Comparison of the structure and cell cycle expression of mRNAs encoded by two histone H3-H4 loci in Saccharomyces cerevisiae

Transcriptional and chromatin-based partitioning mechanisms uncouple protein scaling from cell size

Biosynthesis scales with cell size such that protein concentrations generally remain constant as cells grow. As an exception, synthesis of the cell-cycle inhibitor Whi5 "sub-scales" with cell size so that its concentration is lower in larger cells to promote cell-cycle entry. Here, we find that transcriptional control uncouples Whi5 synthesis from cell size, and we identify histones as the major class of sub-scaling transcripts besides WHI5 by screening for similar genes. Histone synthesis is thereby matched to genome content rather than cell size. Such sub-scaling proteins are challenged by asymmetric cell division because proteins are typically partitioned in proportion to newborn cell volume. To avoid this fate, Whi5 uses chromatin-binding to partition similar protein amounts to each newborn cell regardless of cell size. Disrupting both Whi5 synthesis and chromatin-based partitioning weakens G1 size control. Thus, specific transcriptional and partitioning mechanisms determine protein sub-scaling to control cell size.

Transcription coordinates histone amounts and genome content

Biochemical reactions typically depend on the concentrations of the molecules involved, and cell survival therefore critically depends on the concentration of proteins. To maintain constant protein concentrations during cell growth, global mRNA and protein synthesis rates are tightly linked to cell volume. While such regulation is appropriate for most proteins, certain cellular structures do not scale with cell volume. The most striking example of this is the genomic DNA, which doubles during the cell cycle and increases with ploidy, but is independent of cell volume. Here, we show that the amount of histone proteins is coupled to the DNA content, even though mRNA and protein synthesis globally increase with cell volume. As a consequence, and in contrast to the global trend, histone concentrations decrease with cell volume but increase with ploidy. We find that this distinct coordination of histone homeostasis and genome content is already achieved at the transcript level, and is an intrinsic property of histone promoters that does not require direct feedback mechanisms. Mathematical modeling and histone promoter truncations reveal a simple and generalizable mechanism to control the cell volume- and ploidy-dependence of a given gene through the balance of the initiation and elongation rates.

A CRISPR-Cas9 based shuffle system for endogenous histone H3 and H4 combinatorial mutagenesis

Post-translational modifications of histone proteins greatly impact gene expression and cell fate decisions in eukaryotes. To study these, it is important to develop a convenient, multiplex, and efficient method to precisely introduce mutations to histones. Because eukaryotic cells usually contain multiple copies of histone genes, it is a challenge to mutate all histones at the same time by the traditional homologous recombination method. Here, we developed a CRISPR-Cas9 based shuffle system in Saccharomyces cerevisiae, to generate point mutations on both endogenous histone H3 and H4 genes in a rapid, seamless and multiplex fashion. Using this method, we generated yeast strains containing histone triple H3–K4R–K36R–K79R mutants and histone combinatorial H3–K56Q–H4–K59A double mutants with high efficiencies (70–80%). This CRISPR-Cas9 based mutagenesis system could be an invaluable tool to the epigenetics field.

Loss of chromatin structural integrity is a source of stress during aging

Dysfunction and dysregulation at multiple levels, from organismal to molecular, are associated with the biological process of aging. In a eukaryotic nucleus, multiple lines of evidence have shown that the fundamental structure of chromatin is affected by aging. Not only euchromatic and heterochromatic regions shift locations, global changes, such as reduced levels of histones, have been reported for certain aged cell types and tissues. The physiological effects caused by such broad chromatin changes are complex and the cell's responses to it can be profound and in turn influence the aging process. In this review, we summarize recent findings on the interplay between chromatin architecture and aging with an emphasis on the cellular response to chromatin stress and its antagonistic effects on aging.

Construction of Comprehensive Dosage-Matching Core Histone Mutant Libraries for Saccharomyces cerevisiae

Saccharomyces cerevisiae contains two genes for each core histone, which are presented as pairs under the control of a divergent promoter, i.e., HHT1-HHF1, HHT2-HHF2, HTA1-HTB1 and HTA2-HTB2HHT1-HHF1, and HHT2-HHF2 encode histone H3 and H4 with identical amino acid sequences but under the control of differently regulated promoters. Previous mutagenesis studies were carried out by deleting one pair and mutating the other one. Here, we present the design and construction of three additional libraries covering HTA1-HTB1, HTA2-HTB2, and HHT1-HHF1 respectively. Together with the previously described library of HHT2-HHF2 mutants, a systematic and complete collection of mutants for each of the eight core S. cerevisiae histone genes becomes available. Each designed mutant was incorporated into the genome, generating three more corresponding libraries of yeast strains. We demonstrated that, although, under normal growth conditions, strains with single-copy integrated histone genes lacked phenotypes, in some growth conditions, growth deficiencies were observed. Specifically, we showed that addition of a second copy of the mutant histone gene could rescue the lethality in some previously known mutants that cannot survive with a single copy. This resource enables systematic studies of function of each nucleosome residue in plasmid, single-copy, and double-copy integrated formats.

