Deep Research
Bioreason Pro
(spt16-deep-research-bioreason-sft.md)
Bioreason Pro
(spt16-deep-research-bioreason-sft.md)Thinking Trace
I begin by dissecting the InterPro architecture and its order along the polypeptide. The N-terminus (residues 1โ173) is defined by IPR029149 (Creatinase/Aminopeptidase P/Spt16, N-terminal homologous superfamily) and, within it, a more specific N-terminal lobe module IPR029148 (FACT complex subunit SPT16, N-terminal lobe domain, residues 6โ167/169). Immediately downstream, the core of the protein transitions into an M24-like metallopeptidase fold: IPR036005 (Creatinase/aminopeptidase-like homologous superfamily, residues 174โ442) and its embedded IPR033825 (Spt16, peptidase M24-like domain, residues 181โ427) together with IPR000994 (Peptidase M24 domain, residues 183โ392). In Spt16, this M24-like scaffold is repurposed for histone handling rather than catalysis, creating a rigid cradle that binds histone surfaces. The mid-region is marked by IPR013953 (Spt16 middle domain, residues 540โ652/690), a flexible but structured module that couples the N-terminal histone-binding lobes to distal interaction surfaces. The C-terminal half contains a PH-like fold: IPR056595 (Spt16 PH-like domain, residues 663โ794) followed by a broader PH-like superfamily signature IPR011993 (residues 806โ937) and a histone-chaperone-tuned variant IPR013719 (RTT106/FACT Spt16-like middle domain, residues 813โ903/901). This PH-like platform is specialized to engage histone dimers and nucleosomal DNA, positioning them for transfer. The architecture culminates in IPR048969 (Spt16 C-terminal domain, residues 941โ1018), a module that stabilizes histone contacts and completes the chaperone cradle. The entire sequence is encompassed by IPR040258 (FACT complex subunit Spt16 family, residues 1โ1017), confirming a conserved, multi-lobed histone chaperone design.
This ordered arrangement causes a specific molecular function: the N-terminal lobe and M24-like core form a composite surface that grips H2AโH2B and H3โH3โ dimers, while the PH-like and C-terminal domains position and hand off histones to and from DNA. Such a multi-surface chaperone architecture directly encodes histone chaperone activity (GO:0140713). The extensive interaction surfaces across the middle and C-terminal regions also enable multivalent protein binding (GO:0005515), including contacts with partner proteins and the transcription machinery.
From this molecular activity, the biological processes follow. By binding and transferring H2AโH2B and mediating partial nucleosome disassembly and reassembly, the protein drives nucleosome organization (GO:0034728). During DNA synthesis, the same chaperoning of histones onto nascent DNA supports DNA replication-dependent chromatin assembly (GO:0006335). In addition, by stabilizing and reconfiguring nucleosomes across repetitive and silenced regions, the chaperone contributes to maintaining compacted chromatin states, consistent with constitutive heterochromatin formation (GO:0140719). The ability to transiently destabilize and reassemble nucleosomes also facilitates repair factor access and restoration of chromatin integrity after damage.
The cellular context is dictated by where nucleosomes and transcriptional elongation occur. The chaperoneโs reliance on nucleosomal substrates places it in euchromatin (GO:0000791), where dynamic remodeling is required, and as a core constituent of the FACT complex (GO:0035101). Its operational niche during transcription and replication situates it in the nucleus, and its association with actively transcribed loci explains enrichment on chromosomes. Thus, the most plausible localization is the nucleus (GO:0005634) and chromosome (GO:0005694), with concentration in euchromatic regions and at transcribed genes.
Mechanistically, the protein likely functions as one half of a histone-handling clamp, cooperating with its partner subunit to transiently remove an H2AโH2B dimer during elongation or replication, escort histones via the PH-like and C-terminal domains, and reseat them to maintain chromatin integrity. This mechanism predicts interactions with the cognate FACT partner (pob3) to form the functional heteromer; with the RNA polymerase II machinery (Rpb1 and Rpb3) and elongation factors (Spt4, Spt5, Spt6) to couple histone transfer to polymerase passage; with transcription-coupled chromatin factors such as the Paf1-associated component Cdc73 to coordinate elongation and chromatin maturation; and with scaffold-like adaptors such as TPR-containing proteins and RNA polymerase-associated proteins that stabilize the complex on moving polymerase. Uncharacterized proteins (e.g., C13E7.08c and C651.09c) may act as adaptors or regulators that tune FACT engagement with specific genomic loci, including stress-responsive and telomeric chromatin, thereby linking chaperone activity to gene expression programs and genome maintenance.
