The H1 class of histone and diversity in chromosomal structure

Microheterogeneity in H1 histones and its consequences

The extent of microheterogeneity of H1 histones in individual higher organisms, without considering post-translational modifications, is such that five to eight molecular species can be recognized. The H1 variants differ among themselves in their ability to condense DNA and chromatin fragments, and they are non-uniformly distributed in chromatin. This review assembles data that support the notion that the differences in chromatin condensation (heterochromatization) observed through the microscope are maintained by the non-uniform distribution of H1 variants, and that this pattern of chromatin condensation may determine the dynamics of chromatin during replication and may represent the commitment aspect of differentiation. The differential response of the multiple H1 variants with regard to their synthesis and turnover is consistent with this notion.

Changes in the pattern of histone H1 phosphorylation during Drosophila hydei embryogenesis

Histone H1 phosphorylation was examined during embryonic development of Drosophila hydei. A changing pattern of H1 phosphorylation upon separation on an acid-urea polyacrylamide gel was observed in the course of Drosophila embryogenesis. It is considered to be related to the decrease of the mitotic activity of the cells as development proceeds.

Heterogeneity of high-mobility-group protein 2. Enrichment of a rapidly migrating form in testis

A determination of the absolute amounts of high-mobility-group proteins 1 and 2 (HMG1 and HMG2) in rat tissues demonstrated that amounts of HMG2 were low in non-proliferating tissues, somewhat higher in proliferating and lymphoid tissues, but were extremely elevated in the testis. This increase was due to a germ-cell-specific form of HMG2 with increased mobility relative to somatic HMG2 on acid/urea/polyacrylamide-gel electrophoresis. To determine if the findings in the rat were a general feature of spermatogenesis, testis (germinal), spleen (lymphoid), and liver (non-proliferating) tissues from various vertebrate species were examined for their relative amounts of HMG1 and HMG2, and for HMG2 heterogeneity. Bull, chimpanzee, cynomologus monkey, dog, gopher, guinea pig, hamster, mouse, opossum, rabbit, rat, rhesus monkey, squirrel and toad (Xenopus) tissues were analysed. Nearly all species showed relatively high contents of HMG2 in testis tissue, whereas HMG1 contents were similar in all species and tissues. Ten of thirteen species showed a rapidly migrating HMG2 subtype in testis tissue, separable by acid/urea/polyacrylamide-gel electrophoresis. Xenopus, which lacks HMG2 in somatic tissues, showed an HMG2-like protein in testis tissue. Although the rapidly migrating HMG2 subtype in species other than rat was not testis-specific, it was always enriched in the testis. This study indicates that increased amounts of HMG2 and the enrichment of a rapidly migrating HMG2 subtype are general features of spermatogenic cells.

Changes in the H-1 histone complement during myogenesis. I. Establishment by differential coupling of H-1 species synthesis to DNA replication

Proportions of the four major chicken H-1 histones (referred to as H-1's a-d) change during in vitro skeletal myogenesis. As myoblasts fuse and differentiate into myotubes, the relative amount of H-1c increases dramatically. The change occurs primarily because synthesis of the H-1 species is coupled to DNA synthesis to different extents. H-1c synthesis is least tightly coupled to DNA replication in precursor myoblasts and in differentiated myotubes. Thus H-1c synthesis predominates after dividing myoblasts fuse into postmitotic myotubes. This results in the replacement of pre-existing H-1 and therefore increases the relative amount of H-1c. Differences in the stability of the H-1's are also involved in changing H-1 proportions. The results show that changes in H-1 proportions during myogenesis are a consequence of withdrawal from the cell cycle. The data provides a general mechanistic explanation of how tissue-specific H-1 proportions are established.

