Identification of histone mutants that are defective for transcription-coupled nucleosome occupancy.

Our previous studies of Saccharomyces cerevisiae described a gene repression mechanism where the transcription of intergenic noncoding DNA (ncDNA) (SRG1) assembles nucleosomes across the promoter of the adjacent SER3 gene that interfere with the binding of transcription factors. To investigate the role of histones in this mechanism, we screened a comprehensive library of histone H3 and H4 mutants for those that derepress SER3. We identified mutations altering eight histone residues (H3 residues V46, R49, V117, Q120, and K122 and H4 residues R36, I46, and S47) that strongly increase SER3 expression without reducing the transcription of the intergenic SRG1 ncDNA. We detected reduced nucleosome occupancy across SRG1 in these mutants to degrees that correlate well with the level of SER3 derepression. The histone chromatin immunoprecipitation experiments on several other genes suggest that the loss of nucleosomes in these mutants is specific to highly transcribed regions. Interestingly, two of these histone mutants, H3 R49A and H3 V46A, reduce Set2-dependent methylation of lysine 36 of histone H3 and allow transcription initiation from cryptic intragenic promoters. Taken together, our data identify a new class of histone mutants that is defective for transcription-dependent nucleosome occupancy.