA is likely experimental 
        artifact.
      supported_by:
        - reference_id: PMID:16823961
          supporting_text: "Toward the complete yeast mitochondrial proteome: multidimensional
            separation techniques for mitochondrial proteomics."
  - term:
      id: GO:0006303
      label: double-strand break repair via nonhomologous end joining
    evidence_type: IMP
    original_reference_id: PMID:9501103
    review:
      summary: SIR3 has been implicated in NHEJ through studies with Ku complex,
        but this is likely indirect through telomere maintenance.
      action: REMOVE
      reason: SIR3 itself is not a NHEJ component or catalyst. The association 
        appears to be indirect through telomeric regulation - SIR3 represses 
        telomeric origins and helps maintain telomere structure, which affects 
        DSB signaling. SIR3 has no role in core NHEJ machinery (Ku70/80, Lig IV,
        XLF).
      additional_reference_ids:
        - PMID:9501103
      supported_by:
        - reference_id: PMID:9501103
          supporting_text: Components of the Ku-dependent non-homologous 
            end-joining pathway are involved in telomeric length maintenance and
            telomeric silencing.
  - term:
      id: GO:0030466
      label: silent mating-type cassette heterochromatin formation
    evidence_type: IMP
    original_reference_id: PMID:3297920
    review:
      summary: Classic position-effect data. SIR3 is essential for HML and HMR 
        silencing. Founding function of the SIR genes.
      action: ACCEPT
      reason: SIR3 is required for mating-type silencing at HML and HMR loci. 
        This is a core function defined in the original SIR studies.
      additional_reference_ids:
        - PMID:3297920
        - file:yeast/SIR3/SIR3-deep-research-falcon.md
      supported_by:
        - reference_id: PMID:3297920
          supporting_text: Four genes responsible for a position effect on
            expression from HML and HMR in Saccharomyces cerevisiae.
        - reference_id: file:yeast/SIR3/SIR3-deep-research-falcon.md
          supporting_text: |
            Sir3 functions in the **nucleus** on **chromatin**, with classical sites of action at **HML/HMR** and **telomere-proximal/subtelomeric regions**, where it binds nucleosomes as part of the Sir2/3/4 silencing machinery
  - term:
      id: GO:0097695
      label: establishment of protein-containing complex localization to 
        telomere
    evidence_type: IMP
    original_reference_id: PMID:26218225
    review:
      summary: SIR3 is part of the Sir complex that localizes to telomeres. Ku 
        complex recruitment involves SIR4.
      action: ACCEPT
      reason: SIR3 as part of the Sir complex is required for complex 
        localization to telomeres for telomerase recruitment and telomere 
        maintenance.
      additional_reference_ids:
        - PMID:26218225
      supported_by:
        - reference_id: PMID:26218225
          supporting_text: The Ku subunit of telomerase binds Sir4 to recruit 
            telomerase to lengthen telomeres
  - term:
      id: GO:0034398
      label: telomere tethering at nuclear periphery
    evidence_type: IMP
    original_reference_id: PMID:26399229
    review:
      summary: SIR3 participates in telomere clustering and nuclear 
        organization.
      action: ACCEPT
      reason: SIR3 as component of Sir complex participates in telomere 
        tethering and clustering at nuclear periphery.
      additional_reference_ids:
        - PMID:26399229
      supported_by:
        - reference_id: PMID:26399229
          supporting_text: Spatial reorganization of telomeres in long-lived 
            quiescent cells
  - term:
      id: GO:0034398
      label: telomere tethering at nuclear periphery
    evidence_type: IMP
    original_reference_id: PMID:27122604
    review:
      summary: SIR3 required for telomere hyperclustering and nuclear 
        organization in quiescent cells.
      action: ACCEPT
      reason: Additional evidence for SIR3s role in telomere organization.
      additional_reference_ids:
        - PMID:27122604
      supported_by:
        - reference_id: PMID:27122604
          supporting_text: Quiescent Saccharomyces cerevisiae forms telomere 
            hyperclusters
  - term:
      id: GO:0000781
      label: chromosome, telomeric region
    evidence_type: IDA
    original_reference_id: PMID:16956377
    review:
      summary: SIR3 localization at telomeres documented.
      action: ACCEPT
      reason: Confirms telomeric localization of SIR3 at functional loci.
      supported_by:
        - reference_id: PMID:16956377
