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.     

Uncoordinate synthesis of histone H1 in cells arrested in the G1 phase

It has been known for several years that DNA replication and histone synthesis occur concomitantly in cultured mammalian cells. Normally all five classes of histones are synthesized coordinately. However, mouse myeloma cells, synchronized by starvation for isoleucine, synthesize increased amounts of histone H1 relative to the four nucleosomal core histones. This unscheduled synthesis of histone H1 is reduced within 1 h after refeeding isoleucine, and is not a normal component of G1. The synthesis of H1 increases coordinately again with other histones during the S phase. The DNA synthesis inhibitors, cytosine arabinoside and hydroxyurea, block all histone synthesis in S-phase cells. The levels of histone H1 mRNA, relative to the other histone mRNAs, is increased in isoeleucine-starved cells and decreases rapidly after refeeding isoleucine. The increased incorporation of histone H1 is at least partially due to the low isoleucine content of histone H1. Starvation of cells for lysine resulted in a decrease in H1 synthesis relative to core histones. Again the ratio was altered on refeeding the amino acid. 3T3 cells starved for serum also incorporated only H1 histones into chromatin. The ratio of different H1 proteins also changed. The synthesis of the H1⁰ protein was predominant in G₀ cells, and reduced in S-phase cells. These data indicate the metabolism of H1 is independent of the other histones when cell growth is arrested.     

Rates of histone synthesis during early development of Rana pipiens

Newly synthesized histones have been extracted from Rana pipiens oocytes or cleaving embryos previously injected with [³H]lysine or [³H]arginine. The radioactive proteins were fractionated by cation-exchange chromatography and electrophoresis on acid/urea or SDS-polyacrylamide gels; histones were identified by coelectrophoresis with authentic markers. From percentage total incorporation in the putative histones, and absolute rates of lysine or arginine incorporation, rates of histone synthesis were estimated. Rates of histone synthesis in two-cell embryos were at least 10-fold higher than in maturing oocytes. Between the two-cell and blastula stages, the rate increased an additional threefold, from about 1200 pg hr^?1 per embryo to about 4500 pg hr^?1 per embryo. While all histone classes are synthesized during cleavage, synthesis of the various classes is not coordinated; histones are not synthesized in the same relative proportions at which they are found in blastula chromatin. The synthesis of histone H4 in particular is barely detectable during cleavage. This, and other observations, suggested the existence of cytoplasmic histone pools. In approaching the possible existence of histone pools, the amount of H4 present in oocytes was determined. Oocytes contain about 74 ng of H4, an amount sufficient to allow development to the blastula stage. These data are compared to those reported by others on histone synthesis during cleavage in Xenopus.     

Histone Synthesis inTrypanosoma cruzi

Trypanosoma cruziis an ancient, parasitic eukaryote which does not undergo chromatin condensation during cell division. This behavior may be explained if one considers the strong amino acid sequence divergence ofTrypanosomahistones compared to higher eukaryotes. In the latter organisms histone synthesis is coupled to DNA replication. Considering the nonconserved amino acid sequence ofT. cruzihistones, as well as the absence of chromatin condensation in this organism, we have studied histone synthesis in relation to DNA replication in this parasite. We have found that core histones and a fraction of histone H1 are synthesized concomitantly to DNA replication. However, another fraction of histone H1 is constitutively synthesized.     

Acetylation of histones of rat testis.

When [1-¹⁴C]acetate was injected into rats intratesticularly in the presence of cycloheximide to inhibit protein synthesis, the label was incorporated into histone fractions F2a1 and F3 and into non-histone chromosomal proteins of each of the following stages of spermatogenesis: spermatogonia-preleptotene spermatocytes, leptotene-zygotene-pachytene-diplotene primary spermatocytes, and spermatids. Acetylation of histones was particularly active in the spermatid stages. There was no significant incorporation of acetate into the lysine-rich histone fractions F1 and X₁.In early periods of in vivo incorporation of [³H]amino acids into histones the acetylated histone F2a1 fractions had higher specific activities than the main band of F2a1, but with the passage of time the label moved into the principal band to the extent that specific activities in the acetylated and principal bands were approximately equal at 6 days. However, at 24–36 days the specific activities were again higher in the acetylated bands than in the principal band of F2a1. These data support the conclusions of Candido, Louie, and Dixon, from experiments with trout testis, that acetylation of histone F2a1 may be important in the process of combination of this protein with DNA in chromatin at the spermatogonia-primary spermatocyte stage and also in the subsequent removal of this histone for replacement by protamines at the spermatid stage.[³H]Amino acids were incorporated into histone fractions X₁ and F1 at approximately equal rates, and there was no evidence that one of these fractions was a precursor of the other.Chromatin of the seminiferous epithelial cells of rat testis has a firmly bound acetylase which catalyzes the in vitro acetylation of histones F3 and F2a1 by acetyl CoA.     