Drug-Free Approach To Study the Unusual Cell Cycle of Giardia intestinalis

Giardia intestinalis is a protozoan parasite that causes giardiasis, a form of severe and infectious diarrhea. Despite the importance of the cell cycle in the control of proliferation and differentiation during a giardia infection, it has been difficult to study this process due to the absence of a synchronization procedure that would not induce cellular damage resulting in artifacts. We utilized counterflow centrifugal elutriation (CCE), a size-based separation technique, to successfully obtain fractions of giardia cultures enriched in G₁, S, and G₂. Unlike drug-induced synchronization of giardia cultures, CCE did not induce double-stranded DNA damage or endoreplication. We observed increases in the appearance and size of the median body in the cells from elutriation fractions corresponding to the progression of the cell cycle from early G₁ to late G₂. Consequently, CCE could be used to examine the dynamics of the median body and other structures and organelles in the giardia cell cycle. For the cell cycle gene expression studies, the actin-related gene was identified by the program geNorm as the most suitable normalizer for reverse transcription-quantitative PCR (RT-qPCR) analysis of the CCE samples. Ten of 11 suspected cell cycle-regulated genes in the CCE fractions have expression profiles in giardia that resemble those of higher eukaryotes. However, the RNA levels of these genes during the cell cycle differ less than 4-fold to 5-fold, which might indicate that large changes in gene expression are not required by giardia to regulate the cell cycle. IMPORTANCE Giardias are among the most commonly reported intestinal protozoa in the world, with infections seen in humans and over 40 species of animals. The life cycle of giardia alternates between the motile trophozoite and the infectious cyst. The regulation of the cell cycle controls the proliferation of giardia trophozoites during an active infection and contains the restriction point for the differentiation of trophozoite to cyst. Here, we developed counterflow centrifugal elutriation as a drug-free method to obtain fractions of giardia cultures enriched in cells from the G₁, S, and G₂ stages of the cell cycle. Analysis of these fractions showed that the cells do not show side effects associated with the drugs used for synchronization of giardia cultures. Therefore, counterflow centrifugal elutriation would advance studies on key regulatory events during the giardia cell cycle and identify potential drug targets to block giardia proliferation and transmission.

Asf1 facilitates dephosphorylation of Rad53 after DNA double-strand break repair

In this study, Tsabar et al. investigated how the DNA damage checkpoint is extinguished and found that dissociation of histone H3 from Asf1, a histone chaperone, is required for efficient recovery. They also show that Asf1 is required for complete dephosphorylation of Rad53 when the upstream DNA damage checkpoint signaling is turned off, providing new insights into the mechanisms regulating the response to DNA damage.

To allow for sufficient time to repair DNA double-stranded breaks (DSBs), eukaryotic cells activate the DNA damage checkpoint. In budding yeast, Rad53 (mammalian Chk2) phosphorylation parallels the persistence of the unrepaired DSB and is extinguished when repair is complete in a process termed recovery or when the cells adapt to the DNA damage checkpoint. A strain containing a slowly repaired DSB does not require the histone chaperone Asf1 to resume cell cycle progression after DSB repair. When a second, rapidly repairable DSB is added to this strain, Asf1 becomes required for recovery. Recovery from two repairable DSBs also depends on the histone acetyltransferase Rtt109 and the cullin subunit Rtt101, both of which modify histone H3 that is associated with Asf1. We show that dissociation of histone H3 from Asf1 is required for efficient recovery and that Asf1 is required for complete dephosphorylation of Rad53 when the upstream DNA damage checkpoint signaling is turned off. Our data suggest that the requirements for recovery from the DNA damage checkpoint become more stringent with increased levels of damage and that Asf1 plays a histone chaperone-independent role in facilitating complete Rad53 dephosphorylation following repair.