          supporting_text: The nuclear GTPase Gsp1p can affect proper telomeric 
            function through the Sir4 protein
  - term:
      id: GO:0003682
      label: chromatin binding
    evidence_type: IDA
    original_reference_id: PMID:18195043
    review:
      summary: Long-range communication and chromatin structure study. SIR3 
        directly binds chromatin.
      action: ACCEPT
      reason: Experimental evidence for chromatin binding in HMR silencer 
        analysis.
      additional_reference_ids:
        - PMID:18195043
      supported_by:
        - reference_id: PMID:18195043
          supporting_text: Long-range communication between the silencers of 
            HMR.
  - term:
      id: GO:0003690
      label: double-stranded DNA binding
    evidence_type: IDA
    original_reference_id: PMID:19099415
    review:
      summary: SIR3 binds double-stranded DNA in vitro. Direct biochemical 
        evidence.
      action: ACCEPT
      reason: SIR3 directly binds dsDNA. Core molecular function.
      additional_reference_ids:
        - PMID:19099415
      supported_by:
        - reference_id: PMID:19099415
          supporting_text: Role of nucleic acid binding in Sir3p-dependent 
            interactions with chromatin fibers
  - term:
      id: GO:0003697
      label: single-stranded DNA binding
    evidence_type: IDA
    original_reference_id: PMID:19099415
    review:
      summary: SIR3 binds single-stranded DNA in vitro. Direct biochemical 
        evidence.
      action: ACCEPT
      reason: SIR3 binds both ss and dsDNA. Core molecular function.
      additional_reference_ids:
        - PMID:19099415
      supported_by:
        - reference_id: PMID:19099415
          supporting_text: Role of nucleic acid binding in Sir3p-dependent 
            interactions with chromatin fibers.
  - term:
      id: GO:0005730
      label: nucleolus
    evidence_type: IDA
    original_reference_id: PMID:9150138
    review:
      summary: SIR3 redistributes from telomeres to nucleolus in long-lived 
        yeast strains. Reflects differential regulation in aging.
      action: ACCEPT
      reason: SIR3 can localize to nucleolus where rDNA silencing occurs. 
        Secondary but documented localization site. Important for aging 
        phenotypes.
      additional_reference_ids:
        - PMID:9150138
      supported_by:
        - reference_id: PMID:9150138
          supporting_text: Redistribution of silencing proteins from telomeres 
            to the nucleolus is associated with extension of life span in S. 
            cerevisiae.
  - term:
      id: GO:0030466
      label: silent mating-type cassette heterochromatin formation
    evidence_type: IMP
    original_reference_id: PMID:16581798
    review:
      summary: SIR3 BAH domain structure and function essential for HM 
        silencing.
      action: ACCEPT
      reason: Structural basis of SIR3 function in mating-type silencing 
        documented.
      additional_reference_ids:
        - PMID:16581798
        - file:yeast/SIR3/SIR3-deep-research-falcon.md
      supported_by:
        - reference_id: PMID:16581798
          supporting_text: Structure and function of the Saccharomyces
            cerevisiae Sir3 BAH domain.
        - reference_id: file:yeast/SIR3/SIR3-deep-research-falcon.md
          supporting_text: |
            **BAH domain as a nucleosome reader.** Sir3 contains an N-terminal BAH domain that directly binds nucleosomes and is **sensitive to histone modification state**, a key aspect of how silent chromatin is specified.
  - term:
      id: GO:0030466
      label: silent mating-type cassette heterochromatin formation
    evidence_type: IGI
    original_reference_id: PMID:16581798
    review:
      summary: Genetic interaction studies showing SIR3-SIR1 relationship in 
        silencing.
      action: ACCEPT
      reason: Genetic interactions confirm SIR3s role in mating-type silencing.
      additional_reference_ids:
        - PMID:16581798
      supported_by:
        - reference_id: PMID:16581798
          supporting_text: Structure and function of the Saccharomyces 
            cerevisiae Sir3 BAH domain.
  - term:
      id: GO:0031491
      label: nucleosome binding
    evidence_type: IDA
    original_reference_id: PMID:19217406
    review:
      summary: SIR3 binds nucleosomes in vitro. Histone H4 tail and H3K79 are 
        contact sites. Core mechanism of action.
      action: ACCEPT
      reason: SIR3 directly binds nucleosomes through multiple histone contact 
        sites. Core mechanism. Biochemically validated.
      additional_reference_ids:
        - PMID:19217406
        - file:yeast/SIR3/SIR3-deep-research-falcon.md
      supported_by:
        - reference_id: PMID:19217406
          supporting_text: 'Reconstitution of yeast silent chromatin: multiple contact
            sites...load SIR complexes onto nucleosomes'
        - reference_id: file:yeast/SIR3/SIR3-deep-research-falcon.md
          supporting_text: |
            A landmark structure solved a **3.0 Å crystal structure** of the Sir3 **BAH domain bound to the nucleosome**, showing that Sir3 BAH forms extensive contacts primarily with **histones (not DNA)**, including the **H4 N-terminal tail** and the H3/H4 LRS surface; critically, the structure orders and implicates residues such as **H4K16** and **H3K79**, whose modification state regulates silencing.
        - reference_id: file:yeast/SIR3/SIR3-deep-research-falcon.md
          supporting_text: |
            **BAH domain as a nucleosome reader.** Sir3 contains an N-terminal BAH domain that directly binds nucleosomes and is **sensitive to histone modification state**, a key aspect of how silent chromatin is specified.
  - term:
      id: GO:0031507
      label: heterochromatin formation
    evidence_type: IMP
    original_reference_id: PMID:16908543
    review:
      summary: SIR3 C-terminal domain involvement in heterochromatin initiation 
        and spreading.
      action: ACCEPT
      reason: Functional analysis of SIR3 domains in heterochromatin formation 
        and maintenance.
      additional_reference_ids:
        - PMID:16908543
      supported_by:
        - reference_id: PMID:16908543
          supporting_text: Sir3 C-terminal domain involvement in the initiation 
            and spreading of heterochromatin.
  - term:
      id: GO:0070481
      label: nuclear-transcribed mRNA catabolic process, non-stop decay
    evidence_type: IMP
    original_reference_id: PMID:17660569
    review:
      summary: SIR3 identified in genomic screen for nonstop mRNA decay factors.
        Mechanism unclear and likely indirect.
      action: REMOVE
      reason: SIR3 was recovered in a genomic screen (PMID:17660569) but SIR3s 
        primary functions are in chromatin and heterochromatin formation, not 
        mRNA surveillance. Association is likely indirect through secondary 
        effects on gene expression or cell stress responses, not a core function
        in mRNA decay pathway.
      additional_reference_ids:
        - PMID:17660569
      supported_by:
        - reference_id: PMID:17660569
          supporting_text: A genomic screen in yeast reveals novel aspects of 
            nonstop mRNA metabolism.
core_functions:
  - molecular_function:
      id: GO:0003682
      label: chromatin binding
    directly_involved_in:
      - id: GO:0031507
        label: heterochromatin formation
    description: SIR3 is a structural component that binds chromatin and 
      nucleosomes to establish and maintain heterochromatin domains. This is the
      primary molecular mechanism by which SIR3 functions.
  - molecular_function:
      id: GO:0042802
      label: identical protein binding
    directly_involved_in:
      - id: GO:0030466
        label: silent mating-type cassette heterochromatin formation
    description: SIR3 forms homodimers and interacts with SIR2 and SIR4 to 
      assemble the Sir2-3-4 silent chromatin complex. Homodimerization through 
      the C-terminal winged helix domain is essential for silencing function.
  - molecular_function:
      id: GO:0031491
      label: nucleosome binding
    directly_involved_in:
      - id: GO:0008156
        label: negative regulation of DNA replication
    description: SIR3 directly binds to histone octamers and nucleosomes at 
      mating-type loci, telomeres, rDNA, and euchromatic origins. This 
      nucleosome binding is critical for both establishing transcriptional 
      silencing and suppressing DNA replication initiation at origins.
references:
  - id: GO_REF:0000002
    title: Gene Ontology annotation through association of InterPro records with
      GO terms
    findings: []
  - id: GO_REF:0000033
    title: Annotation inferences using phylogenetic trees
    findings: []
  - id: GO_REF:0000043
    title: Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword 
      mapping
    findings: []
  - id: GO_REF:0000044
    title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular 
      Location vocabulary mapping, accompanied by conservative changes to GO 
      terms applied by UniProt
    findings: []
  - id: GO_REF:0000117
    title: Electronic Gene Ontology annotations created by ARBA machine learning
      models
    findings: []
  - id: PMID:1913809
    title: Modifiers of position effect are shared between telomeric and silent 
      mating-type loci in S. cerevisiae.