Histones and histone synthesis in sea urchin development

Histones are synthesized and become a part of the chromatin as early as the first cleavage in sea urchins. Reproducible changes in relative amounts of individual histone fractions synthesized are observed during development. A new and electrophoretically distinct very lysine rich fraction appears at hatching in Arbacia and in the early gastrula of Lytechinus. When RNA synthesis is blocked by actinomycin D, maternal mRNA alone can direct a quantitatively and qualitatively changing pattern of histone synthesis as cleavage proceeds. Inhibition of DNA synthesis by hydroxyurea reduces synthesis of histones; the arginine-rich histones are more severely affected than the lysine-rich ones.     

Histones from Saccharomyces cerevisiae

Total histones and histone fractions isolated from Saccharomyces cerevisiae chromatin were analysed by polyacrylamide gel electrophoresis. The presence of the four histone fractions H2a, H2b, H3 and H4 was demonstrated. In addition, yeast chromatin contained a protein similar to histone H1 from mammals in molecular weight, charge and association properties with Triton X-100. However, it had a much lower lysine to arginine ratio, equal to about 3, as compared with H1 histones from higher eukaryotes. The order of electrophoretic mobilities of yeast histone fractions in acidic urea-polyacrylamide gels was similar to that observed for histones from plant sources, i.e. H4>H3>H2a>H2b>H1. Previously undetected protein (protein X) was extracted from yeast chromatin with 5 % HClO₄. The properties of this protein are under investigation.     

Developmental Patterns in Histones and Histone Synthesis During Early Embryogenesis of the Sea Urchin Paracentrotus lividus

Abstract. Striking developmental changes in histone and histone synthesis in sea urchin embryos were observed in three histone classes, H1, H2A and H2B. In each case there is a shift in histone synthesis from the early cleavage stage types to other types of histones at the morula stage; Two new forms appear after the blastula stage. In addition, multiple changes in histone types were found during gameto-genesis in the male and female gonads where specific histones, different from the embryonic histones, were observed.     

Histone modifications accompanying the onset of developmental commitment

In the sea urchin, Strongylocentrotus purpuratus, three cell types comprise the 16-cell stage embryo: micromeres, macromeres, and mesomeres. We have analyzed these three cell types for nuclear proteins that were synthesized during the earliest stages of embryonic development. The most striking differences in composition of newly synthesized proteins were found between the micromeres, which are the most committed cell type, and the macromeres and mesomeres. First, the micromeres lacked triply modified forms of histone H3; the levels of doubly modified forms of H3 were also greatly reduced. In contrast, micromeres were enriched in a band which migrated at the position of unmodified, unacetylated, histone H3 protein. Second, the overall distribution of H2A histone variants differed among the three cell types. Compared with macromeres and mesomeres, micromeres had a higher ratio of alpha-stage to cleavage-stage (CS) histone H2A; the micromere nuclei were depleted by 50 and 35%, respectively, in embryonically synthesized histone CS-H2A. Third, micromeres displayed different profiles of H1 histones. (a) They contained a cleavage-stage H1 histone which migrated faster than that of macromeres and mesomeres. This protein displays the electrophoretic behavior expected for a protein with reduced levels of posttranslational covalent modification. (b) Micromeres also had reduced levels of an H1 histone (designated H1 alpha a) band found in the alpha-H1 region of macromeres and mesomeres. These changes in chromatin modification correlate with the degree of commitment of cells in the developing embryo; they may reflect differing activities of the chromatin modifying enzymes in the various cell types at the 16-cell stage. Thus, the newly synthesized chromatin proteins of the individual blastomere types already differ in the developing sea urchin by the 16-cell stage. We suggest that variations in histone subtypes and in the levels of activity of chromatin modifying enzymes, e.g., acetylases and phosphorylases, could be involved in commitment and differentiation of different cell types.     

