Synthesis and turnover of DNA-bound histone during maturation of avian red blood cells

Histone proteolysis: a proposal for categorization into 'clipping' and 'degradation'

We propose for the first time to divide histone proteolysis into "histone degradation" and the epigenetically connoted "histone clipping". Our initial observation is that these two different classes are very hard to distinguish both experimentally and biologically, because they can both be mediated by the same enzymes. Since the first report decades ago, proteolysis has been found in a broad spectrum of eukaryotic organisms. However, the authors often not clearly distinguish or determine whether degradation or clipping was studied. Given the importance of histone modifications in epigenetic regulation we further elaborate on the different ways in which histone proteolysis could play a role in epigenetics. Finally, unanticipated histone proteolysis has probably left a mark on many studies of histones in the past. In conclusion, we emphasize the significance of reviving the study of histone proteolysis both from a biological and an experimental perspective. Also watch the Video Abstract.

Phosphorylation of histone variant regions in chromatin: Unlocking the linker?

Histone variants illuminate the behavior of chromatin through their unique structures and patterns of postsynthetic modification. This review examines the literature on heteromorphous histone structures in chromatin, structures that are primary targets for histone kinases and phosphatases in vivo. Special attention is paid to certain well-studied experimental systems: mammalian culture cells, chicken erythrocytes, sea urchin sperm, wheat sprouts, Tetrahymena, and budding yeast. A common theme emerges from these studies. Specialized, highly basic structures in histone variants promote chromatin condensation in a variety of developmental situations. Before, and sometimes after condensed chromatin is formed, the chromatin is rendered soluble by phosphorylation of the heteromorphous regions, preventing their interaction with linker DNA. A simple structural model accounting for histone variation and phosphorylation is presented.Key words: phosphorylation, histone variants, chromatin, linker DNA.

Assembly into chromatin and subtype-specific transcriptional effects of exogenous linker histones directly introduced into a living Physarum cell

The apparent diversity of linker histone subtypes may be related to their specific roles in defining functional states of chromatin in vivo. We have developed a novel method to study constitutive peptides throughout the cell cycle and have demonstrated that an exogenous linker histone could be introduced into a living cell of the slime mold Physarum polycephalum. Here, we have used this method to assess the functional differences between three somatic linker histone subtypes in vivo, and to demonstrate the general applicability of this method. Exogenous linker histone proteins H1 degrees, H5 and H1 were directly absorbed into living cell segments of the naturally synchronous Physarum macroplasmodia at precise cell cycle stages. Fluorescence microscopy, native nucleoprotein gels and immunoblotting of nuclei and chromatin with subtype-specific antibodies revealed that exogenous linker histones were efficiently transported into nuclei and were integrated into chromatin. The immunoreactivity of a preparation of anti-H1 degrees antibodies that are blocked from binding to specific H1 degrees epitopes in native chromatin indicates that the exogenous linker histones were similarly associated into Physarum chromatin. Interestingly, linker histones were found to be less stably associated with Physarum chromatin during S-phase than during G(2)-phase. Furthermore, we show that exogenous linker histones incorporated in early G(2)-phase inhibited transcription and that the level of inhibition correlates with the apparent role of the linker histone subtype in regulating transcription in cells where it normally occurs.

Effects of H1 histone variant overexpression on chromatin structure

The importance of histone H1 heterogeneity and total H1 stoichiometry in chromatin has been enigmatic. Here we report a detailed characterization of the chromatin structure of cells overexpressing either H1(0) or H1c. Nucleosome spacing was found to change during cell cycle progression, and overexpression of either variant in exponentially growing cells results in a 15-base pair increase in nucleosome repeat length. H1 histones can also assemble on chromatin and influence nucleosome spacing in the absence of DNA replication. Overexpression of H1(0) and, to a lesser extent, H1c results in a decreased rate of digestion of chromatin by micrococcal nuclease. Using green fluorescent protein-tagged H1 variants, we show that micrococcal nuclease-resistant chromatin is specifically enriched in the H1(0) variant. Overexpression of H1(0) results in the appearance of a unique mononucleosome species of higher mobility on nucleoprotein gels. Domain switch mutagenesis revealed that either the N-terminal tail or the central globular domain of the H1(0) protein could independently give rise to this unique mononucleosome species. These results in part explain the differential effects of H1(0) and H1c in regulating chromatin structure and function.