    findings: []
  - id: PMID:3297920
    title: Four genes responsible for a position effect on expression from HML 
      and HMR in Saccharomyces cerevisiae.
    findings: []
  - id: PMID:9122169
    title: "Silent information regulator protein complexes in Saccharomyces cerevisiae: a SIR2/SIR4 complex and evidence for a regulatory domain in SIR4 that inhibits its interaction with SIR3."
    findings: []
  - id: PMID:9150138
    title: Redistribution of silencing proteins from telomeres to the nucleolus 
      is associated with extension of life span in S. cerevisiae.
    findings: []
  - id: PMID:9501103
    title: Components of the Ku-dependent non-homologous end-joining pathway are
      involved in telomeric length maintenance and telomeric silencing.
    findings: []
  - id: PMID:9710643
    title: Sir proteins, Rif proteins, and Cdc13p bind Saccharomyces telomeres 
      in vivo.
    findings: []
  - id: PMID:11689698
    title: Multiple interactions in Sir protein recruitment by Rap1p at 
      silencers and telomeres in yeast.
    findings: []
  - id: PMID:11805837
    title: Systematic identification of protein complexes in Saccharomyces 
      cerevisiae by mass spectrometry.
    findings: []
  - id: PMID:14576278
    title: The proteome of Saccharomyces cerevisiae mitochondria.
    findings: []
  - id: PMID:15282295
    title: Budding yeast silencing complexes and regulation of Sir2 activity by 
      protein-protein interactions.
    findings: []
  - id: PMID:16554755
    title: Global landscape of protein complexes in the yeast Saccharomyces 
      cerevisiae.
    findings: []
  - id: PMID:16581798
    title: Structure and function of the Saccharomyces cerevisiae Sir3 BAH 
      domain.
    findings: []
  - id: PMID:16717101
    title: Domain structure and protein interactions of the silent information 
      regulator Sir3 revealed by screening a nested deletion library of protein 
      fragments.
    findings: []
  - id: PMID:16823961
    title: "Toward the complete yeast mitochondrial proteome: multidimensional separation techniques for mitochondrial proteomics."
    findings: []
  - id: PMID:16908543
    title: Sir3 C-terminal domain involvement in the initiation and spreading of
      heterochromatin.
    findings: []
  - id: PMID:16956377
    title: The nuclear GTPase Gsp1p can affect proper telomeric function through
      the Sir4 protein in Saccharomyces cerevisiae.
    findings: []
  - id: PMID:17176117
    title: Domain organization and quaternary structure of the Saccharomyces 
      cerevisiae silent information regulator 3 protein, Sir3p.
    findings: []
  - id: PMID:17660569
    title: A genomic screen in yeast reveals novel aspects of nonstop mRNA 
      metabolism.
    findings: []
  - id: PMID:18195043
    title: Long-range communication between the silencers of HMR.
    findings: []
  - id: PMID:19099415
    title: Role of nucleic acid binding in Sir3p-dependent interactions with 
      chromatin fibers.
    findings: []
  - id: PMID:19217406
    title: Reconstitution of yeast silent chromatin multiple contact sites and 
      O-AADPR binding load SIR complexes onto nucleosomes in vitro.
    findings: []
  - id: PMID:20176978
    title: An auxiliary silencer and a boundary element maintain high levels of 
      silencing proteins at HMR in Saccharomyces cerevisiae.
    findings: []
  - id: PMID:21179020
    title: Defining the budding yeast chromatin-associated interactome.
    findings: []
  - id: PMID:21217703
    title: A conserved motif within RAP1 has diversified roles in telomere 
      protection and regulation in different organisms.
    findings: []
  - id: PMID:23299941
    title: Dimerization of Sir3 via its C-terminal winged helix domain is 
      essential for yeast heterochromatin formation.
    findings: []
  - id: PMID:26218225
    title: The Ku subunit of telomerase binds Sir4 to recruit telomerase to 
      lengthen telomeres in S. cerevisiae.
    findings: []
  - id: PMID:26399229
    title: Spatial reorganization of telomeres in long-lived quiescent cells.
    findings: []
  - id: PMID:27122604
    title: Quiescent Saccharomyces cerevisiae forms telomere hyperclusters at 
      the nuclear membrane vicinity through a multifaceted mechanism involving 
      Esc1, the Sir complex, and chromatin condensation.
    findings: []
  - id: PMID:29795547
    title: Yeast heterochromatin regulators Sir2 and Sir3 act directly at 
      euchromatic DNA replication origins.
    findings: []
  - id: PMID:31599702
    title: Reciprocal interactions between mtDNA and lifespan control in budding
      yeast.
    findings: []
  - id: file:yeast/SIR3/SIR3-deep-research-falcon.md
