Regulation Of Histone Synthesis Research Articles

Regulation of histone synthesis and nucleosome assembly

Histone deposition onto nascent DNA is the first step in the process of chromatin assembly during DNA replication. The process of nucleosome assembly represents a daunting task for S-phase cells, partly because cells need to rapidly package nascent DNA into nucleosomes while avoiding the generation of excess histones. Consequently, cells have evolved a number of nucleosome assembly factors and regulatory mechanisms that collectively function to coordinate the rates of histone and DNA synthesis during both normal cell cycle progression and in response to conditions that interfere with DNA replication.

Hir proteins are required for position-dependent gene silencing in Saccharomyces cerevisiae in the absence of chromatin assembly factor I.

Chromatin assembly factor I (CAF-I) is a three-subunit histone-binding complex conserved from the yeast Saccharomyces cerevisiae to humans. Yeast cells lacking CAF-I (cacDelta mutants) have defects in heterochromatic gene silencing. In this study, we showed that deletion of HIR genes, which regulate histone gene expression, synergistically reduced gene silencing at telomeres and at the HM loci in cacDelta mutants, although hirDelta mutants had no silencing defects when CAF-I was intact. Therefore, Hir proteins are required for an alternative silencing pathway that becomes important in the absence of CAF-I. Because Hir proteins regulate expression of histone genes, we tested the effects of histone gene deletion and overexpression on telomeric silencing and found that alterations in histone H3 and H4 levels or in core histone stoichiometry reduced silencing in cacDelta mutants but not in wild-type cells. We therefore propose that Hir proteins contribute to silencing indirectly via regulation of histone synthesis. However, deletion of combinations of CAC and HIR genes also affected the growth rate and in some cases caused partial temperature sensitivity, suggesting that global aspects of chromosome function may be affected by the loss of members of both gene families.

The Regulation Of Histone Synthesis In The Cell Cycle

The Regulation Of Histone Synthesis In The Cell Cycle

Regulation of histone synthesis during early Urechis caupo (Echiura) development

The histones present in mature oocytes and embryos of Urechis caupo and their pattern of synthesis during early development have been characterized. Acid-soluble proteins extracted from mature oocyte germinal vesicles and from embryonic nuclei were analyzed by two-dimensional polyacrylamide gel electrophoresis. Histones are accumulated in the mature oocytes in amounts sufficient to provide for the assembly of chromatin through the 32- to 64-cell stage of embryogenesis. Two H1 histones, which appear to be variants, were found. Germinal vesicles and cleavage-stage nuclei are enriched in H1M (maternal). During late cleavage a faster-migrating H1, H1E (embryonic), appears among the nuclear histones and, as embryogenesis continues, replaces H1M as the predominant H1. No new core histone variants are detected during early development. Examination of [ 3H]lysine-labeled histones from germinal vesicles and embryonic nuclei reveals stage-specific patterns of histone synthesis. H1M is the major H1 species synthesized in mature oocytes. After fertilization, a switch to the predominant synthesis of H1E occurs. Comparison of the [ 3H]lysine incorporated into H1E and core histones indicates that H1E synthesis is disproportionately high from midcleavage through the midblastula stage. By the gastrula stage, a balanced synthesis of H1E and each core histone is established. The results indicate that there is noncoordinate regulation of H1 and core histone synthesis during Urechis development.

Nonrandom distribution of histone mRNAs into polysomes and nonpolysomal ribonucleoprotein particles in sea urchin embryos.

In the early sea urchin embryo, newly synthesized cytoplasmic histone mRNA is found both on polysomes and free of ribosomes as nonpolysomal messenger ribonucleoprotein particles (free RNPs). The distribution of newly synthesized histone mRNAs between translating and nontranslating compartments is nonrandom and dependent on the developmental stage. Gel electrophoresis and autoradiography of polysomal and free RNP RNA from embryos at various stages show that (i) the fraction of total newly synthesized histone mRNA that is in polysomes is greater than the fraction of total newly synthesized poly(A)+mRNA that is in polysomes, at all stages examined, and (ii) among the five histone mRNAs. H1 mRNA and H4 mRNA are relatively more enriched in the free RNPs than are the mRNAs for H2A, H2B, and H3. These data suggest that histone mRNA, as a class, is more efficiently utilized as a template than the average mRNA and, of the cytoplasmic histone mRNAs, the mRNAs for histones H2A, H2B, and possibly H3 are selected more frequently for translation than those for H1 and H4. Cell-free translations of polysomal and free RNP RNAs yield different ratios of in vitro histone products, consistent with the RNA distribution data. To test the possibility that the in vivo distribution of the histone mRNAs is the consequence of different intrinsic initiation capabilities of the individual mRNAs, ribosome-binding assays were carried out and unequal binding abilities of the histone mRNAs in the reticulocyte lysate were shown. A translational level component in the regulation of histone synthesis in the sea urchin embryo is indicated.

Noncoincidence of histone and DNA synthesis in cleavage cycles of early development.

The times of histone and DNA synthesis do not coincide in early cleavage of the sea urchin embryo. In fact, the production of histones increases during the interval after DNA synthesis (G2). Enucleate merogones, parthenogenetically activated, synthesize histones encoded upon maternal histone messenger RNA. The pattern of protein synthesis changes following fertilization, in part, but not solely, due to the increasing synthesis of histones relative to other proteins. Regulation of histone synthesis and the loading of newly replicated DNA with histones must themselves undergo change at the time of transition from cleavage cycles to cycles more typical of somatic cells.

On the translational control of histone synthesis. Quantitation of biologically active histone mRNA from synchronized HeLa cells and its translation in different cell-free systems.

Histone mRNA was isolated from polyribosomes and other cell fractions of synchronized HeLa cells and quantitated by its translational capacity in a rabbit reticulocyte lysate. Inhibition of DNA synthesis with hydroxyurea (5 mM) results within 30 to 60 min in a loss of 80–85% of biologically active histone mRNA from polyribosomes. In contrast, actinomycin D (5 μg/ml) during the first hour of its action, only stops further accumulation of histone mRNA on polyribosomes in early S‐phase cells. Under conditions of interruption of DNA replication biologically active histone mRNA does not appear in other cell fractions suggesting that it is immediately destroyed in the cytoplasm.The possible translational control of histone synthesis in HeLa cells was investigated with special consideration of the existence of messenger‐specific initiation factors. Cell‐free extracts from synchronized HeLa cells in which DNA and histone synthesis were inhibited with hydroxyurea, fully retain their capacity to translate added homologous histone mRNA as compared with extracts from synchronized S‐phase cells. Using a partially purified protein‐synthesizing system from rat liver supplemented with initiation factors from synchronized HeLa cells differing in their histone‐synthesizing capacity, it was shown that factors from hydroxyurea‐treated cells are as active in stimulating the translation of histone mRNA, globin mRNA and tobacco mosaic virus RNA as are initiation factors from S‐phase cells. The results suggest that messenger discriminating translation factors are not involved in the regulation of histone synthesis in HeLa cells or they are very labile and inactivated during preparation.