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Research Report: Saccharomyces cerevisiae SPT16 (UniProt P32558) — FACT complex subunit Spt16/Cdc68

1. Gene/protein identification and disambiguation (mandatory verification)

The target protein described by UniProt accession P32558 is Saccharomyces cerevisiae Spt16, also known as Cdc68, and is the large subunit of the FACT (FAcilitates Chromatin Transcription) histone chaperone complex. Multiple independent sources describe Spt16/Cdc68 as a yeast FACT subunit that forms a heterodimer with Pob3 (the yeast homolog of metazoan SSRP1), with nucleosome-binding supported by the HMGB protein Nhp6 in budding yeast. (schweighofer2025biochemicalinsightsinto pages 22-25, jang2025abo1atpasefacilitates pages 1-5, takahata2023opposingrolesof pages 2-4)

The domain/mechanistic descriptions in the literature (notably an N-terminal peptidase-like “pita-bread” fold that has lost catalytic activity and is repurposed for histone/chromatin interactions, plus additional histone-binding modules) are consistent with the UniProt-provided domain/family context (peptidase M24-like fold within Spt16). (schweighofer2025biochemicalinsightsinto pages 25-28, schweighofer2025biochemicalinsightsintoa pages 25-28)

Disambiguation note: “SPT16” is also used for conserved orthologs in other eukaryotes (e.g., mammalian SPT16 partners with SSRP1), but the yeast-specific pairing with Pob3/Nhp6, and the alias Cdc68, strongly identify the S. cerevisiae protein targeted here rather than a different organism’s SPT16. (takahata2023opposingrolesof pages 2-4, jang2025abo1atpasefacilitates pages 1-5)

2. Key concepts and definitions (current understanding)

2.1 FACT and Spt16 definition

FACT is a conserved histone chaperone that was originally identified as an RNA polymerase II (RNAPII) transcription elongation factor and is now understood to function broadly in chromatin transactions including transcription and DNA replication. In budding yeast, FACT is built around an Spt16–Pob3 heterodimer and relies on Nhp6 for efficient nucleosome binding and reorganization. (takahata2023opposingrolesof pages 1-2, takahata2023opposingrolesof pages 2-4)

A core conceptual point emphasized in recent syntheses is that FACT is ATP-independent (i.e., it lacks an intrinsic ATPase) and instead remodels nucleosomes through multivalent binding and stabilization of intermediate nucleosome states. (takahata2023opposingrolesof pages 1-2)

2.2 “Substrate specificity” for a chromatin factor

SPT16 is not an enzyme with a small-molecule substrate; its relevant “substrates/targets” are nucleosomes and subnucleosomal intermediates, particularly via interactions with:
- H3–H4 (tetramer/dimers) and
- H2A–H2B dimers
within partially unwrapped nucleosomes. (schweighofer2025biochemicalinsightsinto pages 25-28, takahata2023opposingrolesof pages 2-4)

A mechanistic theme is that FACT preferentially recognizes and stabilizes partially unwrapped nucleosomes rather than fully wrapped nucleosomes, enabling controlled nucleosome disruption and restoration during polymerase passage. (hou2025characterizingthefunctionala pages 20-23, hou2025characterizingthefunctional pages 20-23)

3. Molecular function and domain architecture of Spt16

3.1 Domain architecture (working model)

Across the retrieved sources, Spt16 is described as a modular protein with:
- an N-terminal peptidase-like (pita-bread fold) domain that has lost catalytic activity (repurposed for chromatin function),
- a dimerization domain supporting stable complex formation with Pob3/SSRP1,
- a middle domain that contacts (H3–H4)₂,
- an acidic/disordered C-terminal region that binds H2A–H2B and can act as a DNA placeholder on partially unwrapped nucleosomes. (schweighofer2025biochemicalinsightsinto pages 25-28, hou2025characterizingthefunctional pages 20-23, schweighofer2025biochemicalinsightsintoa pages 25-28)

