The Linker Histone Plays a Dual Role during Gametogenesis in Saccharomyces cerevisiae

Abstract

fThe differentiation of gametes involves dramatic changes to chromatin, affecting transcription, meiosis, and cell morphology. Sporulation in Saccharomyces cerevisiae shares many chromatin features with spermatogenesis, including a 10-fold compaction of the nucleus. To identify new proteins involved in spore nuclear organization, we purified chromatin from mature spores and discovered a significant enrichment of the linker histone (Hho1). The function of Hho1 has proven to be elusive during vegetative growth, but here we demonstrate its requirement for efficient sporulation and full compaction of the spore genome. Hho1 chromatin immunoprecipitation followed by sequencing (ChIP-seq) revealed increased genome-wide binding in mature spores and provides novel in vivo evidence of the linker histone binding to nucleosomal linker DNA. We also link Hho1 function to the transcription factor Ume6, the master repressor of early meiotic genes. Hho1 and Ume6 are depleted during meiosis, and analysis of published ChIP-chip data obtained during vegetative growth reveals a high binding correlation of both proteins at promoters of early meiotic genes. Moreover, Ume6 promotes binding of Hho1 to meiotic gene promoters. Thus, Hho1 may play a dual role during sporulation: Hho1 and Ume6 depletion facilitates the onset of meiosis via activation of Ume6-repressed early meiotic genes, whereas Hho1 enrichment in mature spores contributes to spore genome compaction. Gametogenesis is a complex, highly regulated differentiation program that is integral to the survival of higher eukaryotes. Mammalian spermatogenesis begins with DNA replication and recombination, ultimately generating four genetically unique haploid cells. The postmeiotic differentiation of spermatozoa involves a complete reorganization of the chromatin and a dramatic compaction of the nucleus that prepare the genome for passage to a new generation (16,30). In fact, specific combinations of histone posttranslational modifications (PTMs) are found at genes encoding important developmental regulators (5,20).Gametogenesis in Saccharomyces cerevisiae, or sporulation, shares several chromatin features with mammalian spermatogenesis, namely, drastic compaction of the nucleus and stage-specific appearance of histone PTMs such as phosphorylation of histone H3 at serine 10 (H3S10ph), H3T11ph, and H4S1ph (17, 31). Sporulation takes place in diploid yeast deprived of nitrogen and a fermentable carbon source (15,39). Under these starvation conditions, yeast cells enter a meiotic program similar to that found in mammalian spermatogenesis. During the postmeiotic phase, cell and nuclear volumes are substantially decreased, the genome is compacted, transcription is silenced, and a protective spore wall is synthesized. The resultant quiescent spore is able to survive unfavorable growth conditions of heat, dehydration, and chemical insult. When a fermentable carbon source is reintroduced, spores are induced to reenter the cell cycle through a process called germination, characterized by a spec...

Saccharomyces cerevisiaeLinker Histone—Hho1p Maintains Chromatin Loop Organization during Ageing

“…This is expected having in mind that with the time of the ageing process chromatin starts losing its normal characteristics and becomes more relaxed (Figure 2(b), WT DNase I), and the logarithmically growing mutant cells (6th and 48th hour) produced longer comets than the wild type Figure 2(b). This proved that more chromatin loops were relaxed and extended form the mutant nucleoids than in the wild type, which is in good correlation with other data showing that the yeast linker histone is necessary for chromatin compaction during the overall growth of these cells and its lack leads to total rearrangement of chromatin loop structures making them less compacted and more susceptible to the action of nucleases [18, 31]. Surprisingly, though, after the 72nd hour (3rd day) till the last 16th day the mutant comets started to decrease in size till 16th day when empty and faintly visible nuclear “shades” appeared.…”

Chromatin Compaction Protects Genomic DNA from Radiation Damage

“…In eukaryotic organisms, the maintenance of genetic information would be especially critical in germline cells for the next generation. For example, yeast spores have ∼10-fold more condensed genomic DNA than somatic cells [60]. Human primary oocytes, which take 20–40 years to complete meiosis I [61], have compact chromosomes, while mouse and rat have highly extended (dictyate-type) chromatin in their primary oocytes [62].…”

Sequestration of mRNAs Modulates the Timing of Translation during Meiosis in Budding Yeast

“…In midsporulation, the yeast cell switches from fine-grained control of gene expression based primarily on differential transcription to differential translation (12). One possible reason for this change is that the nuclear chromatin undergoes compaction, mediated by both modification of core histones and increased levels of histone H1, as cells progress through meiosis (3,34). It may be that transcriptional regulation becomes more problematic in this more compacted chromatin so that translational control is preferred.…”