Elements regulating differential activity of chicken histone H1 gene promoters

The chicken genome contains six closely related histone H1 genes, each of which encodes a different H1 protein. The four common regulatory elements previously identified in H1 histone promoters are very similar in sequence and location in all chicken H1 genes, which gives rise to the question of how the six H1 variants are expressed at significantly different levels. Transient transfections of reporter gene transcriptional fusions indicate that approximately 200 bp of each promoter is sufficient to generate the observed spectrum of H1 promoter activity. The differences in H1 promoter-driven expression are shown to be explained by the relative activity of the previously characterized G box region and that of a novel element found between CCAAT and TATA that we have termed differential upstream sequence (Dus). Gel shift analysis indicated that the primary nuclear binding protein to the G box is one or more avian homologues of the Sp1 transcription factor. The Dus region binds multiple nuclear proteins, one of which is the recently described IBR/IBF factor. The differential affinities of the G box and Dus sequences of the H1 promoters for their respective nuclear binding factors correlate well with their relative promoter activities.

Histone H1t: a tissue-specific model used to study transcriptional control and nuclear function during cellular differentiation

One of the most prominent and best studied family of genes is the histone gene family. In recent years, histone gene regulation during the cell cycle of somatic cells has been studied extensively. This paper is intended to highlight and emphasize recent data concerning the tissue-specific expression of histone H1t using spermatogenesis as a model system. In this article we describe a unique DNA element within the proximal promoter of the histone H1t gene. This element has been shown to bind exclusively to nuclear proteins from pachytene spermatocytes and early spermatids. Thus, there is a strong temporal correlation between the appearance of the testis-specific DNA-binding protein and the onset of transcription of the testis-specific histone H1t gene.

Crystal structure of globular domain of histone H5 and its implications for nucleosome binding

The structure of GH5, the globular domain of the linker histone H5, has been solved to 2.5 A resolution by multiwavelength anomalous diffraction on crystals of the selenomethionyl protein. The structure shows a striking similarity to the DNA-binding domain of the catabolite gene activator protein CAP, thereby providing a possible model for the binding of GH5 to DNA.

Immunochemical studies of histone H5 from Halobatrachus didactylus

1. Histone H5 from Halobatrachus didactylus was isolated by using perchloric acid (PCA) extraction of fish liver nuclei and trichloroacetic acid (TCA) precipitation. 2. A polyclonal antiserum was generated by immunizing rabbits with the antigen purified from SDS-PAGE. 3. By immunofluorescence the serum stains erythrocyte nuclei from H. didactylus but it does not react with mammalian cells. 4. By Western blotting, the anti-H5 antibody reacts with the isolated antigen at high titers. 5. Digestion of histone H5 with pepsin and cyanogen bromide suggests that the epitopes are located in the globular and C-terminal region of the H5 molecule excluding the N-terminal.