In a yeast-focused review, Spt16’s N-terminal peptidase-like domain is discussed in the context of H3/H4 binding, while a central PH-like region is described as providing H2A/H2B chaperone activity and the acidic C-terminal tail is discussed as tethering H2A/H2B (with phosphorylation implicated in regulating this binding). (takahata2023opposingrolesof pages 2-4)

3.2 Histone-binding specificity

FACT/Spt16 is described as atypical because it engages both major histone modules, enabling coordinated nucleosome disassembly/reassembly:
- H3/H4 contact is linked to the Spt16 N-terminal and middle modules (including PH-like binding sites described in yeast contexts). (takahata2023opposingrolesof pages 2-4, dolson2024impactofthe pages 16-19)
- H2A/H2B binding is linked to the acidic/disordered C-terminal region and other binding motifs, supporting retention and redeposition of H2A/H2B during nucleosome reassembly. (schweighofer2025biochemicalinsightsinto pages 25-28, hou2025characterizingthefunctional pages 20-23)

4. Biological processes and pathways involving Spt16 in S. cerevisiae

4.1 Transcription through chromatin (RNAPII elongation, promoter function, and chromatin integrity)

Evidence across yeast-focused sources supports that Spt16/FACT:
- is required for efficient RNAPII transcription on chromatin templates and functions as a nucleosome-templated elongation factor, (schweighofer2025biochemicalinsightsinto pages 22-25, hou2025characterizingthefunctional pages 20-23)
- helps manage nucleosome dynamics (both disassembly and reassembly) as RNAPII traverses genes, (byrd2024assessingcontributionsof pages 1-2)
- suppresses cryptic initiation and supports chromatin integrity, consistent with its chaperone role maintaining proper nucleosome organization. (hou2025characterizingthefunctional pages 20-23)

A 2024 yeast study operationalizes these concepts by analyzing yFACT component Spt16 occupancy across gene regions and testing how DNA sequence at the 3′ end contributes to dissociation after transcription termination; the authors interpret a modest 3′ retention phenotype as reflecting inefficient dissociation linked with altered chromatin architecture. (byrd2024assessingcontributionsof pages 1-2)

4.2 Replication-coupled chromatin assembly and parental histone recycling

Multiple sources describe roles for Spt16/FACT at replication forks, including physical coupling to replisome components:
- The Spt16 N-terminal domain binds Tof1 (fork protection complex) and MCM2-7 helicase in yeast, linking FACT to fork-associated chromatin transactions. (schweighofer2025biochemicalinsightsintoa pages 22-25, dolson2024impactofthe pages 16-19)
- FACT is reported to physically interact with DNA polymerase α in yeast. (schweighofer2025biochemicalinsightsinto pages 22-25, schweighofer2025biochemicalinsightsintoa pages 22-25)

A 2024 preprint provides mechanistic evidence that FACT influences replication on chromatin templates and reports S-phase-specific detection of a Spt16::Pol1::H3 ternary interaction signal (BiFC-FRET), consistent with replication-coupled histone handling. (zhang2024fact(h3h4)complexstimulates pages 1-4)

A 2024 thesis-style synthesis of replication-coupled chromatin processes cites recent in vivo findings that deletion of the Spt16 N-terminal domain (spt16ΔN) disrupts aspects of parental histone recycling and reduces parental H3K4me3 recycling globally; it also cites cryo-EM placement of FACT at the front of the fork, where Spt16’s middle and C-terminal modules contact H3–H4 and H2A–H2B, respectively, consistent with a model in which FACT helps disassemble and reassemble nucleosomes during fork progression. (dolson2024impactofthe pages 16-19, dolson2024impactoftheb pages 16-19)