Accumulation of the early histone messenger RNAs during the development of Strongylocentrotus purpuratus

The developmental profile of amounts of the mRNAs for the early histones was determined. Sea urchin embryos (Strongylocentrotus purpuratus) were labeled with ³H nucleoside, and the RNA was extracted and fractionated by electrophoresis on polyacrylamide gels. Radioactivity in the resolved mRNA bands was determined and converted to molar quantities by knowledge of the precursor pool-specific activity. Between the 16- and the 200-cell stages 7–10 × 10⁶ molecules of each core histone mRNA, and 2.5 × 10⁵ molecules of H1 mRNA accumulate. During the subsequent few hours of cleavage the accumulated mRNAs disappear with a half-life of 1.5–2 hr. It is argued that mRNA half-life may be regulated during cleavage.     

On the existence of polyadenylated histone mRNA in Xenopus laevis oocytes

In a variety of systems, histone mRNA has been shown to lack poly(A) (Adesnik and Darnell, 1972;Grunstein et al., 1973). We have found, however, that in Xenopus laevis oocytes, poly(A)-containing mRNA codes for histones, in a wheat germ cell-free system, based on the following criteria: first, co-migration with authentic X. laevis oocyte histones on polyacrylamide gels; second, no detectable incorporation of tryptophan; third, differential incorporation of lysine and methionine into histone fraction H2A; fourth, resistance of histone fraction H2A to cleavage with cyanogen bromide; and fifth, correspondence of tryptic peptide maps of partially purified cell-free products with authentic X. laevis oocyte histone. RNA which directs the synthesis of histones in the cell-free system is retained on oligo(dT)-cellulose, even after denaturation in 80% DMSO at 70°C, thereby demonstrating the covalent attachment of polyadenylic acid sequences to the mRNA. Poly(A)^? RNA (7S–14S fraction) was also found to code for histones using the same criteria. We discuss the significance of the finding that X. laevis oocytes contain two classes of histone mRNA as well as the potential developmental implications of this observation.     

Nucleosomes and subnucleosomes: heterogeneity and composition

Previous studies (Varshavsky, Bakayev and Georgiev, 1976a) have shown that chromatin subunits (mononucleosomes) and their oligomers in a mild staphylococcal nuclease digest of chromatin display a heterogeneous content of histone H1. We now report that a mild staphylococcal nuclease digest of either chromatin or nuclei from mouse Ehrlich tumor cells contains mononucleosomes of three discrete kinds. The smallest mononucleosome (MN₁) contains all histones except H1 and a DNA fragment 140 base pairs (bp) long. The intermediate mononucleosome (MN₂) contains all five histones and a DNA fragment 170 bp long. The third mononucleosome (MN₃) also contains all five histones, but its DNA fragment is longer and more heterogeneous in size (180–200 bp). Most of the MN₃ particles are rapidly converted by nuclease into mononucleosomes MN₁ and MN₂ There exists, however, a relatively nuclease-resistant subpopulation of the MN₃ mononucleosomes. These 200 bp MN₁ particles contain not only histones but also nonhistone proteins, and are significantly more resistant to nuclease than the bulk of MN₃ particles and the smaller mononucleosomes MN₁ and MN₂.There are eight major kinds of staphylococcal nuclease-produced soluble subnucleosomes (SN). The SN₁ is a set of naked double-stranded DNA fragments ～20 bp long. The SN₂ is a complex of a specific basic nonhistone protein (molecular weight ～16,000 daltons) and a DNA fragment ～27 bp long. The SN₃ contains histone H4, the above-mentioned specific nonhistone protein and a DNA fragment ～27 bp long. The SN₄ contains histones H2a, H2b, H4 and a DNA fragment ～45 bp long. The SN₅ contains histones H2a, H2b, H3 and a DNA fragment ～55 bp long. The SN₆ is a complex of histone H1 and a DNA fragment ～35 bp long. Subnucleosomes SN₇ and SN₈ each contain all the histones except H1, and DNA fragments ～100 and ～120 bp long, respectively.Nuclease digestion of isolated mono- or dinucleosomes does not produce some of the subnucleosomes. These and related findings indicate that the cleavage required to generate these subnucleosomes result from some aspect of chromatin structure which is lost upon digestion to mono- and dinucleosomes.     