Regulation of histone H5 and H1 zero gene expression under the control of vaccinia virus-specific sequences in interferon-treated chick embryo fibroblasts

The duck histone H5 and human H1 zero were inserted into the thymidine kinase (TK) gene of vaccinia virus and the interferon sensitivity of their expression under the control of the viral TK and P7.5 promoters in chick embryo fibroblasts (CEF) was compared to the interferon sensitivity of vaccinia virus WR specific TK induction. Expression and transport of these histones to the nucleus in CEF infected with the appropriate vaccinia virus recombinants could be detected with antisera raised against chick histone H5. In CEF cultivated for 3 days, interferon treatment that completely inhibited TK synthesis had no or only a marginal inhibitory effect on the expression of the histone genes. Inhibition of the expression of the histones could be detected under conditions of increased interferon sensitivity in aged CEF. The magnitude of inhibition was, however, less pronounced than the inhibition of viral TK synthesis. These data indicate that flanking vaccinia virus DNA regions confer interferon sensitivity to the expression of these histone genes, but that they contain structural information that partially exempts their expression from the inhibitory activity of the interferon-induced regulatory system.

The expression of the histone H1 (0) gene in the human hepatoma cell line HepG2 is independent of the state of cell proliferation

The H1 histone subtype H1 (0) is a characteristic component of the chromatin of several mammalian tissues. Since H1 (0) is synthesized in nondividing cells upon terminal differentiation, it has been mostly considered either as a prerequisite for or as a consequence of an arrest of DNA replication during the process of differentiation. In several H1 (0)-expressing systems studied until now, inducers of differentiation or inhibitors of DNA synthesis cause an increase of the ratio between H1 (0) and the other H1 proteins. We have studied the steady-state levels of histone H1 (0) mRNA under varied growth conditions in the human hepatoma cell lines HepG2 and Hep3B, and we show in the HepG2 system that H1 (0) is not confined to resting cells, that the H1 (0) gene appears to be expressed throughout the cell cycle and that established inducers of de novo H1 (0) synthesis fail to cause a further increase of the high H1 (0) level. This constitutive expression of H1 (0) appears to reflect the chromatin structure of the liver cells, from which the HepG2 hepatoblastoma cells initially may have evolved. In contrast to the situation in nondividing adult liver cells, the H1 (0) gene is transcribed in HepG2 at a high level, and this expression is compatible with DNA replication.

Histone H5 in the control of DNA synthesis and cell proliferation

The linker histones (H1, H5, H1 degrees) are involved in the condensation of chromatin into the 30-nanometer fiber. This supranucleosome organization correlates with the resting state of chromatin, and it is therefore possible that the linker histones play an active role in the control of chromatin activity. The effect of H5 has been directly determined by expression of an inducible transfected H5 gene in rat sarcoma cells, which do not produce H5. Transfection resulted in the reversible inhibition of DNA replication and arrest of cells in G1, at which time H5 concentrations approached that of terminally differentiated avian erythrocytes. The arrest of proliferation was accompanied by specific changes in gene expression probably related to the cell cycle block. The selectivity of these effects suggest that H5 plays an active role in the control of DNA replication and cell proliferation.

Homology of histone H1 variants with adenine nucleotide-binding proteins

Significant homology was observed between the adenine nucleotide-binding domain in the catalytic subunit of bovine protein kinase A and the carboxy-terminal half of the globular domain of histone H1. A consensus sequence deducible from several previously characterized adenine nucleotide-binding sites is totally conserved in H1. In addition, several putative phosphate binding-sites were observed within the carboxyterminal tail and one in the cluster of basic amino acids in the aminoterminal tail. Both the putative adenine and phosphate-binding sites are well conserved through evolution in various species and in different H1 variants. The present data thus suggest that histone H1 variants may bind to adenine derivatives and imply that they may recognize a specific nucleotide sequence in DNA.