4.3 Euchromatin vs. heterochromatin roles (yeast-specific nuances)

An authoritative 2023 review emphasizes an important distinction for functional annotation:
- In budding yeast, FACT is presented primarily as acting in euchromatin, including promoter chromatin activation via interactions with SBF/MBF in regulation of G1/S cell cycle genes; the review notes a lack of evidence for roles in SIR-dependent heterochromatin silencing in budding yeast. (takahata2023opposingrolesof pages 1-2)
- In fission yeast, FACT plays roles in heterochromatin formation/stability through binding HP1/Swi6, highlighting that heterochromatin-associated FACT functions may be species/context dependent. (takahata2023opposingrolesof pages 1-2, takahata2023opposingrolesof pages 2-4)

5. Cellular localization (where Spt16 acts)

The evidence supports that Spt16 is nuclear and chromatin-associated, acting at:
- promoters/+1 nucleosomes and transcribed gene bodies (tracking with transcription), (byrd2024assessingcontributionsof pages 1-2, hou2025characterizingthefunctional pages 20-23)
- replication forks, via interactions with Tof1 and MCM2-7 and with polymerase α components, (dolson2024impactofthe pages 16-19, zhang2024fact(h3h4)complexstimulates pages 1-4)
- centromeric chromatin based on enrichment in proteomics/co-IP studies, although its specific centromeric role is described as still debated. (schweighofer2025biochemicalinsightsintoa pages 22-25)

6. Recent developments and latest research (prioritizing 2023–2024)

6.1 2023: Mechanistic synthesis of FACT’s nucleosome-reorganization logic

The 2023 Biomolecules review consolidates key mechanistic ideas: FACT is an ATP-independent histone chaperone, budding yeast FACT comprises Spt16/Pob3 plus Nhp6, and nucleosome reorganization is explained by coordinated binding to histones and DNA, including functions of Spt16’s peptidase-like domain, PH-like regions, and acidic tails. (takahata2023opposingrolesof pages 1-2, takahata2023opposingrolesof pages 2-4)

Publication details: Takahata & Murakami, 2023-02, Biomolecules. URL: https://doi.org/10.3390/biom13020377 (takahata2023opposingrolesof pages 1-2, takahata2023opposingrolesof pages 2-4)

6.2 2023: Histone residue-dependent recruitment of Spt16 to regulate a signaling pathway output

A 2023 Scientific Reports paper links Spt16 recruitment to transcription of the pheromone response pathway scaffold gene STE5 via a specific histone residue (H2B Arg95). The authors report that an H2B R95A mutant weakens H2B–Spt16 association and “lost the ability to express 26 genes of the pheromone response pathway,” connecting histone interface chemistry to FACT-mediated gene expression control. (sulaiman2023thehistoneh2b pages 1-2)

Publication details: Sulaiman et al., 2023-06, Scientific Reports. URL: https://doi.org/10.1038/s41598-023-37339-y (sulaiman2023thehistoneh2b pages 1-2)

6.3 2024: Gene-end sequence features and yFACT dissociation dynamics in vivo

A 2024 study used engineered deletions across the 3′ end of PMA1 to test determinants of yFACT dissociation after termination. The design included fourteen 68-bp deletions, and one allele caused a modest increase in Spt16 occupancy at the gene’s 3′ end along with minor Pol II retention and altered nucleosome occupancy, supporting a model where DNA elements at gene ends contribute to proper dissociation and chromatin architecture. (byrd2024assessingcontributionsof pages 1-2)

Publication details: Byrd et al., 2024-08, BMC Research Notes. URL: https://doi.org/10.1186/s13104-024-06872-y (byrd2024assessingcontributionsof pages 1-2)

6.4 2024: FACT coupling to Pol α and chromatin replication

A 2024 bioRxiv preprint reports that FACT (Spt16/Pob3) interacts with Pol α catalytic subunit Pol1 and that a Spt16::Pol1::H3 ternary interaction signal is detectable by BiFC-FRET specifically in S phase; it also notes that FACT is essential for helicase activity on a nucleosomal template but not on naked DNA, consistent with roles in disassembling nucleosomes ahead of the fork and coordinating synthesis with nucleosome assembly. The paper also reiterates primer length biochemistry for Pol α/primase (primase makes 8–10 nt RNA extended to ~20–35 nt total primer length). (zhang2024fact(h3h4)complexstimulates pages 1-4)