## Functional Summary
A nuclear chromatin chaperone that operates within the FACT complex to reorganize nucleosomes during DNA-templated processes. It transiently destabilizes a nucleosome by removing an H2AโH2B dimer to allow RNA polymerase II to pass, then restores the nucleosome to preserve chromatin integrity. The same histone-handling mechanism supports replication-coupled chromatin assembly and contributes to the stability of compacted chromatin domains, thereby influencing transcriptional output, DNA repair, and chromosome stability. It concentrates on actively transcribed chromosomal regions and likely coordinates with elongation factors and the polymerase to couple histone transfer to transcriptional progression.
## UniProt Summary
Component of the FACT complex, a general chromatin factor that acts to reorganize nucleosomes. The FACT complex is involved in multiple processes that require DNA as a template such as mRNA elongation, DNA replication and DNA repair. During transcription elongation the FACT complex acts as a histone chaperone that both destabilizes and restores nucleosomal structure. It facilitates the passage of RNA polymerase II and transcription by promoting the dissociation of one histone H2A-H2B dimer from the nucleosome, then subsequently promotes the reestablishment of the nucleosome following the passage of RNA polymerase II. The FACT complex is also required for replication and repair of damaged DNA. The FACT complex activity is required in transcriptional regulation of a subset of genes involved in critical cellular functions such as DNA replication, DNA repair, and cellular stress responses. The FACT complex changes in response to environmental stress, and contributes to the adaptive transcriptional reprogramming of the cell. May also play a role in maintaining the stability of telomeres.
## InterPro Domains
- IPR029149: Creatinase/Aminopeptidase P/Spt16, N-terminal (homologous_superfamily) [1-173]
- IPR040258: FACT complex subunit Spt16 (family) [1-1017]
- IPR029148: FACT complex subunit SPT16, N-terminal lobe domain (domain) [6-167]
- IPR029148: FACT complex subunit SPT16, N-terminal lobe domain (domain) [6-169]
- IPR036005: Creatinase/aminopeptidase-like (homologous_superfamily) [174-442]
- IPR036005: Creatinase/aminopeptidase-like (homologous_superfamily) [176-429]
- IPR033825: FACT complex subunit Spt16, peptidase M24-like domain (domain) [181-427]
- IPR000994: Peptidase M24 (domain) [183-392]
- IPR013953: FACT complex subunit SPT16, middle domain (domain) [540-652]
- IPR013953: FACT complex subunit SPT16, middle domain (domain) [540-690]
- IPR056595: FACT complex subunit SPT16, PH-like domain (domain) [663-794]
- IPR011993: PH-like domain superfamily (homologous_superfamily) [806-937]
- IPR013719: Histone chaperone RTT106/FACT complex subunit SPT16-like, middle domain (domain) [813-903]
- IPR013719: Histone chaperone RTT106/FACT complex subunit SPT16-like, middle domain (domain) [815-901]
- IPR048969: FACT complex subunit SPT16, C-terminal domain (domain) [941-1018]
## GO Term Predictions
### Molecular Function
### Biological Process
### Cellular Component
Falcon
(spt16-deep-research-falcon.md)
Falcon
(spt16-deep-research-falcon.md)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: Functional annotation of spt16 (UniProt O94267) in Schizosaccharomyces pombe (strain 972)
0) Target verification (mandatory)
The target protein Spt16 in S. pombe is the large, essential subunit of the conserved FACT (FAcilitates Chromatin Transactions) histone-chaperone complex. In yeasts, FACT is composed of an Spt16/Pob3 heterodimer with an HMGB accessory module (Nhp6 in S. pombe), which matches the UniProt description for O94267 (RecName: FACT complex subunit Spt16; gene: spt16, ORF: SPBP8B7.19). (takahata2023opposingrolesof pages 2-4, takahata2024thehmgโboxmodule pages 1-2)
