Identification of a new set of cell cycle-regulatory genes that regulate S-phase transcription of histone genes in Saccharomyces cerevisiae.

Abstract

Histone mRNA synthesis is tightly regulated to S phase of the yeast Saccharomyces cerevisiae cell cycle as a result of transcriptional and posttranscriptional controls. Moreover, histone gene transcription decreases rapidly if DNA replication is inhibited by hydroxyurea or if cells are arrested in G1 by the mating pheromone a-factor. To identify the transcriptional controls responsible for cycle-specific histone mRNA synthesis, we have developed a selection for mutations which disrupt this process. Using this approach, we have isolated five mutants (hpcl, hpc2, hpc3, hpc4, and Histone mRNAs are synthesized and accumulated preferentially in S phase of the eukaryotic cell cycle, partly as a result of transcriptional controls which cause the rate of synthesis of histone mRNAs to increase during S phase. Also, posttranscriptional regulation results in the preferential degradation of histone mRNAs outside of S (reviewed in reference 17). These two mechanisms of regulation function to decrease mammalian histone mRNA levels after DNA synthesis is blocked in the presence of hydroxyurea (HU) (17). In this report, we focus on cell cycle-specific, transcriptional control of histone mRNA synthesis.The human histone H4 gene is transcribed more efficiently (3-to 10-fold) in S-phase extracts than in extracts taken from cells at other stages of the cell cycle (18). Mammalian upstream promoter elements responsible for this regulation have been identified (1,6,16,28,42). Moreover, fusion of the 5' end of the mouse histone H3 gene to the bacterial neomycin resistance (neo) gene causes the bacterial mRNA to be synthesized in a cell cycle-specific manner (3). cisacting regulatory sequences and their trans-acting protein factors are likely responsible for regulating histone mRNA synthesis (7, 10, 11, 14, 50; reviewed in reference 33); however, the molecular details of the regulatory mechanism by which this occurs are still under investigation.The yeast Saccharomyces cerevisiae provides a system which allows the identification genetically of trans-acting factors that regulate cycle-specific histone mRNA synthesis. Yeast

Regulation of Histone Gene Expression in Budding Yeast

“…Although the levels of histone mRNA are increased in hir mutants relative to wild type, expression is still cell cycle dependent in that a peak is observed in S phase Xu et al 1992). In contrast, depletion of Asf1 from cells has little effect on the levels of histone transcripts during the cell cycle (Zabaronick and Tyler 2005), suggesting that the HIR complex and associated Asf1 do not have redundant functions at the histone promoters.…”

“…Mutations in HIR genes derepress all the major histone genes except HTA2-HTB2, as shown by significant expression of the histone genes outside S phase Xu et al 1992). Although the levels of histone mRNA are increased in hir mutants relative to wild type, expression is still cell cycle dependent in that a peak is observed in S phase Xu et al 1992).…”

“…This reduction is transient; the message levels are restored as cells adapt to replication stress (Libuda and Winston 2010). Histone gene expression is rendered immune to hydroxyurea-mediated repression by deletion of the NEG region, by hir mutations, and by asf1 mutations, again with the exception of HTA2-HTB2 Osley and Lycan 1987;Xu et al 1992;Spector and Osley 1993;Spector et al 1997;Sutton et al 2001;Ng et al 2002).…”

“…Instead, we believe that the activating system is the primary source of specificity for the following reasons: (1) Only the histone UAS element confers cell-cycle-dependent regulation with the correct timing (Osley et al 1986); (2) mutations in the negative regulatory sequence of HHT1-HHF1 do not affect cycling, whereas mutations in the UAS elements do (Freeman et al 1992); (3) HTA2-HTB2 cycles similarly to the other major core histone loci but has no NEG system Xu et al 1992); (4) Spt10 is the only known sequence-specific factor dedicated to histone gene expression (Eriksson et al 2005)-none of the known NEG system components are specific for the histone genes; (5) mutations in the UAS elements in HTA1-HTB1 Figure 5 Model for the activated and repressed states of the HTA1-HTB1 promoter. In late G1 and S phase, the histone genes are transcriptionally active.…”

“…The histone regulatory complex: a negative regulator of the histone genes Two independent genetic screens for factors that negatively regulate the expression of the histone genes identified the histone regulatory genes (HIR1, HIR2, and HIR3) and the histone periodic control gene (HPC2) Xu et al 1992). The Hir1, Hir2, Hir3, and Hpc2 proteins together form the HIR complex, which acts as a histone chaperone (Green et al 2005;Prochasson et al 2005).…”