Targeted in Situ Mutagenesis of Histone Genes in Budding Yeast

We describe a PCR- and homologous recombination-based system for generating targeted mutations in histone genes in budding yeast cells. The resulting mutant alleles reside at their endogenous genomic sites and no exogenous DNA sequences are left in the genome following the procedure. Since in haploid yeast cells each of the four core histone proteins is encoded by two non-allelic genes with highly homologous open reading frames (ORFs), targeting mutagenesis specifically to one of two genes encoding a particular histone protein can be problematic. The strategy we describe here bypasses this problem by utilizing sequences outside, rather than within, the ORF of the target genes for the homologous recombination step. Another feature of this system is that the regions of DNA driving the homologous recombination steps can be made to be very extensive, thus increasing the likelihood of successful integration events. These features make this strategy particularly well-suited for histone gene mutagenesis, but can also be adapted for mutagenesis of other genes in the yeast genome.

The Major Replicative Histone Chaperone CAF-1 Suppresses the Activity of the DNA Mismatch Repair System in the Cytotoxic Response to a DNA-methylating Agent

The DNA mismatch repair (MMR) system corrects DNA mismatches in the genome. It is also required for the cytotoxic response of O⁶-methylguanine-DNA methyltransferase (MGMT)-deficient mammalian cells and yeast mgt1Δ rad52Δ cells to treatment with S_n1-type methylating agents, which produce cytotoxic O⁶-methylguanine (O⁶-mG) DNA lesions. Specifically, an activity of the MMR system causes degradation of irreparable O⁶-mG-T mispair-containing DNA, triggering cell death; this process forms the basis of treatments of MGMT-deficient cancers with S_n1-type methylating drugs. Recent research supports the view that degradation of irreparable O⁶-mG-T mispair-containing DNA by the MMR system and CAF-1-dependent packaging of the newly replicated DNA into nucleosomes are two concomitant processes that interact with each other. Here, we studied whether CAF-1 modulates the activity of the MMR system in the cytotoxic response to S_n1-type methylating agents. We found that CAF-1 suppresses the activity of the MMR system in the cytotoxic response of yeast mgt1Δ rad52Δ cells to the prototypic S_n1-type methylating agent N-methyl-N'-nitro-N-nitrosoguanidine. We also report evidence that in human MGMT-deficient cell-free extracts, CAF-1-dependent packaging of irreparable O⁶-mG-T mispair-containing DNA into nucleosomes suppresses its degradation by the MMR system. Taken together, these findings suggest that CAF-1-dependent incorporation of irreparable O⁶-mG-T mispair-containing DNA into nucleosomes suppresses its degradation by the MMR system, thereby defending the cell against killing by the S_n1-type methylating agent.

Unexpected function of the glucanosyltransferase Gas1 in the DNA damage response linked to histone H3 acetyltransferases in Saccharomyces cerevisiae

Chromatin organization and structure are crucial for transcriptional regulation, DNA replication, and damage repair. Although initially characterized in remodeling cell wall glucans, the β-1,3-glucanosyltransferase Gas1 was recently discovered to regulate transcriptional silencing in a manner separable from its activity at the cell wall. However, the function of Gas1 in modulating chromatin remains largely unexplored. Our genetic characterization revealed that GAS1 had critical interactions with genes encoding the histone H3 lysine acetyltransferases Gcn5 and Sas3. Specifically, whereas the gas1 gcn5 double mutant was synthetically lethal, deletion of both GAS1 and SAS3 restored silencing in Saccharomyces cerevisiae. The loss of GAS1 also led to broad DNA damage sensitivity with reduced Rad53 phosphorylation and defective cell cycle checkpoint activation following exposure to select genotoxins. Deletion of SAS3 in the gas1 background restored both Rad53 phosphorylation and checkpoint activation following exposure to genotoxins that trigger the DNA replication checkpoint. Our analysis thus uncovers previously unsuspected functions for both Gas1 and Sas3 in DNA damage response and cell cycle regulation.

Regulation of histone gene expression in budding yeast

We discuss the regulation of the histone genes of the budding yeast Saccharomyces cerevisiae. These include genes encoding the major core histones (H3, H4, H2A, and H2B), histone H1 (HHO1), H2AZ (HTZ1), and centromeric H3 (CSE4). Histone production is regulated during the cell cycle because the cell must replicate both its DNA during S phase and its chromatin. Consequently, the histone genes are activated in late G1 to provide sufficient core histones to assemble the replicated genome into chromatin. The major core histone genes are subject to both positive and negative regulation. The primary control system is positive, mediated by the histone gene-specific transcription activator, Spt10, through the histone upstream activating sequences (UAS) elements, with help from the major G1/S-phase activators, SBF (Swi4 cell cycle box binding factor) and perhaps MBF (MluI cell cycle box binding factor). Spt10 binds specifically to the histone UAS elements and contains a putative histone acetyltransferase domain. The negative system involves negative regulatory elements in the histone promoters, the RSC chromatin-remodeling complex, various histone chaperones [the histone regulatory (HIR) complex, Asf1, and Rtt106], and putative sequence-specific factors. The SWI/SNF chromatin-remodeling complex links the positive and negative systems. We propose that the negative system is a damping system that modulates the amount of transcription activated by Spt10 and SBF. We hypothesize that the negative system mediates negative feedback on the histone genes by histone proteins through the level of saturation of histone chaperones with histone. Thus, the negative system could communicate the degree of nucleosome assembly during DNA replication and the need to shut down the activating system under replication-stress conditions. We also discuss post-transcriptional regulation and dosage compensation of the histone genes.