Recent fission-yeast FACT literature depicts Spt16 with a peptidase-like (peptidase M24-like) fold and PH-domain architecture (peptidase-like โPLโ domain; tandem PH modules; acidic tail/cluster), consistent with the domain/family labels provided in UniProt/InterPro for O94267. (takahata2023opposingrolesof pages 2-4, takahata2024thehmgโboxmodule pages 4-5)
URLs / publication dates (key S. pombe-focused sources used here):
- Takahata & Murakami, Biomolecules (review), Feb 2023. https://doi.org/10.3390/biom13020377 (takahata2023opposingrolesof pages 1-2)
- Takahata et al., Genes to Cells (primary), Jun 2024. https://doi.org/10.1111/gtc.13132 (takahata2024thehmgโboxmodule pages 1-2)
- Nathanailidou et al., PNAS (primary), Jan 2024. https://doi.org/10.1073/pnas.2315596121 (nathanailidou2024specializedreplicationof pages 9-10)
1) Key concepts and definitions (current understanding)
1.1 FACT and Spt16
FACT is a conserved, abundant chromatin factor initially identified as an RNA polymerase II elongation factor and now broadly understood as an ATP-independent histone chaperone that promotes transcription and other chromatin transactions by reorganizing nucleosomes while preserving chromatin integrity. (takahata2023opposingrolesof pages 1-2, takahata2023opposingrolesof pages 2-4)
In S. pombe, FACT includes Spt16 (large subunit) and Pob3 (partner subunit); Nhp6 provides an HMGB DNA-binding/bending activity as an accessory module. Importantly, in fission yeast spt16+ is essential, while pob3+ and nhp6+ are nonessential, allowing genetic dissection of submodules. (takahata2023opposingrolesof pages 2-4)
1.2 Molecular function vs โpeptidase familyโ annotation
Although Spt16 is classified in some annotations as peptidase M24 family / peptidase-like, the relevant fission-yeast literature describes this region primarily as a peptidase-like fold used for protein/histone interactions, not as a characterized protease that catalyzes peptide hydrolysis in vivo. (takahata2023opposingrolesof pages 2-4, jang2025abo1atpasefacilitates pages 1-5)
2) Domain architecture and mechanistic model (structure-to-function)
Spt16/FACT is modular and binds multiple nucleosomal components. In fission-yeast-focused summaries and schematics, Spt16 is described with:
- An N-terminal peptidase-like (PL; peptidase-fold) domain with histone H3/H4-binding capacity and (in S. pombe) specific interaction capacity with HP1/Swi6 (see below). (takahata2023opposingrolesof pages 2-4, takahata2023opposingrolesof pages 8-10)
- A central tandem PH domain in Spt16 associated with H2A/H2B chaperone activity (deposition/repositioning/removal). (takahata2023opposingrolesof pages 2-4, takahata2023opposingrolesof pages 8-10)
- Dimerization domain(s) that mediate the stable Spt16โPob3 heterodimer. (takahata2023opposingrolesof pages 2-4, jang2025abo1atpasefacilitates pages 1-5)
- Acidic tail/cluster(s) that tether H2A/H2B and are described as phosphorylation-regulated for nucleosome binding in yeast FACT models. (takahata2023opposingrolesof pages 2-4)
A working model in yeasts is that FACT binds nucleosomes and can promote transient conversion among octasome/hexasome/tetrasome-like intermediates by controlling H2A/H2B association, aided by Nhp6 DNA binding and FACT conformational opening. (takahata2023opposingrolesof pages 2-4)
3) Biological roles and pathways in S. pombe
3.1 Euchromatin: transcription elongation and chromatin integrity
FACT is widely described as a transcription-stimulating factor that relaxes chromatin through its H2A/H2B chaperone function, enabling polymerase progression through nucleosomes while helping restore/maintain nucleosome structure afterward. (takahata2023opposingrolesof pages 1-2, takahata2023opposingrolesof pages 2-4)
A 2024 S. pombe study analyzing the FACT HMGB module emphasizes that transcriptional perturbations in pob3ฮ resemble those in spt4ฮ (an elongation-related factor), whereas nhp6ฮ shows comparatively minimal global expression change, supporting the view that the Spt16/Pob3 core is the principal transcription module, with Nhp6 tuning chromatin binding/engagement rather than driving transcriptional output alone. (takahata2024thehmgโboxmodule pages 3-3, takahata2024thehmgโboxmodule pages 4-5)