    title: Falcon deep research report on SIR3
    findings:
      - statement: |
          Sir3 is the principal nucleosome-binding/structural subunit of the
          Sir2/3/4 silencing complex. It binds chromatin via its conserved
          N-terminal BAH domain and oligomerizes and participates in
          spreading/compaction through additional conserved regions, including a
          C-terminal AAA-like domain that lacks canonical ATPase activity.
        supporting_text: |
          Across classic biochemistry and structural work, **Sir3 is the principal nucleosome-binding/structural subunit** of the Sir2/3/4 complex. It binds chromatin via its conserved **N-terminal BAH (bromo-adjacent homology) domain**, and it oligomerizes and participates in spreading/compaction through additional conserved regions, including a **C-terminal AAA-like domain** (lacking canonical ATPase activity).
        reference_section_type: RESULTS
      - statement: |
          The Sir3 BAH domain is a histone-modification-sensitive nucleosome
          reader; its association with nucleosomes is impaired by H4K16
          acetylation and H3K79 methylation.
        supporting_text: |
          **BAH domain as a nucleosome reader.** Sir3 contains an N-terminal BAH domain that directly binds nucleosomes and is **sensitive to histone modification state**, a key aspect of how silent chromatin is specified.
        reference_section_type: RESULTS
      - statement: |
          A 3.0 Angstrom crystal structure of the Sir3 BAH domain bound to the
          nucleosome shows two Sir3 BAH domains binding one nucleosome (one per
          face) making extensive contacts primarily with the core histones
          (notably the H4 N-terminal tail and the H3/H4 LRS surface) rather than
          DNA, implicating H4K16 and H3K79.
        supporting_text: |
          A landmark structure solved a **3.0 Å crystal structure** of the Sir3 **BAH domain bound to the nucleosome**, showing that Sir3 BAH forms extensive contacts primarily with **histones (not DNA)**, including the **H4 N-terminal tail** and the H3/H4 LRS surface; critically, the structure orders and implicates residues such as **H4K16** and **H3K79**, whose modification state regulates silencing.
        reference_section_type: RESULTS
      - statement: |
          Read-write spreading logic: Sir2 deacetylates histone H4K16 to create
          binding-competent nucleosomes; Sir3 preferentially recognizes
          deacetylated H4K16 nucleosomes; Sir3 dimers linked through Sir4 dimers
          support cooperative binding to paired nucleosomes and cis-spreading of
          the silent domain from silencers.
        supporting_text: |
          A widely used mechanistic model is an iterative “read–write” logic in which **Sir2 deacetylates histone H4K16**, creating binding-competent nucleosomes; **Sir3 preferentially recognizes deacetylated H4K16 nucleosomes** and, together with Sir4-mediated assemblies, supports cooperative binding and **cis-spreading** of the silent domain outward from silencers.
        reference_section_type: RESULTS
      - statement: |
          Sir3 functions in the nucleus on chromatin, with classical sites of
          action at HML/HMR and telomere-proximal/subtelomeric regions, where it
          binds nucleosomes as part of the Sir2/3/4 silencing machinery.
        supporting_text: |
          Sir3 functions in the **nucleus** on **chromatin**, with classical sites of action at **HML/HMR** and **telomere-proximal/subtelomeric regions**, where it binds nucleosomes as part of the Sir2/3/4 silencing machinery
        reference_section_type: RESULTS
      - statement: |
          Beyond canonical silent domains, Sir3 also makes transient direct
          contacts with euchromatin at large scale; Nanopore-MetID identified
          1,197 genes (~19% of genes) as Sir3 contact targets, with high-density
          occupancy concentrated about +/-2 kb around subtelomeric nucleation
          sites.
        supporting_text: |
          Recent mapping adds that Sir3 also makes **transient direct contacts with euchromatin** at large scale, beyond canonical silent domains.
        reference_section_type: RESULTS
      - statement: |
          Sir3 chromatin contacts depend on the Sir2/3/4 complex: subtelomeric
          Sir3 methylation is abolished in sir2-delta cells and reduced about
          2-fold in sir4-delta cells.
        supporting_text: |
          Subtelomeric Sir3 methylation is abolished in *sir2Δ* and reduced about 2-fold in *sir4Δ*, consistent with Sir3 acting within the Sir2/3/4 complex
        reference_section_type: RESULTS