Histone dosage regulates DNA damage sensitivity in a checkpoint-independent manner by the homologous recombination pathway

In eukaryotes, multiple genes encode histone proteins that package genomic deoxyribonucleic acid (DNA) and regulate its accessibility. Because of their positive charge, ‘free’ (non-chromatin associated) histones can bind non-specifically to the negatively charged DNA and affect its metabolism, including DNA repair. We have investigated the effect of altering histone dosage on DNA repair in budding yeast. An increase in histone gene dosage resulted in enhanced DNA damage sensitivity, whereas deletion of a H3–H4 gene pair resulted in reduced levels of free H3 and H4 concomitant with resistance to DNA damaging agents, even in mutants defective in the DNA damage checkpoint. Studies involving the repair of a HO endonuclease-mediated DNA double-strand break (DSB) at the MAT locus show enhanced repair efficiency by the homologous recombination (HR) pathway on a reduction in histone dosage. Cells with reduced histone dosage experience greater histone loss around a DSB, whereas the recruitment of HR factors is concomitantly enhanced. Further, free histones compete with the HR machinery for binding to DNA and associate with certain HR factors, potentially interfering with HR-mediated repair. Our findings may have important implications for DNA repair, genomic stability, carcinogenesis and aging in human cells that have dozens of histone genes.

The mitotic Clb cyclins are required to alleviate HIR-mediated repression of the yeast histone genes at the G1/S transition

The histone genes are an important group of cell cycle regulated genes whose transcription is activated during the G1/S transition and repressed in early G1, late S, and G2/M. The HIR complex, comprised of Hir1, Hir2, Hir3 and Hpc2, regulates three of the four histone gene loci. While relief of repression at the G1/S boundary involves the HIR complex, as well as other cofactors, the mechanism by which this derepression occurs remains unknown. To better understand how transcriptional repression contributes to periodic expression in the cell cycle, we sought to identify the cell cycle signals required to alleviate HIR-mediated repression of the histone genes. By measuring histone gene transcription in strains with various combinations of clb mutations, we found that the mitotic Clb1/Clb2 cyclins are required to alleviate Hir-mediated repression during the G1/S transition and that Clb2 physically interacts with the HIR complex. While the HIR complex regulates histone gene transcription in combination with two other histone H3/H4 chaperones, Asf1 and Rtt106, our data demonstrate that the mitotic Clb cyclins are necessary to specifically alleviate the repressive action of the HIR complex itself in order to allow proper expression of the histone genes in late G1/early S phase.

Alterations in DNA replication and histone levels promote histone gene amplification in Saccharomyces cerevisiae

Gene amplification, a process that increases the copy number of a gene or a genomic region to two or more, is utilized by many organisms in response to environmental stress or decreased levels of a gene product. Our previous studies in Saccharomyces cerevisiae identified the amplification of a histone H2A-H2B gene pair, HTA2-HTB2, in response to the deletion of the other H2A-H2B gene pair, HTA1-HTB1. This amplification arises from a recombination event between two flanking Ty1 elements to form a new, stable circular chromosome and occurs at a frequency higher than has been observed for other Ty1-Ty1 recombination events. To understand the regulation of this amplification event, we screened the S. cerevisiae nonessential deletion set for mutations that alter the amplification frequency. Among the deletions that increase HTA2-HTB2 amplification frequency, we identified those that either decrease DNA replication fork progression (rrm3Delta, dpb3Delta, dpb4Delta, and clb5Delta) or that reduce histone H3-H4 levels (hht2-hhf2Delta). These two classes are related because reduced histone H3-H4 levels increase replication fork pauses, and impaired replication forks cause a reduction in histone levels. Consistent with our mutant screen, we found that the introduction of DNA replication stress by hydroxyurea induces the HTA2-HTB2 amplification event. Taken together, our results suggest that either reduced histone levels or slowed replication forks stimulate the HTA2-HTB2 amplification event, contributing to the restoration of normal chromatin structure.