Localization (inference from function): These transcription-associated functions place Spt16 on nuclear chromatin, particularly at actively transcribed regions, consistent with FACTโs chromatin-binding roles. (takahata2023opposingrolesof pages 1-2)
3.2 Heterochromatin: HP1/Swi6-dependent recruitment, condensation, and silencing
A distinctive and well-supported S. pombe role for FACT is in constitutive heterochromatin, where it contributes to higher-order chromatin structure and transcriptional repression by binding to Swi6 (HP1 homolog). (takahata2023opposingrolesof pages 1-2, takahata2023opposingrolesof pages 8-10)
Direct recruitment mechanism (key finding):
- The peptidase-like domain of Spt16 directly binds the dimerized chromo-shadow domain (CSD) of Swi6 (but not the related HP1-family protein Chp2), establishing a molecular recruitment route for FACT to heterochromatin. (takahata2023opposingrolesof pages 8-10)
- This interaction depends on a charge-biased โRKDDโ cluster in the Swi6 CSD ฮฒ1โฮฒ2 loop; mutating RKDDโAAAA (Swi6-4A) abolishes Spt16 binding in vitro and is associated with disordered heterochromatin in vivo. (takahata2023opposingrolesof pages 8-10)
Proposed mechanistic consequence in heterochromatin:
After Swi6-mediated recruitment, models propose a scaffold shift wherein the Spt16 peptidase-like domain transitions from recognizing Swi6-CSD to contacting histone H3/H4, enabling either (i) cooperative bridging/condensation of dinucleosomes with Swi6 or (ii) stable binding to mononucleosomes, followed by H2A/H2B repositioning via Spt16โs chaperone modules. (takahata2023opposingrolesof pages 8-10)
Quantitative/phenotypic data point (recently summarized): In pob3ฮ strains, Spt16 binding to heterochromatin is reduced to ~50% of wild type, indicating that Pob3 supportsโbut is not strictly required forโSpt16 recruitment, consistent with additional Swi6-dependent recruitment routes. (takahata2023opposingrolesof pages 6-8)
3.3 Replication-coupled chromatin inheritance: linking FACT to the replisome
A major 2024 development is direct integration of FACT into replication-coupled parental histone recycling mechanisms supporting heterochromatin inheritance.
Nathanailidou et al. (PNAS, Jan 2024) propose that replication proteins enriched across heterochromatin cooperate with FACT to retain parental histones and maintain H3K9 methylation patterns through S phase, and show that Mcl1 (Ctf4 ortholog) is required to connect the replisome to FACT, implicating FACT as a histone-capture/redeposition factor at replication forks. (nathanailidou2024specializedreplicationof pages 9-10)
The paperโs Figure 6/7 provides figure-backed evidence and a conceptual model: co-immunoprecipitation assays track association of FACT (via Pob3) with replisome components (e.g., MCM/GINS) and a model in which Swi6/HP1 creates a high-concentration microenvironment that helps retain FACT and parental histones near the replication fork to support epigenetic propagation. (nathanailidou2024specializedreplicationof media 4febe89e, nathanailidou2024specializedreplicationof media 040f23c1)
4) Recent developments and โlatest researchโ emphasis (2023โ2024)
- Mechanistic clarity of heterochromatin recruitment: The 2023 synthesis highlights a specific Spt16โSwi6 CSD binding interface (RKDD loop dependence) and proposes how FACT could cooperate with HP1 bridging to compact and stabilize heterochromatin. (takahata2023opposingrolesof pages 8-10)
- Module dissection of FACT accessory HMG-box activity (2024): Nhp6 is not simply redundant with Pob3; engineering a Pob3โNhp6 fusion (SSRP1-like) increases chromatin binding and promotes heterochromatin formation and stability, suggesting tunable chromatin affinity/retention is a key variable controlling epigenetic โvariegation.โ (takahata2024thehmgโboxmodule pages 1-2)