Separation of histones by reverse-phase high-performance liquid chromatography: analysis of the binding of carcinogens to histones

Reverse-phase high-performance liquid chromatography (RP-HPLC) has been examined as an approach to the rapid analysis of carcinogen-modified histones. H1 and core histone fractions were prepared by differential acid extraction of 0.35 M NaCl-extracted rat liver nuclei previously exposed to [3H]-7r,8t-dihydroxy-9t, 10t-oxy-7,8,9, 10-tetrahydrobenzo(a)pyrene [( 3H]BPDE-I). Using a sodium perchlorate-phosphate (PCP)/acetonitrile solvent system, the H1 histone fraction was eluted from an Aquapore RP-300 column in five peaks (P1-P5). The core histone fraction was resolved into eight peaks (C1-C8) using a PCP/acetonitrile-methanol solvent system. The histones of each peak were identified by sodium dodecyl sulfate and Triton/acid/urea gel electrophoresis or amino acid analysis as follows: P1, H1 degrees; P2-P5, four different H1 variant fractions; C1, H4 + A24; C2, H2B; C3, H2A X 2 + to one H2A variant; C4, H2A.1; C5, H2A.1 + two H2A variants; C6, H3.2; C7, H3.3; C8, H3.1. The bulk of radioactivity was covalently bound to histone H2A, which had higher specific activities of BPDE-I than other histones. Significant amounts of radioactivity were observed in histones H3 and H1, but not in histones H2B and H4. These RP-HPLC systems have the advantages of an analysis time within 60 min, the identification of H1, H2A, and H3 variants, and the quantitative analysis of radioactive histones. These results indicate that these RP-HPLC systems are very useful to analyze the binding of carcinogens to histones.

Proteolytic processing of h1-like histones in chromatin: a physiologically and developmentally regulated event in Tetrahymena micronuclei

Micronuclei isolated from growing cells of Tetrahymena thermophila contain three H1-like polypeptides alpha, beta, and gamma. Micronuclei isolated from young conjugating cells (3-7 h) also contain a larger molecular weight polypeptide, X, which is being actively synthesized and deposited into these nuclei (Allis, C. D., and J. C. Wiggins, 1984, Dev. Biol., 101:282-294). Pulse-chase experiments (with growing and conjugating cells) suggested that X is a precursor to alpha and that alpha is further processed to gamma and a previously undescribed and relatively minor species, delta. These precursor-product relationships were supported by cross-reactivity with polyclonal antibodies raised against alpha and peptide mapping. While beta consistently became labeled under chase conditions (both in growing and mating cells), it was not clear whether it is part of the vivo processing event(s) which interrelates X, alpha, gamma, and delta. Beta was not recognized by alpha antibodies. Despite this uncertainty, these results suggest that proteolytic processing serves to generate significant changes in the complement of H1-like histones present in this nucleus.

Uncoupled synthesis of H1o-like histone H1s during late erythropoiesis in Xenopus laevis

This study investigated the synthesis of Xenopus histones during erythropoiesis. Although cessation of DNA replication in the mid-stages of erythroid maturation is accompanied by arrested synthesis of histone H1 and core histones, synthesis of H1o (an H1o-like histone) was found to continue into late stages of erythropoiesis, as has been reported for avian erythrocyte histone H5. This was accompanied by a threefold increase in the relative amount of Xenopus H1s, similar to the accumulation reported for H5 during avian erythropoiesis and for H1o in some differentiated mammalian cells. The structural and metabolic homologies of avian H5, mammalian H1o, and Xenopus H1s imply that these lysine-rich histones have closely related functions distinct from those of H1, and thus represent a subclass of lysine-rich histones.

Changes in nucleosomal core histone variants during chicken development and maturation

