Mutant versions of histone H3 are defective for nucleosome reassembly and histone chaperone recruitment during transcription (944.3)

Expression of the Saccharomyces cerevisiae SER3 gene is regulated by transcription interference. In the presence of serine, transcription of SRG1 ncDNA is initiated upstream of the adjacent SER3 gene and extends across the SER3 promoter. Our previous studies provided evidence that during SRG1 transcription, Spt6 and Spt16 histone chaperones reassemble nucleosomes over the SER3 promoter after the passage of RNA Pol II, which then interfere with transcription factor binding resulting in SER3 repression. In response to serine starvation, SRG1 transcription is reduced causing nucleosome depletion over the SER3 promoter, which in turn allows transcription factors to bind the SER3 promoter and activate SER3 transcription. We recently reported the identification of mutations altering eight histone residues that strongly increase SER3 expression without reducing transcription of intergenic SRG1 ncDNA. Nucleosome occupancy across SRG1 in these mutants was reduced to degrees that correlated well with the level of SER3 derepression. Additional analyses of these mutants indicated a broader defect in transcription‐coupled nucleosome occupancy at highly transcribed genes, but not lowly transcribed genes. Taken together, these data defined a new class of histone mutants that are defective for transcription‐dependent nucleosome occupancy. Further analysis focused on a subset of these histone mutants revealed a reduced rate of nucleosome reassembly during transcription in vivo. Moreover, we provide evidence that Spt6 and Spt16 histone chaperones fail to be recruited to genes being transcribed by RNA Pol II in these mutants. These results suggest that amino acids in histone H3 and H4 promote transcription‐coupled nucleosome assembly by facilitating the recruitment or retention of Spt6 and/or Spt16 histone chaperones. Grant Funding Source : Supported by NIGMS