Differential regulation of repeated histone genes during the fission yeast cell cycle

The histone genes are highly reiterated in a wide range of eukaryotic genomes. The fission yeast, Schizosaccharomyces pombe, has three pairs of histone H3-H4 genes: hht1⁺-hhf1⁺, hht2⁺-hhf2⁺ and hht3⁺-hhf3⁺. While the deduced amino acid sequences are identical, it remains unknown whether transcriptional regulation differs among the three pairs. Here, we report the transcriptional properties of each H3-H4 gene pair during the cell cycle. The levels of transcripts of hht1⁺-hhf1⁺ and hht3⁺-hhf3⁺ pairs and hhf2⁺ are increased at S-phase, while that of hht2⁺ remains constant throughout the cell cycle. We showed that the GATA-type transcription factor, Ams2, binds to the promoter regions of core histone genes in an AACCCT-box-dependent manner and is required for activation of S-phase-specific transcription. Furthermore, we found that Ams2-depletion stimulates feedback regulation of histone transcripts, mainly up-regulating the basal levels of hht2⁺-hhf2⁺ transcription, which are normally down-regulated by Hip1 and Slm9, homologs of the human histone chaperone, HIRA. These observations provide insight into the molecular mechanisms of differential regulation of transcripts from repeated histone genes in the fission yeast.

Contribution of Trf4/5 and the nuclear exosome to genome stability through regulation of histone mRNA levels in Saccharomyces cerevisiae

Balanced levels of histones are crucial for chromosome stability, and one major component of this control regulates histone mRNA amounts. The Saccharomyces cerevisiae poly(A) polymerases Trf4 and Trf5 are involved in a quality control mechanism that mediates polyadenylation and consequent degradation of various RNA species by the nuclear exosome. None of the known RNA targets, however, explains the fact that trf mutants have specific cell cycle defects consistent with a role in maintaining genome stability. Here, we investigate the role of Trf4/5 in regulation of histone mRNA levels. We show that loss of Trf4 and Trf5, or of Rrp6, a component of the nuclear exosome, results in elevated levels of transcripts encoding DNA replication-dependent histones. Suggesting that increased histone levels account for the phenotypes of trf mutants, we find that TRF4 shows synthetic genetic interactions with genes that negatively regulate histone levels, including RAD53. Moreover, synthetic lethality of trf4Delta rad53Delta is rescued by reducing histone levels whereas overproduction of histones is deleterious to trf's and rrp6Delta mutants. These results identify TRF4, TRF5, and RRP6 as new players in the regulation of histone mRNA levels in yeast. To our knowledge, the histone transcripts are the first mRNAs that are upregulated in Trf mutants.

Multiple independent evolutionary solutions to core histone gene regulation

BACKGROUND

Core histone genes are periodically expressed along the cell cycle and peak during S phase. Core histone gene expression is deeply evolutionarily conserved from the yeast Saccharomyces cerevisiae to human.

RESULTS

We evaluated the evolutionary dynamics of the specific regulatory mechanisms that give rise to the conserved histone regulatory phenotype. In contrast to the conservation of core histone gene expression patterns, the core histone regulatory machinery is highly divergent between species. There has been substantial evolutionary turnover of cis-regulatory sequence motifs along with the transcription factors that bind them. The regulatory mechanisms employed by members of the four core histone families are more similar within species than within gene families. The presence of species-specific histone regulatory mechanisms is opposite to what is seen at the protein sequence level. Core histone proteins are more similar within families, irrespective of their species of origin, than between families, which is consistent with the shared common ancestry of the members of individual histone families. Structure and sequence comparisons between histone families reveal that H2A and H2B form one related group whereas H3 and H4 form a distinct group, which is consistent with the nucleosome assembly dynamics.

CONCLUSION

The dissonance between the evolutionary conservation of the core histone gene regulatory phenotypes and the divergence of their regulatory mechanisms indicates a highly dynamic mode of regulatory evolution. This distinct mode of regulatory evolution is probably facilitated by a solution space for promoter sequences, in terms of functionally viable cis-regulatory sites, that is substantially greater than that of protein sequences.

Yeast chromatin assembly complex 1 protein excludes nonacetylatable forms of histone H4 from chromatin and the nucleus

