title: Defects in SPT16 or POB3 (yFACT) in Saccharomyces cerevisiae cause dependence on the Hir/Hpc pathway: polymerase passage may degrade chromatin structure.
abstract: Spt16/Cdc68, Pob3, and Nhp6 collaborate in vitro and in vivo as the yeast factor SPN, which is homologous to human FACT. SPN/FACT complexes mediate passage of polymerases through nucleosomes and are important for both transcription and replication. An spt16 mutation was found to be intolerable when combined with a mutation in any member of the set of functionally related genes HIR1, HIR2/SPT1, HIR3/HPC1, or HPC2. Mutations in POB3, but not in NHP6A/B, also display strong synthetic defects with hir/hpc mutations. A screen for other mutations that cause dependence on HIR/HPC genes revealed genes encoding members of the Paf1 complex, which also promotes transcriptional elongation. The Hir/Hpc proteins affect the expression of histone genes and also promote normal deposition of nucleosomes; either role could explain an interaction with elongation factors. We show that both spt16 and pob3 mutants respond to changes in histone gene numbers, but in opposite ways, suggesting that Spt16 and Pob3 each interact with histones but perhaps with different subsets of these proteins. Supporting this, spt16 and pob3 mutants also display different sensitivities to mutations in the N-terminal tails of histones H3 and H4 and to mutations in enzymes that modulate acetylation of these tails. Our results support a model in which SPN/FACT has two functions: it disrupts nucleosomes to allow polymerases to access DNA, and it reassembles the nucleosomes afterward. Mutations that impair the reassembly activity cause chromatin to accumulate in an abnormally disrupted state, imposing a requirement for a nucleosome reassembly function that we propose is provided by Hir/Hpc proteins.
authors.affiliation: Department of Biochemistry, University of Utah School of Medicine, Salt Lake City, Utah 84132, USA. Tim@biochem.utah.edu
authors.forename: Tim
authors.initials: T
authors.lastname: Formosa
authors.forename: Susan
authors.initials: S
authors.lastname: Ruone
authors.forename: Melissa D
authors.initials: MD
authors.lastname: Adams
authors.forename: Aileen E
authors.initials: AE
authors.lastname: Olsen
authors.forename: Peter
authors.initials: P
authors.lastname: Eriksson
authors.forename: Yaxin
authors.initials: Y
authors.lastname: Yu
authors.forename: Alison R
authors.initials: AR
authors.lastname: Rhoades
authors.forename: Paul D
authors.initials: PD
authors.lastname: Kaufman
authors.forename: David J
authors.initials: DJ
authors.lastname: Stillman
mesh_terms: D000123:Acetyltransferases / Q000235:genetics / Q000378:metabolism; D002352:Carrier Proteins / Q000235:genetics* / Q000378:metabolism*; D018797:Cell Cycle Proteins / Q000235:genetics* / Q000378:metabolism*; D002843:Chromatin / Q000378:metabolism*; D056504:Chromatin Assembly Factor-1; D002868:Chromosomal Proteins, Non-Histone; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D018628:Gene Dosage; D005800:Genes, Fungal; D024001:HMGN Proteins; D051548:Histone Acetyltransferases; D006657:Histones / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D008954:Models, Biological; D009154:Mutation; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism; D010641:Phenotype; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000235:genetics* / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism*; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism; D014157:Transcription Factors / Q000235:genetics* / Q000378:metabolism*; D015533:Transcriptional Activation; D035602:Transcriptional Elongation Factors
chemical_list: D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D002843:Chromatin; D056504:Chromatin Assembly Factor-1; D002868:Chromosomal Proteins, Non-Histone; D004268:DNA-Binding Proteins; C078808:HIR1 protein, S cerevisiae; C078809:HIR2 protein, S cerevisiae; D024001:HMGN Proteins; C077306:HPC2 protein, S cerevisiae; D006657:Histones; C086471:NHP6A protein, S cerevisiae; C086472:NHP6B protein, S cerevisiae; D009687:Nuclear Proteins; C106768:POB3 protein, S cerevisiae; D012097:Repressor Proteins; C071238:SPT16 protein, S cerevisiae; C489489:SUPT16H protein, human; D029701:Saccharomyces cerevisiae Proteins; D015534:Trans-Activators; D014157:Transcription Factors; D035602:Transcriptional Elongation Factors; D000123:Acetyltransferases; D051548:Histone Acetyltransferases