- Replication coupling (2024): FACT is integrated into replisome-based models of parental histone recycling and heterochromatin propagation, linking transcription-coupled nucleosome handling to replication-associated epigenetic inheritance. (nathanailidou2024specializedreplicationof pages 9-10, nathanailidou2024specializedreplicationof media 4febe89e)
5) Current applications and real-world implementations
Although S. pombe Spt16 is not directly a therapeutic target, the real-world implementation of Spt16 biology is substantial in experimental chromatin biology, where FACT/Spt16 is used as:
- A mechanistic model to dissect histone chaperone function, nucleosome intermediate handling (hexasomes/tetrasomes), and the coupling of transcription/replication to chromatin restoration. (takahata2023opposingrolesof pages 2-4, jang2025abo1atpasefacilitates pages 1-5)
- A genetically tractable system to study heterochromatin formation and epigenetic stability, including how increased or decreased chromatin binding of a histone chaperone modulates silencing variegation. (takahata2024thehmgโboxmodule pages 1-2)
- A component in models of replication-coupled epigenetic inheritance, relevant to broader eukaryotic chromatin replication principles. (nathanailidou2024specializedreplicationof pages 9-10)
6) Expert opinions and analysis (authoritative synthesis)
A 2023 peer-reviewed review emphasizes FACTโs โdouble-edgedโ roles: supporting euchromatin accessibility/transcription while also contributing to heterochromatin repression/condensation in fission yeast through partner-dependent targeting (e.g., transcription activators vs HP1/Swi6). (takahata2023opposingrolesof pages 1-2, takahata2023opposingrolesof pages 6-8)
This partner-dependent โcontext switchingโ provides a coherent explanation for how a single conserved histone chaperone complex can both promote transcription and stabilize repressed chromatin: the molecular outcome depends strongly on recruitment interfaces (Swi6-binding in heterochromatin) and on which nucleosomal intermediates are being stabilized. (takahata2023opposingrolesof pages 8-10, takahata2023opposingrolesof pages 2-4)
7) Relevant statistics and data highlights (recent studies)
- Heterochromatin recruitment dependency: In S. pombe pob3ฮ, Spt16 binding at heterochromatin is reduced to approximately 50% of wild-type levels, indicating partial Pob3 dependence and supporting an additional recruitment pathway (e.g., via Swi6). (takahata2023opposingrolesof pages 6-8)
- Figure-backed replisome association: Co-IP assays and model schematics in PNAS 2024 present evidence that FACT associates with replisome components in an Mcl1-dependent manner, supporting a replication-coupled histone recycling mechanism (qualitative/interaction data; see Figure 6/7). (nathanailidou2024specializedreplicationof media 4febe89e, nathanailidou2024specializedreplicationof media 040f23c1)
8) Summary of functional annotation (actionable narrative)
Spt16 (UniProt O94267) in Schizosaccharomyces pombe is the essential large subunit of the FACT histone chaperone complex. It is best annotated as a chromatin-associated histone chaperone whose principal biochemical โsubstratesโ are nucleosomes and histone components (H2A/H2B and H3/H4), rather than a small-molecule enzymatic substrate. Spt16 uses a peptidase-like fold and PH-domain modules to engage histones and remodel nucleosome structure in an ATP-independent manner, enabling transcription through chromatin while preserving nucleosome integrity. In fission yeast it has a particularly prominent role in constitutive heterochromatin, where it is recruited via a direct binding interaction between its peptidase-like domain and the Swi6/HP1 chromo-shadow domain, supporting chromatin condensation, suppressed histone turnover, and silencing. Recent 2024 work further integrates FACT/Spt16 into replication-coupled parental histone recycling at heterochromatin domains by linking FACT to the replisome through factors such as Mcl1 and Mcm2, providing a mechanistic route for epigenetic inheritance across cell divisions. (takahata2023opposingrolesof pages 2-4, takahata2023opposingrolesof pages 8-10, nathanailidou2024specializedreplicationof pages 9-10)
