In Saccharomyces cerevisiae, the histone chaperone Rtt106 binds newly synthesized histone proteins and mediates their delivery into chromatin during transcription, replication, and silencing. Rtt106 is also recruited to histone gene regulatory regions by the HIR histone chaperone complex to ensure S-phase-specific expression. Here we showed that this Rtt106:HIR complex included Asf1 and histone proteins. Mutations in Rtt106 that reduced histone binding reduced Rtt106 enrichment at histone genes, leading to their increased transcription. Deletion of the chromatin boundary element Yta7 led to increased Rtt106:H3 binding, increased Rtt106 enrichment at histone gene regulatory regions, and decreased histone gene transcription at the HTA1-HTB1 locus. These results suggested a unique regulatory mechanism in which Rtt106 sensed the level of histone proteins to maintain the proper level of histone gene transcription. The role of these histone chaperones and Yta7 differed markedly among the histone gene loci, including the two H3-H4 histone gene pairs. Defects in silencing in rtt106 mutants could be partially accounted for by Rtt106-mediated changes in histone gene repression. These studies suggested that feedback mediated by histone chaperone complexes plays a pivotal role in regulating histone gene transcription.
Cell cycle-regulated transcription of the canonical histone genes is a hallmark of eukaryotic organisms. During S phase, a coordinated burst of histone gene transcription is required to double the level of histone proteins to package newly replicated DNA into chromatin (13,22). The tight coupling of both the timing and the level of histone gene expression with DNA synthesis is critical for cell viability. Mutations that perturb DNA replication lead to altered histone gene transcription (3, 23). Conversely, misregulation of the timing and/or level of histone gene expression leads to genomic instability and cell cycle defects (23,43). Consequently, cells have evolved a complex regulatory mechanism to repress histone gene transcription outside S phase and to promote a precise level of transcription during S phase.The Saccharomyces cerevisiae genome contains two nonallelic copies of each canonical histone gene which are organized in head-to-head pairs of H2A-H2B (HTA1-HTB1 and HTA2-HTB2) and H3-H4 (HHT1-HHF1 and HHT2-HHF2). Nucleosomes, the fundamental units of chromatin structure, contain two H2A/H2B dimers, one H3/H4 tetramer, and ϳ147 bp of DNA wrapped around the outer surface (37). Repression of histone transcription outside S phase is maintained by the HIR H3/H4 histone-protein chaperone complex, referred to here as the HIR complex (Hir1, Hir2, Hir3, and Hpc2). The HIR complex localizes to negative (NEG) sequence elements within the regulatory regions of HTA1-HTB1, 45,59). Although HTA2-HTB2 transcription is S-phase specific, its promoter does not contain NEG elements, and the factors involved in its HIRindependent repression are unknown (45). A recent study implicated two additional H3/H4 chaperones, Rtt106 ...