Gene Description

Existing Annotations Review

Core Functions

SIR3 is a structural component that binds chromatin and nucleosomes to establish and maintain heterochromatin domains. This is the primary molecular mechanism by which SIR3 functions.

SIR3 forms homodimers and interacts with SIR2 and SIR4 to assemble the Sir2-3-4 silent chromatin complex. Homodimerization through the C-terminal winged helix domain is essential for silencing function.

SIR3 directly binds to histone octamers and nucleosomes at mating-type loci, telomeres, rDNA, and euchromatic origins. This nucleosome binding is critical for both establishing transcriptional silencing and suppressing DNA replication initiation at origins.

References

Deep Research

Falcon

Research report: Saccharomyces cerevisiae SIR3 (UniProt P06701; ORF YLR442C)

0) Mandatory identity verification (correct gene/protein)

1) Key concepts, definitions, and current understanding

1.1 Yeast “heterochromatin” and SIR-dependent silencing

1.2 Sir3’s primary molecular function

1.3 Domain architecture and nucleosome recognition (mechanistic definition)

1.4 How Sir3 participates in initiation and spreading

2) Recent developments and latest research (prioritizing 2023–2024)

2.1 Quantitative limits of silencing and gene-level escape (2023)

2.2 Sir3 dynamics and genome-wide transient contacts (2023)

2.3 Current “expert view” of silencing mechanisms (2024 review)

2.4 Epigenetic inheritance and bistability framing (2024)

3) Current applications and real-world implementations

3.1 Sir3 as a model for epigenetic domain assembly and inheritance

3.2 Tools and assays enabled by Sir3 biology

4) Expert opinions and analysis (authoritative sources)

4.1 How silencers “work” and why silencing is probabilistic

4.2 Structural interpretation of “histone mark dependency”

5) Statistics and data highlights (recent studies)

6) Subcellular localization and sites of action

7) Pathways and molecular interactions (high-confidence)

8) Notes on evidence quality and recency

Artifacts

Citations

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