The nucleosomal core histones H2A, H2B, and H3 of the chicken can be resolved by polyacrylamide gel electrophoresis in the presence of nonionic detergents into two primary structure variants each, which occur in different relative amounts in various adult tissues. Quantitative analysis of the histone components throughout embryonic development and posthatching maturation of the chicken revealed that the proportions of the three pairs of variants change independently. Thus, the two H2A variants occur in similar proportions throughout embryonic development and in all adult tissues. In contrast, only one variant each of H2B and H3 is detectable at the earliest stages (primitive streak). The second variant of these histones becomes detectable and increases gradually during somite formation (2-12 days of incubation) to reach a plateau at a level of about 3 and 10% of total H2B and H3 histones, respectively. After hatching, the relative amounts of the minor H2B and H3 variants remain at embryonic levels in those tissues which maintain a high mitotic activity such as blood-forming tissues, but increase with different kinetics in tissues which essentially stop cell division in adults (e.g., liver, kidney, etc.). However, while H2B.2 remains a very minor component in all tissues, H3.3 increases at a relatively high rate for more than a year to become the predominant H3 variant in the liver and kidney of older chickens. The changes in chicken core histone variant proportions appear to be related to changes in growth rate rather than cell differentiation. The extensive change of H3 variant proportions in nondividing adult tissues is most likely due to replication-independent incorporation of H3.3 into nucleosomes.

Participation of core histone "tails" in the stabilization of the chromatin solenoid

We show here that the solenoid is maintained by the combination of linker histones and the nonglobular, highly basic "tails" of the core histones, which play only a minor part in the formation of the nucleosome core (Whitlock and Simpson, 1977. J. Biol. Chem. 252:6,516--6,520; Lilley and Tatchell, 1977. Nucleic Acids Res. 4:2,039--2,055; and Whitlock and Stein, 1978. J. Biol. Chem. 253:3,857--3,861). Polynucleosomes that contain core histones devoid of tails remain substantially unfolded under conditions otherwise favorable for the formation of solenoids. The tails can be replaced by extraneous basic polypeptides and in the presence of the linker histones the solenoid structure is then spontaneously recovered, as judged by a wide variety of structural criteria. The inference is that the core histone tail segments function by providing electrostatic shielding of the DNA charge and at the same time bridging adjacent nucleosomes in the solenoid. Our results carry the further implication that posttranscriptional modifications, such as acetylation of epsilon-amino groups, that reduce the positive charge of the core histone tails will tend to destabilize the higher-order structure and could thus render the DNA with which they are associated more readily available for transcription.

Regulation of the higher-order structure of chromatin by histones H1 and H5

Chicken erythrocyte chromatins containing a single species of linker histone, H1 or H5, have been prepared, using reassembly techniques developed previously. The reconstituted complexes possess the conformation of native chicken erythrocyte chromatin, as judged by chemical and structural criteria; saturation is reached when two molecules of linker histone are bound per nucleosome, as in native erythrocyte chromatin, which the resulting material resembles in its appearance in the electron microscope and quantitatively in its linear condensation factor relative to free DNA. The periodicity of micrococcal nuclease-sensitive sites in the linker regions associated with histone H1 or H5 is 10.4 base pairs, suggesting that the spatial organization of the linker region in the higher-order structure of chromatin is similar to that in isolated nucleosomes. The susceptible sites are cut at differing frequencies, as previously found for the nucleosome cores, leading to a characteristic distribution of intensities in the digests. The scission frequency of sites in the linker DNA depends additionally on the identity of the linker histone, suggesting that the higher-order structure is subject to secondary modulation by the associated histones.

Phosphorylation states of different histone 1 subtypes and their relationship to chromatin functions during the HeLa S-3 cell cycle

The histone 1 (H1) fraction of HeLa S-3 cells contains two principal subtypes, H1A (Mr approximately 21 000) and H1B (Mr approximately 22 000). In G1 cells, the H1 molecules are distributed among several phosphorylation states, most H1A molecules containing 0 or 1 phosphate groups and most H1B molecules containing 0, 1, 2, or 3 phosphate groups. Both subtypes undergo a general increase in phosphorylation levels of approximately 1 P/mol during the S phase and a further increase or 3--4 P/mol during mitosis. These two increases affect most of the H1 molecules and thus reflect phosphorylations occurring widely throughout the chromatin, presumably in association with replication and mitotic chromosome condensation. During all these periods, multiple phosphorylation levels of H1 molecules persist, as does the phosphorylation differential between H1A and H1B. Thus, there appear to be phosphorylation states that only some of the H1 molecules occupy, a fact that may be related to the conformational diversity in interphase and mitotic chromatin. The existence of differences between H1A and H1B phosphorylation states throughout the cell cycle, and within a single cell type, is in accord with the hypothesis that the H1 subtypes are functionally distinct, such that subtype-specific phosphorylations contribute to the control of chromatin organization.