Publication details: Zhang et al., 2024-08, bioRxiv. URL: https://doi.org/10.1101/2024.08.08.607175 (zhang2024fact(h3h4)complexstimulates pages 1-4)

6.5 2024: Replication-coupled epigenetic stability assays involving Spt16 N-terminal truncation

A 2024 thesis-style work examined spt16ΔN and related genetic backgrounds and reports that spt16ΔN shifts the percentage of yEGFP+ cells downward in a variegation reporter assay and that combining spt16ΔN with asf1Δ further reduces yEGFP+ frequency, supporting functional coupling of FACT/Spt16 with replication-coupled histone chaperoning pathways in maintaining epigenetic states. (dolson2024impactoftheb pages 30-33)

7. Current applications and real-world implementations (methods and experimental use)

In practice, SPT16/FACT is used as a probe and mechanistic handle for chromatin biology in yeast through:

1. Chromatin occupancy mapping (ChIP/qPCR) to quantify Spt16 distribution along genes and diagnose defects in dissociation, elongation-coupled chromatin reassembly, and termination-associated chromatin states. (byrd2024assessingcontributionsof pages 1-2)

2. Genetic allele and suppressor analysis (e.g., spt16-11, spt16 E857K, spt16ΔN) to connect specific histone interfaces and FACT domains to pathway outputs (e.g., pheromone response gene expression and rapamycin response). (sulaiman2023thehistoneh2b pages 1-2, dolson2024impactoftheb pages 30-33)

3. Cell-cycle-resolved protein interaction imaging (e.g., BiFC-FRET) to detect replication-coupled complexes involving Spt16/FACT and replisome factors in vivo. (zhang2024fact(h3h4)complexstimulates pages 1-4)

4. Structure-guided mechanistic inference (cryo-EM) where FACT’s stabilization of partially unwrapped nucleosomes and fork-proximal placement inform domain-function models and guide mutational strategies. (hou2025characterizingthefunctional pages 20-23, dolson2024impactofthe pages 16-19)

8. Expert opinions and analysis (authoritative synthesis)

A key expert-level interpretation from the yeast-focused 2023 review is that FACT’s “opposing roles” across yeast species are largely driven by context-specific binding partners and chromatin state: budding yeast FACT is framed as a promoter/chromatin activator in euchromatin (especially cell-cycle genes), whereas fission yeast FACT is positioned as a heterochromatin stabilizer through Swi6/HP1 interactions. This underscores that functional annotation for S. cerevisiae Spt16 should emphasize transcription/replication-coupled nucleosome handling in euchromatin and genome-wide chromatin integrity, rather than Sir-dependent heterochromatin silencing. (takahata2023opposingrolesof pages 1-2, takahata2023opposingrolesof pages 2-4)

9. Relevant statistics and data (from available 2023–2024 studies)

The accessible full-text evidence contained a limited set of explicit numerical results; key ones include:
- 26 genes in the pheromone response pathway reported as not expressed in an H2B R95A mutant linked to weakened recruitment/association of Spt16. (sulaiman2023thehistoneh2b pages 1-2)
- Rapamycin response: approximately 3–4-fold induction of CLN2 in ste5Δ within 30 min (used to interpret pathway control). (sulaiman2023thehistoneh2b pages 1-2)
- Replication primer lengths: primase makes 8–10 nt RNA primers extended by Pol α to ~20–35 nt primers (context for Pol α function in the study linking Pol α to FACT). (zhang2024fact(h3h4)complexstimulates pages 1-4)
- Experimental scale of gene-end deletion series: 14 internal deletions of 68 bp across the 3′ end of PMA1 used to test sequence contributions to Spt16 occupancy/dissociation. (byrd2024assessingcontributionsof pages 1-2)