Table summary (evidence-backed)
| Functional facet | Summary | Key domains / inferred activity | Major partners | Key evidence | Representative recent sources |
|---|---|---|---|---|---|
| Identity / verification | UniProt O94267 matches Schizosaccharomyces pombe spt16+, the essential large subunit of the conserved FACT histone-chaperone complex; in fission yeast FACT is built from an Spt16/Pob3 heterodimer plus the HMGB protein Nhp6. This matches the user-provided protein description and excludes confusion with unrelated genes. (takahata2023opposingrolesof pages 2-4, takahata2023opposingrolesof pages 1-2, takahata2024thehmgโboxmodule pages 1-2) | Conserved Spt16 architecture in yeast FACT; essential chromatin factor rather than enzyme with a small-molecule substrate. (takahata2023opposingrolesof pages 2-4, jang2025abo1atpasefacilitates pages 1-5) | Pob3, Nhp6 | โSpt16/Pob3 heterodimerโ; โspt16+ is essentialโ; โNhp6 accessory HMGB moduleโ. (takahata2023opposingrolesof pages 2-4, takahata2024thehmgโboxmodule pages 1-2) | Takahata & Murakami, Feb 2023, Biomolecules, DOI: https://doi.org/10.3390/biom13020377; Takahata et al., Jun 2024, Genes to Cells, DOI: https://doi.org/10.1111/gtc.13132 |
| Domain architecture | Recent fission-yeast literature depicts Spt16/FACT with a peptidase-like N-terminal fold, dimerization domain, tandem PH domains, and acidic tail(s); these align well with UniProt/InterPro annotations for O94267. (takahata2023opposingrolesof pages 2-4, takahata2023opposingrolesof pages 1-2, takahata2024thehmgโboxmodule pages 4-5) | Peptidase-like/peptidase M24-like fold: H3/H4 binding and protein interactions; tandem PH domain(s): H2A/H2B chaperone function; acidic tail: H2A/H2B tethering, DNA-mimic-like behavior; DD: Spt16-Pob3 assembly. (takahata2023opposingrolesof pages 2-4, jang2025abo1atpasefacilitates pages 1-5, takahata2024thehmgโboxmodule pages 4-5) | Pob3, core histones | โPL domainโ; โPH1/PH2โ; โacidic clusterโ; โC-terminal acidic tail required for H2A/H2B bindingโ. (takahata2023opposingrolesof pages 2-4, takahata2024thehmgโboxmodule pages 4-5) | Takahata & Murakami, Feb 2023, https://doi.org/10.3390/biom13020377; Takahata et al., Jun 2024, https://doi.org/10.1111/gtc.13132 |
| Core molecular function | Spt16 is best understood as a histone chaperone/chromatin transaction factor, not a peptidase enzyme: it promotes nucleosome reorganization, especially H2A-H2B dimer handling and stabilization of partially disrupted nucleosomes during transcription and replication. (takahata2023opposingrolesof pages 2-4, jang2025abo1atpasefacilitates pages 1-5) | H3/H4 engagement via N-terminal peptidase-like region; H2A/H2B deposition/removal via PH and acidic regions; ATP-independent chromatin remodeling support. (takahata2023opposingrolesof pages 2-4, jang2025abo1atpasefacilitates pages 1-5) | Histones H3/H4 and H2A/H2B | โhistone H2A/H2B chaperoneโ; โdisplace H2A/H2B dimersโ; โpreserve nucleosome integrityโ. (takahata2023opposingrolesof pages 2-4, jang2025abo1atpasefacilitates pages 1-5) | Takahata & Murakami, Feb 2023, https://doi.org/10.3390/biom13020377; Jang et al., preprint posted Jun 2024, later NAR, https://doi.org/10.1101/2024.06.17.599424 |