In yeast, the establishment and maintenance of a transcriptionally silent chromatin state are dependent upon the acetylation state of the N terminus of histone proteins. Histone H4 proteins that contain mutations in N-terminal lysines disrupt heterochromatin and result in yeast that cannot mate. Introduction of a wild-type copy of histone H4 restores mating, despite the presence of the mutant protein, suggesting that mutant H4 protein is either excluded from, or tolerated in, chromatin. To understand how the cell differentiates wild-type histone and mutant histone in which the four N-terminal lysines were replaced with alanine (H4-4A), we analyzed silencing, growth phenotypes, and the histone composition of chromatin in yeast strains coexpressing equal amounts of wild-type and mutant H4 proteins (histone H4 heterozygote). We found that histone H4 heterozygotes have defects in heterochromatin silencing and growth, implying that mutations in H4 are not completely recessive. Nuclear preparations from histone H4 heterozygotes contained less mutant H4 than wild-type H4, consistent with the idea that cells exclude some of the mutant histone. Surprisingly, the N-terminal nuclear localization signal of H4-4A fused to green fluorescent protein was defective in nuclear localization, while a mutant in which the four lysines were replaced with arginine (H4-4R) appeared to have normal nuclear import, implying a role for the charged state of the acetylatable lysines in the nuclear import of histones. The biased partial exclusion of H4-4A was dependent upon Cac1p, the largest subunit of yeast chromatin assembly factor 1 (CAF-1), as well as upon the karyopherin Kap123p, but was independent of Cac2p, another CAF-1 component, and other chromatin assembly proteins (Hir3p, Nap1p, and Asf1p). We conclude that N-terminal lysines of histone H4 are important for efficient histone nuclear import. In addition, our data support a model whereby Cac1p and Kap123 cooperate to ensure that only appropriately acetylated histone H4 proteins are imported into the nucleus.

Histone-histone interactions and centromere function

Cse4p is a structural component of the core centromere of Saccharomyces cerevisiae and is a member of the conserved CENP-A family of specialized histone H3 variants. The histone H4 allele hhf1-20 confers defects in core centromere chromatin structure and mitotic chromosome transmission. We have proposed that Cse4p and histone H4 interact through their respective histone fold domains to assemble a nucleosome-like structure at centromeric DNA. To test this model, we targeted random mutations to the Cse4p histone fold domain and isolated three temperature-sensitive cse4 alleles in an unbiased genetic screen. Two of the cse4 alleles contain mutations at the Cse4p-H4 interface. One of these requires two widely separated mutations demonstrating long-range cooperative interactions in the structure. The third cse4 allele is mutated at its helix 2-helix 3 interface, a region required for homotypic H3 fold dimerization. Overexpression of wild-type Cse4p and histone H4 confer reciprocal allele-specific suppression of cse4 and hhf1 mutations, providing strong evidence for Cse4p-H4 protein interaction. Overexpression of histone H3 is dosage lethal in cse4 mutants, suggesting that histone H3 competes with Cse4p for histone H4 binding. However, the relative resistance of the Cse4p-H4 pathway to H3 interference argues that centromere chromatin assembly must be highly regulated.

SPT10 and SPT21 are required for transcription of particular histone genes in Saccharomyces cerevisiae

The Saccharomyces cerevisiae genome contains four loci that encode histone proteins. Two of these loci, HTA1-HTB1 and HTA2-HTB2, each encode histones H2A and H2B. The other two loci, HHT1-HHF1 and HHT2-HHF2, each encode histones H3 and H4. Because of their redundancy, deletion of any one histone locus does not cause lethality. Previous experiments demonstrated that mutations at one histone locus, HTA1-HTB1, do cause lethality when in conjunction with mutations in the SPT10 gene. SPT10 has been shown to be required for normal levels of transcription of several genes in S. cerevisiae. Motivated by this double-mutant lethality, we have now investigated the interactions of mutations in SPT10 and in a functionally related gene, SPT21, with mutations at each of the four histone loci. These experiments have demonstrated that both SPT10 and SPT21 are required for transcription at two particular histone loci, HTA2-HTB2 and HHF2-HHT2, but not at the other two histone loci. These results suggest that under some conditions, S. cerevisiae may control the level of histone proteins by differential expression of its histone genes.

SPT10 and SPT21 are required for transcription of particular histone genes in Saccharomyces cerevisiae

The Saccharomyces cerevisiae genome contains four loci that encode histone proteins. Two of these loci, HTA1-HTB1 and HTA2-HTB2, each encode histones H2A and H2B. The other two loci, HHT1-HHF1 and HHT2-HHF2, each encode histones H3 and H4. Because of their redundancy, deletion of any one histone locus does not cause lethality. Previous experiments demonstrated that mutations at one histone locus, HTA1-HTB1, do cause lethality when in conjunction with mutations in the SPT10 gene. SPT10 has been shown to be required for normal levels of transcription of several genes in S. cerevisiae. Motivated by this double-mutant lethality, we have now investigated the interactions of mutations in SPT10 and in a functionally related gene, SPT21, with mutations at each of the four histone loci. These experiments have demonstrated that both SPT10 and SPT21 are required for transcription at two particular histone loci, HTA2-HTB2 and HHF2-HHT2, but not at the other two histone loci. These results suggest that under some conditions, S. cerevisiae may control the level of histone proteins by differential expression of its histone genes.