Heat shock and deciliation induce phosphorylation of histone H1 in T. pyriformis

Both heat shock and decilliation of Tetrahymena pyriformis lead to an increase in the level of histone H1 phosphorylation. After heat shock, starved or growing cells reach the same maximum level of H1 phosphorylation, although the increase is more easily detected in starved cells because of their relatively low initial level of phosphorylation. In starved cells, stress-induced phosphorylation is rapid, involves a large percentage of the H1, occurs at multiple sites on the H1 molecule and is inhibited by cycloheximide. Stress-induced phosphorylation of H1 in Tetrahymena thus has many properties in common with cell-cycle-dependent H1 phosphorylation although it is not coupled to the cell cycle.

Expression of H1 histone genes in mouse-human somatic cell hybrids

The synthesis of H1 histones was studied in nine mouse-human somatic cell hybrid clones containing reduced numbers of human chromosomes. The entire human genome could be accounted for karyologically and by the use of functional assays for specific enzyme markers encoded by human chromosomes. Chromatographic resolution and peptide mapping of species-specific H1 histones failed to reveal human H1 histones to a level of about 1% of total in the nine clones. In addition to the species-specific extinction of human H1 histones, effects were seen on the quantity of mouse H1 histone subtypes produced in four of the nine clones. The remaining five clones produced H1 histones qualitatively and quantitatively identical with those of the mouse parent, which was common to all nine clones. The results suggest at least two levels of control for H1 histone gene expression.

Nucleosome repeat lengths do not change during in vitro differentiation of erythroleukemia cells

It has been previously demonstrated that nucleosome repeat lengths change during avian erythroid development and that repeat lengths correlate with histone H5 levels. Chromatin condensation also occurs during this process. In order to further investigate the relationship between histone H5 and/or chromatin condensation and nucleosome structure, repeat lengths were examined during in vitro differentiation of mouse erythroleukemia cells in which chromatin condensation occurs but in which histone H5 is absent. Our finding that repeat lengths do not change during this process supports the hypothesis that H5 plays a role in the mechanism which determines nucleosome repeat lengths.

Nuclear protein kinase activities during the cell cycle of HeLa S3 cells

To ascertain the activity and substrate specificity of nuclear protein kinases during various stages of the cell cycle of HeLa S3 cells, a nuclear phospho-protein-enriched sample was extracted from synchronised cells and assayed in vitro in the presence of homologous substrates. The nuclear protein kinases increased in activity during S and G2 phase to a level that was twice that of kinases from early S phase cells. The activity was reduced during mitosis but increased again in G1 phase. When the phosphoproteins were separated into five fractions by cellulose-phosphate chromatography each fraction, though not homogenous, exhibited differences in activity. Variations in the activity of the protein kinase fractions were observed during the cell cycle, similar to those observed for the unfractionated kinases. Sodium dodecyl sulfate polyacrylamide gel electrophoretic analysis of the proteins phosphorylated by each of the five kinase fractions demonstrated a substrate specificity. The fractions also exhibited some cell cycle stage-specific preference for substrates; kinases from G1 cells phosphorylated mainly high molecular weight polypeptides, whereas lower molecular weight species were phosphorylated by kinases from the S, G2 and mitotic stages of the cell cycle. Inhibition of DNA and histone synthesis by cytosine arabinoside had no effect on the activity or substrate specificity of S phase kinases. Some kinase fractions phosphorylated histones as well as non-histone chromosomal proteins and this phosphorylation was also cell cycle stage dependent. The presence of histones in the in vitro assay influenced the ability of some fractions to phosphorylate particular non-histone polypeptides; non-histone proteins also appeared to affect the in vitro phosphorylation of histones.