Where quantitative genome-wide statistics (e.g., fraction of genome bound, occupancy fold-changes) were mentioned conceptually (e.g., ChIP-seq/MNase-seq as approaches), they were not present with numeric values in the accessible snippets and therefore are not reproduced here. (takahata2023opposingrolesof pages 1-2)

10. Summary functional annotation statement (for curation)

SPT16 (UniProt P32558) encodes the essential large subunit of the yeast FACT histone chaperone (Spt16–Pob3, aided by Nhp6), an ATP-independent chromatin factor that binds both H3–H4 and H2A–H2B to stabilize partially unwrapped nucleosome intermediates. In S. cerevisiae, Spt16 promotes transcription through chromatin (supporting RNAPII elongation, nucleosome disassembly/reassembly, and suppression of cryptic initiation) and participates in replication-coupled chromatin maintenance via interactions with replisome factors including Tof1 and MCM2-7 and reported interactions with polymerase α, consistent with roles in parental histone recycling and nucleosome reassembly at the replication fork. (takahata2023opposingrolesof pages 2-4, hou2025characterizingthefunctional pages 20-23, dolson2024impactofthe pages 16-19)

Evidence map table

Table: This table summarizes the supported functional annotation of Saccharomyces cerevisiae Spt16/Cdc68 as the FACT complex subunit, emphasizing domains, binding specificity, transcription and replication roles, localization, and recent 2023–2024 findings. It is useful as a compact evidence map tied directly to available citation IDs and source URLs.

References

1. (schweighofer2025biochemicalinsightsinto pages 22-25): J Schweighofer. Biochemical insights into the role of histone chaperone fact in kinetochore stability. Unknown journal, 2025.

2. (jang2025abo1atpasefacilitates pages 1-5): Juwon Jang, Yujin Kang, Martin Zofall, Carol Cho, Shiv Grewal, Ja Yil Lee, and Ji-Joon Song. Abo1 atpase facilitates the dissociation of fact from chromatin. Nucleic Acids Research, Jun 2025. URL: https://doi.org/10.1101/2024.06.17.599424, doi:10.1101/2024.06.17.599424. This article has 6 citations and is from a highest quality peer-reviewed journal.

3. (takahata2023opposingrolesof pages 2-4): Shinya Takahata and Yota Murakami. Opposing roles of fact for euchromatin and heterochromatin in yeast. Biomolecules, Feb 2023. URL: https://doi.org/10.3390/biom13020377, doi:10.3390/biom13020377. This article has 4 citations.

4. (schweighofer2025biochemicalinsightsinto pages 25-28): J Schweighofer. Biochemical insights into the role of histone chaperone fact in kinetochore stability. Unknown journal, 2025.

5. (schweighofer2025biochemicalinsightsintoa pages 25-28): J Schweighofer. Biochemical insights into the role of histone chaperone fact in kinetochore stability. Unknown journal, 2025.

6. (takahata2023opposingrolesof pages 1-2): Shinya Takahata and Yota Murakami. Opposing roles of fact for euchromatin and heterochromatin in yeast. Biomolecules, Feb 2023. URL: https://doi.org/10.3390/biom13020377, doi:10.3390/biom13020377. This article has 4 citations.

7. (hou2025characterizingthefunctionala pages 20-23): P Hou. Characterizing the functional interaction between h2bub1 and fact in rnapii elongation. Unknown journal, 2025.

8. (hou2025characterizingthefunctional pages 20-23): P Hou. Characterizing the functional interaction between h2bub1 and fact in rnapii elongation. Unknown journal, 2025.

9. (dolson2024impactofthe pages 16-19): A Dolson. Impact of the histone chaperone fact, rif1p, and tof1p on the epigenetic stability of variegating loci in s. cerevisiae. Unknown journal, 2024.