Independent evolutionary origin of histone H3.3-like variants of animals and Tetrahymena

All three genes encoding histone H3 proteins were cloned and sequenced from Tetrahymena thermophila. Two of these genes encode a major H3 protein identical to that of T. pyriformis and 87% identical to the major H3 of vertebrates. The third gene encodes hv2, a quantitatively minor replication independent (replacement) variant. The sequence of hv2 is only 85% identical to the animal replacement variant H3.3 and is the most divergent H3 replacement variant described. Phylogenetic analysis of 73 H3 protein sequences suggests that hv2, H3.3, and the plant replacement variant H3.III evolved independently, and that H3.3 is not the ancestral H3 gene, as was previously suggested (Wells, D., Bains, W., and Kedes, L. 1986, J. Mol. Evol., 23: 224-241). These results suggest it is the replication independence and not the particular protein sequence that is important in the function of H3 replacement variants.

FAR1 is required for posttranscriptional regulation of CLN2 gene expression in response to mating pheromone

Yeast cells arrest during the G1 interval of the cell cycle in response to peptide mating pheromones. The FAR1 gene is required for cell cycle arrest but not for a number of other aspects of the pheromone response. Genetic evidence suggests that FAR1 is required specifically for inactivation of the G1 cyclin CLN2. From these observations, the FAR1 gene has been proposed to encode an element of the interface between the mating pheromone signal transduction pathway and the cell cycle regulatory apparatus. We show here that FAR1 is necessary for the decrease in CLN1 and CLN2 transcript accumulation observed in response to mating pheromone but is unnecessary for regulation of the same transcripts during vegetative growth. However, the defect in regulation of CLN1 expression is dependent upon CLN2. We show that pheromone regulates the abundance of Cln2 at a posttranscriptional level and that FAR1 is required for that regulation. From these observations, we suggest that FAR1 function is limited to posttranscriptional regulation of CLN2 expression by mating pheromone. The failure of mating pheromone to repress CLN2 transcript levels in far1 mutants can be explained by the stimulatory effect of the persistent Cln2 protein on CLN2 transcription via the CLN/CDC28-dependent feedback pathway.

Characterization of HIR1 and HIR2, two genes required for regulation of histone gene transcription in Saccharomyces cerevisiae

The products of the HIR1 and HIR2 genes have been defined genetically as repressors of histone gene transcription in S. cerevisiae. A mutation in either gene affects cell cycle regulation of three of the four histone gene loci; transcription of these loci occurs throughout the cell cycle and is no longer repressed in response to the inhibition of DNA replication. The same mutations also eliminate autogenous regulation of the HTA1-HTB1 locus by histones H2A and H2B. The HIR1 and HIR2 genes have been isolated, and their roles in the transcriptional regulation of the HTA1-HTB1 locus have been characterized. Neither gene encodes an essential protein, and null alleles derepress HTA1-HTB1 transcription. Both HIR genes are expressed constitutively under conditions that lead to repression or derepression of the HTA1 gene, and neither gene regulates the expression of the other. The sequence of the HIR1 gene predicts an 88-kDa protein with three repeats of a motif found in the G beta subunit of retinal transducin and in a yeast transcriptional repressor, Tup1. The sequence of the HIR2 gene predicts a protein of 98 kDa. Both gene products contain nuclear targeting signals, and the Hir2 protein is localized in the nucleus.

Characterization of HIR1 and HIR2, two genes required for regulation of histone gene transcription in Saccharomyces cerevisiae

The products of the HIR1 and HIR2 genes have been defined genetically as repressors of histone gene transcription in S. cerevisiae. A mutation in either gene affects cell cycle regulation of three of the four histone gene loci; transcription of these loci occurs throughout the cell cycle and is no longer repressed in response to the inhibition of DNA replication. The same mutations also eliminate autogenous regulation of the HTA1-HTB1 locus by histones H2A and H2B. The HIR1 and HIR2 genes have been isolated, and their roles in the transcriptional regulation of the HTA1-HTB1 locus have been characterized. Neither gene encodes an essential protein, and null alleles derepress HTA1-HTB1 transcription. Both HIR genes are expressed constitutively under conditions that lead to repression or derepression of the HTA1 gene, and neither gene regulates the expression of the other. The sequence of the HIR1 gene predicts an 88-kDa protein with three repeats of a motif found in the G beta subunit of retinal transducin and in a yeast transcriptional repressor, Tup1. The sequence of the HIR2 gene predicts a protein of 98 kDa. Both gene products contain nuclear targeting signals, and the Hir2 protein is localized in the nucleus.