| Euchromatin / transcription role | In euchromatin, FACT/Spt16 supports transcription elongation by helping RNA polymerase traverse chromatin while maintaining nucleosome integrity; pob3ฮ and spt4ฮ show correlated transcriptional defects, whereas nhp6ฮ is much milder, implying the Spt16/Pob3 core is the main transcriptional module. (takahata2023opposingrolesof pages 1-2, takahata2024thehmgโboxmodule pages 3-3) | Nucleosome engagement and H2A/H2B exchange/repositioning during elongation. (takahata2023opposingrolesof pages 2-4, takahata2024thehmgโboxmodule pages 3-3) | Pob3, Spt5/DSIF, RNAP-associated chromatin | โFACT functions as a transcription elongation factorโ; โpob3ฮ and spt4ฮ transcriptomes correlatedโ; โSpt16/Pob3 interacts with Spt5โ. (takahata2023opposingrolesof pages 1-2, takahata2024thehmgโboxmodule pages 3-3) | Takahata & Murakami, Feb 2023, https://doi.org/10.3390/biom13020377; Takahata et al., Jun 2024, https://doi.org/10.1111/gtc.13132 |
| Heterochromatin role | In fission yeast heterochromatin, Spt16 has a prominent silencing and chromatin-condensation role: FACT is recruited to H3K9me/Swi6 chromatin, suppresses histone turnover, promotes proper H2A/H2B maintenance, and supports establishment/maintenance of silenced chromatin. (takahata2023opposingrolesof pages 8-10, takahata2023opposingrolesof pages 1-2, takahata2024thehmgโboxmodule pages 2-2) | Peptidase-like domain recruitment to HP1/Swi6 followed by scaffold shift to histone H3/H4; H2A/H2B repositioning on heterochromatic nucleosomes. (takahata2023opposingrolesof pages 8-10) | Swi6/HP1, Pob3, histones | โFACT strongly suppresses histone turnoverโ; โcritical role in establishment and maintenance of heterochromatinโ. (takahata2023opposingrolesof pages 8-10, takahata2024thehmgโboxmodule pages 2-2) | Takahata & Murakami, Feb 2023, https://doi.org/10.3390/biom13020377; Takahata et al., Jun 2024, https://doi.org/10.1111/gtc.13132 |
| Direct HP1/Swi6 interaction | A key S. pombe-specific finding is that the peptidase-like domain of Spt16 directly binds the dimerized chromo-shadow domain of Swi6/HP1, via an unusual interaction dependent on the RKDD loop in Swi6; mutating this loop abolishes Spt16 binding and perturbs heterochromatin. (takahata2023opposingrolesof pages 8-10) | Peptidase-like domain acts as a protein-interaction module in addition to histone binding. (takahata2023opposingrolesof pages 8-10) | Swi6/HP1 | โdirectly binds dimerized Swi6-CSDโ; โSwi6-4A lost Spt16 bindingโ; โheterochromatin significantly disorderedโ. (takahata2023opposingrolesof pages 8-10) | Takahata & Murakami, Feb 2023, https://doi.org/10.3390/biom13020377 |
| Pob3 dependence / independence | Pob3 promotes Spt16 recruitment and function, but Spt16 also shows Pob3-independent activity in fission yeast. In pob3ฮ, Spt16 occupancy at heterochromatin falls to about 50% of wild type, indicating partial dependence but not full loss of recruitment. (takahata2023opposingrolesof pages 6-8) | DD-mediated core FACT assembly; Pob3 tandem PH contributes H3/H4 recognition in some models. (takahata2023opposingrolesof pages 8-10) | Pob3, Swi6 | โSpt16 binding reduced to ~50% in pob3ฮโ; โadditive defect in pob3ฮ swi6ฮโ. (takahata2023opposingrolesof pages 6-8) | Takahata & Murakami, Feb 2023, https://doi.org/10.3390/biom13020377 |
| Nhp6 / HMG-box module | Nhp6 provides an HMGB DNA-binding/bending module that enhances FACT chromatin engagement. In 2024 work, Nhp6 alone had minor expression effects, but fusing Nhp6 to Pob3 increased FACT chromatin binding and promoted heterochromatin formation, indicating the HMG module tunes chromatin affinity and epigenetic stability. (takahata2024thehmgโboxmodule pages 1-2, takahata2024thehmgโboxmodule pages 3-3) | HMGB-mediated DNA binding/bending; promotes FACT opening/nucleosome recognition. (takahata2023opposingrolesof pages 2-4, takahata2024thehmgโboxmodule pages 1-2) | Pob3, DNA, nucleosomes | โPob3-Nhp6 fusion increased chromatin bindingโ; โpromoted heterochromatinโ; โNhp6 mutants had little effect aloneโ. (takahata2024thehmgโboxmodule pages 1-2) | Takahata et al., Jun 2024, https://doi.org/10.1111/gtc.13132; Takahata & Murakami, Feb 2023, https://doi.org/10.3390/biom13020377 |