10. (byrd2024assessingcontributionsof pages 1-2): Samuel E. Byrd, Brianna Hoyt, Sydney A. Ozersky, Alex W. Crocker, Daniel Habenicht, Mattie R. Nester, Heather Prowse, Claire E. Turkal, Lauren Joseph, and Andrea A. Duina. Assessing contributions of dna sequences at the 3’ end of a yeast gene on yfact, rna polymerase ii, and nucleosome occupancy. BMC Research Notes, Aug 2024. URL: https://doi.org/10.1186/s13104-024-06872-y, doi:10.1186/s13104-024-06872-y. This article has 0 citations and is from a peer-reviewed journal.

11. (schweighofer2025biochemicalinsightsintoa pages 22-25): J Schweighofer. Biochemical insights into the role of histone chaperone fact in kinetochore stability. Unknown journal, 2025.

12. (zhang2024fact(h3h4)complexstimulates pages 1-4): Wenshuo Zhang, Jiawei Xu, Jiayi Yang, Guojun Shi, Jiale Wu, Ning Gao, Jianxun Feng, and Qing Li. Fact-(h3-h4) complex stimulates pol α activity to coordinate dna synthesis with nucleosome assembly. bioRxiv, Aug 2024. URL: https://doi.org/10.1101/2024.08.08.607175, doi:10.1101/2024.08.08.607175. This article has 0 citations.

13. (dolson2024impactoftheb pages 16-19): A Dolson. Impact of the histone chaperone fact, rif1p, and tof1p on the epigenetic stability of variegating loci in s. cerevisiae. Unknown journal, 2024.

14. (sulaiman2023thehistoneh2b pages 1-2): Abdallah Alhaj Sulaiman, Reem Ali, and Dindial Ramotar. The histone h2b arg95 residue efficiently recruits the transcription factor spt16 to mediate ste5 expression of the pheromone response pathway. Scientific Reports, Jun 2023. URL: https://doi.org/10.1038/s41598-023-37339-y, doi:10.1038/s41598-023-37339-y. This article has 3 citations and is from a peer-reviewed journal.

15. (dolson2024impactoftheb pages 30-33): A Dolson. Impact of the histone chaperone fact, rif1p, and tof1p on the epigenetic stability of variegating loci in s. cerevisiae. Unknown journal, 2024.

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Citations

1. takahata2023opposingrolesof pages 1-2
2. takahata2023opposingrolesof pages 2-4
3. byrd2024assessingcontributionsof pages 1-2
4. hou2025characterizingthefunctional pages 20-23
5. schweighofer2025biochemicalinsightsintoa pages 22-25
6. dolson2024impactoftheb pages 30-33
7. schweighofer2025biochemicalinsightsinto pages 22-25
8. schweighofer2025biochemicalinsightsinto pages 25-28
9. schweighofer2025biochemicalinsightsintoa pages 25-28
10. hou2025characterizingthefunctionala pages 20-23
11. dolson2024impactofthe pages 16-19
12. dolson2024impactoftheb pages 16-19
13. https://doi.org/10.3390/biom13020377
14. https://doi.org/10.1038/s41598-023-37339-y
15. https://doi.org/10.1186/s13104-024-06872-y
16. https://doi.org/10.1101/2024.08.08.607175
17. https://doi.org/10.3390/biom13020377;
18. https://doi.org/10.1101/2024.06.17.599424
19. https://doi.org/10.1186/s13104-024-06872-y;
20. https://doi.org/10.1101/2024.08.08.607175;
21. https://doi.org/10.1038/s41598-023-37339-y;
22. https://doi.org/10.1101/2024.06.17.599424,
23. https://doi.org/10.3390/biom13020377,
24. https://doi.org/10.1186/s13104-024-06872-y,
25. https://doi.org/10.1101/2024.08.08.607175,
26. https://doi.org/10.1038/s41598-023-37339-y,