Histone H3 transcription in Saccharomyces cerevisiae is controlled by multiple cell cycle activation sites and a constitutive negative regulatory element

The promoters of the Saccharomyces cerevisiae histone H3 and H4 genes were examined for cis-acting DNA sequence elements regulating transcription and cell division cycle control. Deletion and linker disruption mutations identified two classes of regulatory elements: multiple cell cycle activation (CCA) sites and a negative regulatory site (NRS). Duplicate 19-bp CCA sites are present in both the copy I and copy II histone H3-H4 promoters arranged as inverted repeats separated by 45 and 68 bp. The CCA sites are both necessary and sufficient to activate transcription under cell division cycle control. A single CCA site provides cell cycle control but is a weak transcriptional activator, while an inverted repeat comprising two CCA sites provides both strong transcriptional activation and cell division cycle control. The NRS was identified in the copy I histone H3-H4 promoter. Deletion or disruption of the NRS increased the level of the histone H3 promoter activity but did not alter the cell division cycle periodicity of transcription. When the CCA sites were deleted from the histone promoter, the NRS element was unable to confer cell division cycle control on the remaining basal level of transcription. When the NRS element was inserted into the promoter of a foreign reporter gene, transcription was constitutively repressed and did not acquire cell cycle regulation.

Histone H3 transcription in Saccharomyces cerevisiae is controlled by multiple cell cycle activation sites and a constitutive negative regulatory element

The promoters of the Saccharomyces cerevisiae histone H3 and H4 genes were examined for cis-acting DNA sequence elements regulating transcription and cell division cycle control. Deletion and linker disruption mutations identified two classes of regulatory elements: multiple cell cycle activation (CCA) sites and a negative regulatory site (NRS). Duplicate 19-bp CCA sites are present in both the copy I and copy II histone H3-H4 promoters arranged as inverted repeats separated by 45 and 68 bp. The CCA sites are both necessary and sufficient to activate transcription under cell division cycle control. A single CCA site provides cell cycle control but is a weak transcriptional activator, while an inverted repeat comprising two CCA sites provides both strong transcriptional activation and cell division cycle control. The NRS was identified in the copy I histone H3-H4 promoter. Deletion or disruption of the NRS increased the level of the histone H3 promoter activity but did not alter the cell division cycle periodicity of transcription. When the CCA sites were deleted from the histone promoter, the NRS element was unable to confer cell division cycle control on the remaining basal level of transcription. When the NRS element was inserted into the promoter of a foreign reporter gene, transcription was constitutively repressed and did not acquire cell cycle regulation.

Protein synthesis requirements for nuclear division, cytokinesis, and cell separation in Saccharomyces cerevisiae

Protein synthesis inhibitors have often been used to identify regulatory steps in cell division. We used cell division cycle mutants of the yeast Saccharomyces cerevisiae and two chemical inhibitors of translation to investigate the requirements for protein synthesis for completing landmark events after the G1 phase of the cell cycle. We show, using cdc2, cdc6, cdc7, cdc8, cdc17 (38 degrees C), and cdc21 (also named tmp1) mutants, that cells arrested in S phase complete DNA synthesis but cannot complete nuclear division if protein synthesis is inhibited. In contrast, we show, using cdc16, cdc17 (36 degrees C), cdc20, cdc23, and nocodazole treatment, that cells that arrest in the G2 stage complete nuclear division in the absence of protein synthesis. Protein synthesis is required late in the cell cycle to complete cytokinesis and cell separation. These studies show that there are requirements for protein synthesis in the cell cycle, after G1, that are restricted to two discrete intervals.

Protein synthesis requirements for nuclear division, cytokinesis, and cell separation in Saccharomyces cerevisiae

Protein synthesis inhibitors have often been used to identify regulatory steps in cell division. We used cell division cycle mutants of the yeast Saccharomyces cerevisiae and two chemical inhibitors of translation to investigate the requirements for protein synthesis for completing landmark events after the G1 phase of the cell cycle. We show, using cdc2, cdc6, cdc7, cdc8, cdc17 (38 degrees C), and cdc21 (also named tmp1) mutants, that cells arrested in S phase complete DNA synthesis but cannot complete nuclear division if protein synthesis is inhibited. In contrast, we show, using cdc16, cdc17 (36 degrees C), cdc20, cdc23, and nocodazole treatment, that cells that arrest in the G2 stage complete nuclear division in the absence of protein synthesis. Protein synthesis is required late in the cell cycle to complete cytokinesis and cell separation. These studies show that there are requirements for protein synthesis in the cell cycle, after G1, that are restricted to two discrete intervals.