Histone acetylation in chicken erythrocytes. Rates of acetylation and evidence that histones in both active and potentially active chromatin are rapidly modified

Of the modifiable histone lysine sites, 2-4% participate in dynamic acetylation in chicken erythrocytes, suggesting the involvement of no more than 1-2% of the total genome. The rates and chromatin locality of this dynamic acetylation were studied in both chicken mature and immature red blood cells. In mature erythrocytes, two rates of acetylation of radiolabelled, monoacetylated H4 are observed, with half-lives of approximately 12 and approximately 300 min. In contrast, only one rate with a half-life (t1/2) of 12 min is observed in immature cells, and further experiments rule out the possibility of a slow rate of acetylation (with a t1/2 of approximately 300 min) for any form of H4 in this cell type. The simplest interpretation of these quantitative results, taken together with the behaviour of H3, H2B and H4 observed on the fluorograms used for rate analysis, is that a portion of the rapidly acetylated histone is converted to a more slowly acetylated form during erythrocyte maturation. The transcriptionally active adult beta-globin and H5 nucleohistone, which are presumably converted to potentially active chromatin during the maturation process, remain of the rapidly acetylated form in the mature cell.

Regulation of histone and beta A-globin gene expression during differentiation of chicken erythroid cells

The expression of the genes for several histones and beta A-globin was examined in the chicken erythroid cells lineage. During the transition from CFU-(E) to the mature erythrocyte, histone H5 gradually increased fourfold in nuclei with little concomitant displacement of the H1 histones. This resulted in a 70% net increase in linker histone (H1 plus H5) content. The differential accumulation of H5 reflected (i) an increase in the transcriptional activity of the H5 gene occurring at the erythroblast stage, (ii) an apparent longer half-life of H5 mRNA, and (iii) a higher stability of the protein. Although the transcriptional activity of the histone genes (except H5) decreased with cell age, it was not tightly coupled to the S phase. On the other hand, the mRNA levels for these histones were tightly regulated during the cell cycle. Use of protein and DNA synthesis inhibitors indicated that the content of H5 mRNA was regulated at the posttranscriptional level by a control mechanism(s) differing from those for the other histones. Although the transcription rates of the H5 and beta A-globin genes were comparable, differential accumulation of beta A-globin mRNA led to a 30- to 170-fold-higher copy number of the beta A-globin mRNA as the cell matured.

The H1 and core histone subtypes: differential gene expression and varied primary structures

The patterns of chromosomal proteins reflect in many cases the functional state of the respective cell type. The H1 histone group is particularly important in this respect, since these histones are involved in the higher order chromatin organization above the level of chains of nucleosomes. In mammals, the H1 histone family comprises at least five main subtypes (H1a-H1e), a testicular variant (H1t) and, thirdly, a subtype H1(0), which is found only in terminally differentiated cells. The H1(0) variant is structurally related to the avian red blood cell specific histone H5, which was the basis for our recent isolation of the human H1(0) gene. Changes of H1 histone patterns may be crucial events in modulating local chromatin arrangements, since the formation of higher order chromatin structures depends on a cooperative interaction of the H1 histones. Variations in their patterns can be studied in vivo during several developmental processes (such as spermatogenesis, erythropoiesis, maturation of several cell types) or in vitro in several tumor cell lines upon treatment with several inducers or upon inhibition of cell division. The differential regulation of the individual H1 subtypes is reflected in the gene and mRNA structures coding for the respective proteins. The cell cycle regulated histones are mostly encoded by non-polyadenylated mRNAs, whereas H5 as well as H1(0) mRNA shows a poly(A) tail at its 3' end. In conclusion, gene activity may not only be controlled at the level of RNA polymerases and their regulatory transcription factors. The varied patterns of chromosomal proteins at different stages during development and differentiation suggest that the local or overall organization of chromatin plays an additional role in these regulatory programs. Hence, the analysis of variations in patterns of chromosomal proteins is an integral part of the investigation of gene regulation mechanisms.