| Replication-coupled chromatin role | Beyond transcription, recent work places FACT/Spt16 at the replisome, where it helps retain/recycle parental histones during replication of heterochromatin. FACT associates with replisome components, and models propose it captures released histones and redeposits them on daughter strands to preserve H3K9me inheritance. (nathanailidou2024specializedreplicationof pages 9-10, nathanailidou2024specializedreplicationof media 4febe89e) | Histone-chaperone role during S phase; interfaces with replisome-linked histone recycling pathways. (nathanailidou2024specializedreplicationof pages 9-10, yu2024areplisomeassociatedhistone pages 10-11) | Mcl1/Ctf4 ortholog, Mcm2, GINS, Polฮฑ, Swi6/HP1 | โFACT association with the replisomeโ; โhelp retain parental histonesโ; โco-IP of FACT (Pob3) with Mcm2/Psf3 depends on Mcl1โ. (nathanailidou2024specializedreplicationof pages 9-10, nathanailidou2024specializedreplicationof media 4febe89e) | Nathanailidou et al., Jan 2024, PNAS, https://doi.org/10.1073/pnas.2315596121 |
| Localization / cellular context | Functionally, Spt16 acts on nuclear chromatin in both transcribed euchromatin and H3K9-methylated heterochromatin, and during S phase it is also linked to replication forks/replisome-enriched heterochromatin domains. (takahata2023opposingrolesof pages 1-2, nathanailidou2024specializedreplicationof pages 9-10) | Chromatin-bound histone chaperone; no evidence here for extracellular or membrane localization. (takahata2023opposingrolesof pages 1-2, nathanailidou2024specializedreplicationof pages 9-10) | Chromatin, heterochromatin, replisome | โbinds heterochromatic regionsโ; โenriched replisome components together with FACTโ; โretains parental histones during DNA replicationโ. (nathanailidou2024specializedreplicationof pages 9-10, takahata2023opposingrolesof pages 6-8) | Takahata & Murakami, Feb 2023, https://doi.org/10.3390/biom13020377; Nathanailidou et al., Jan 2024, https://doi.org/10.1073/pnas.2315596121 |
Table: This table verifies that UniProt O94267 corresponds to the Schizosaccharomyces pombe FACT subunit Spt16 and summarizes its domain organization, major partners, and experimentally supported roles in transcription, heterochromatin, and replication. It emphasizes recent 2023โ2024 literature and short evidence phrases to support each functional facet.
Figure evidence cited
Replication-coupled association of FACT with replisome components and the proposed model for FACT-mediated parental histone recycling during heterochromatin replication are supported by extracted figure crops from Nathanailidou et al. (PNAS 2024). (nathanailidou2024specializedreplicationof media 4febe89e, nathanailidou2024specializedreplicationof media 040f23c1)
Notes on evidence limits
This report was constrained to full-text evidence retrievable via the current toolchain. Some mechanistic details (e.g., precise numeric ChIP-seq fold-changes for H3K9me3 in specific mutants) appear primarily in figures/supplements beyond the extracted text snippets; where such details were not directly accessible as text, I cited the relevant figure evidence instead. (nathanailidou2024specializedreplicationof media 4febe89e, nathanailidou2024specializedreplicationof media 040f23c1)
References
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(takahata2024thehmgโboxmodule pages 3-3): Shinya Takahata, Asahi Taguchi, Ayaka Takenaka, Miyuki Mori, Yuji Chikashige, Chihiro Tsutsumi, Yasushi Hiraoka, and Yota Murakami. The hmgโbox module in fact is critical for suppressing epigenetic variegation of heterochromatin in fission yeast. Genes to Cells, 29:567-583, Jun 2024. URL: https://doi.org/10.1111/gtc.13132, doi:10.1111/gtc.13132. This article has 2 citations and is from a peer-reviewed journal.
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Artifacts
Citations
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- nathanailidou2024specializedreplicationof pages 9-10
- takahata2023opposingrolesof pages 2-4
- takahata2023opposingrolesof pages 8-10
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