Closely spaced nucleosome cores in reconstituted histone.DNA complexes and histone-H1-depleted chromatin

It has been demonstrated by digestion studies with micrococcal nuclease that reconstitution of complexes from DNA and a mixture of the four small calf thymus histones H2A, H2B, H3, and H4 leads to subunits closely spaced in a 137 +/- 7-nucleotide-pair register. Subunits isolated from the reconstituted complex contain nearly equimolar amounts of the four histones and sediment at 11.6S. On DNase I digestion both the reconstituted complex and the separated subunits gave rise to series of single-stranded DNA fragments with a 10-nucleotide periodicity. This indicates that the reconstitution leads to subunits very similar to nucleosome cores. Nucleosome cores closely spaced in a 140-nucleotide-pair register were also obtained upon removal of histone H1 from chromatin by dissociation with 0.63 M NaCl and subsequent ultracentrifugation. In reconstitution experiments with all five histones (including histone H1) our procedure did not lead to tandemly arranged nucleosomes containing about 200 nucleotide pairs of DNA. In the presence of EDTA, DNase II cleaved calf thymus nuclei and chromatin at about 200-nucleotide-pair intervals whereas in the presence of Mg2+ cleavage at intervals of approximately half this size was observed. The change in the nature of the cleavage pattern, however, was no longer found after removal of histone H1 from chromatin. This indicates that H1 influences the accessibility of DNase II cleavage sites in chromatin. This finding is discussed with respect to the influence of histone H1 on chromatin superstructure.     

Isolation of sea urchin embryo histone H2Aα1 and immunological identification of other stage-specific H2A proteins

H2A_α1, the principal H2A histone synthesized prior to the blastula stage of the sea urchin, was isolated free of other putative H2A subtypes and other histones. Its amino acid composition provides confirmation that H2A_α1 is the H2A protein encoded in the histone gene cluster carried by pCO2. An antibody prepared against this protein cross-reacts strongly with CS2A (a putative H2A synthesized only during the cleavage stage) as well as with H2A_β, H2A_γ, and H2A_δ (putative H2As synthesized principally after the blastula stage) but not with non-H2A core histones or other nuclear proteins. The data support the view that CS2A, H2A_α1, H2A_β, H2A_γ, and H2A_δ are all H2A proteins.     

Histone synthesis in early amphibian development: histone and DNA syntheses are not co-ordinated

The synthesis of basic proteins has been studied in the oocytes, eggs and embryos of the South African clawed frog, Xenopus laevis. A group of newly synthesized proteins has been identified as histones by the following criteria: solubility properties; incorporation of [³H]lysine and [³H]arginine in the correct proportions, but lack of incorporation of [³H]tryptophan; co-cleotrophoresis with marker histones in various types of polyacrylamide gels, including a type run in two dimensions; peptide analysis of the arginine-rich fraction, F2A1. The four main histone fractions other than F1 were found to be synthesized at all stages of development. F1 histone synthesis was first detected at the late blastula stage.Rates of histone synthesis were estimated for the different stages of development and it was concluded that histone synthesis was not co-ordinated with DNA synthesis either temporally or quantitatively. Histone synthesis was unusual in the following major respects: histones were synthesized in oocytes, and yet in these cells DNA replication had not occurred for several months; histones were synthesized in activated or fertilized eggs at a rate far in excess (about 500 times) of the immediate requirements. We suggest that in order to provide enough histones for the late blastula embryo a store of histone is accumulated during the early cleavage stages and possibly during oogenesis.     

Heterogeneity of the histone-containing chromatin in sea cucumber spermatozoa : Distribution of the basic protein Φ0 and absence of non-histone proteins

Nuclei of spermatozoa of the sea cucumber Holothuria tubulosa contain the five somatic-type histones plus a sperm-specific histone H1 and a unique basic protein ₀, which is related to H1 in amino acid composition. No proteins of the High Mobility Group (HMG) type have been detected. The structure of this chromatin has been probed by nuclease digestion. Its behaviour is anomalous, since two distinct fractions of chromatin are recovered from these spermatozoa, which differ either in the presence or absence of the sperm-specific proteins H1 and ₀. This heterogeneous distribution is not found in conventional materials, such as calf thymus or chicken erythrocytes. Proteins H1 and ₀ are not uniformly distributed and may be localized in special regions of chromatin. Fragments containing long stretches of nucleosomes lacking both proteins can be recovered. At the same time, the chromatin fractions which contain these two proteins are shown to be less soluble. When an extensive digestion of chromatin is carried out yielding only nucleosomes and small oligomers, the H1 and ₀ proteins redistribute themselves on chromatin, the two proteins acting in a cooperative fashion in this process. Cross-linking experiments carried out in whole cells indicate a proximity of ₀ and H1, whereas no crosslinks have been detected between ₀ and any of the four nucleosomal histones. The ₀ protein may thus play a role similar to histone H1 and be only loosely associated with nucleosomal histones, but contribute to the structuration of chromatin during spermiogenesis.