Regulation of histone and beta A-globin gene expression during differentiation of chicken erythroid cells

The expression of the genes for several histones and beta A-globin was examined in the chicken erythroid cells lineage. During the transition from CFU-(E) to the mature erythrocyte, histone H5 gradually increased fourfold in nuclei with little concomitant displacement of the H1 histones. This resulted in a 70% net increase in linker histone (H1 plus H5) content. The differential accumulation of H5 reflected (i) an increase in the transcriptional activity of the H5 gene occurring at the erythroblast stage, (ii) an apparent longer half-life of H5 mRNA, and (iii) a higher stability of the protein. Although the transcriptional activity of the histone genes (except H5) decreased with cell age, it was not tightly coupled to the S phase. On the other hand, the mRNA levels for these histones were tightly regulated during the cell cycle. Use of protein and DNA synthesis inhibitors indicated that the content of H5 mRNA was regulated at the posttranscriptional level by a control mechanism(s) differing from those for the other histones. Although the transcription rates of the H5 and beta A-globin genes were comparable, differential accumulation of beta A-globin mRNA led to a 30- to 170-fold-higher copy number of the beta A-globin mRNA as the cell matured.

A highly conserved sequence in H1 histone genes as an oligonucleotide hybridization probe: isolation and sequence of a duck H1 gene

A 3.5-kb HindIII fragment of a histone gene cluster was isolated from a recombinant phage out of a duck genomic library. This DNA contains a duck H1 gene and its flanking sequences. The hybridization probe, which was used to screen for the H1 gene, had been designed on the basis of a comparative analysis of available H1 gene and protein data. Most H1 histones contain repeated motifs in their C-terminal domain, and these form part of an octapeptide (ser pro lys lys ala lys lys pro) that is highly conserved in many H1 histone proteins. A comparison of the duck H1 described here with two different published chicken H1 histone sequences reveals conservative amino acid exchanges at 22 (of 217 and 218, respectively) positions. The homology is maintained at the flanking sequences, and includes the putative H1 histone gene-specific signal structures and the established 3' stem and loop structures and the CAAGA box. The duck H1 gene and its flanking sequence have been found in identical arrangements in two recombinant bacteriophages, but minor sequence variations and genomic Southern blotting after HindIII digestion suggest that we have either isolated alleles of this genome segment or that the gene described may occur twice per haploid duck genome.

Fine analysis of the active H5 gene chromatin of chicken erythroid cells at different stages of differentiation

We have analyzed the chromatin structure of a region that encompasses 14.4 X 10(3) base-pairs of the chicken histone H5 locus in adult erythroid cells at different stages of maturation. Seven of eight major lineage-specific DNase I-hypersensitive sites, some of which show complex substructure, were found in the flanking regions of the gene. The hypersensitivity of some of these sites is modulated during erythrocyte maturation in a way that parallels the transcriptional activity of the gene. DNase I, micrococcal nuclease, and S1 nuclease recognize the same regions, which differ from those cleaved by S1 on supercoiled plasmid DNA. This suggests that hypersensitivity of DNA in chromatin reflects a greater accessibility of the DNA rather than its altered conformation. The DNA sequence of some of the DNase I target sites contains repeated motifs, (T-C-C-C)2, (T-C-C)2, (T-G-G-G-G)2, which are found in the hypersensitive sites of other genes. Detailed analysis across sections of the H5 gene and flanking sequences revealed differences in the DNase I sensitivity of the different regions examined. Notably, the first one-third of the gene is more sensitive than the rest. The sequences downstream from the region where most RNA polymerases terminate transcription were found to be the most resistant.

Differential distribution of lysine and arginine residues in the closely related histones H1 and H5. Analysis of a human H1 gene

A human H1 histone gene and its flanking sequences were isolated from a human gene library using a fragment of the duck H5 histone gene as a hybridization probe. The primary structure of this human H1 histone (as deduced from the nucleotide sequence of the gene) reveals a close homology of H1 and H5 histones and fits the three-domain organization of all members of the H1 histone family. Within this protein organization, the C-terminal domain of H1 differs from the arginine-rich H5 in its distribution of the basic amino acids: the C-terminal domain of the human H1 shows only one arginine and most of the H5 specific arginine positions show lysine instead.

H1 variant synthesis in proliferating and quiescent human cells

The synthesis of histone H1 isoprotein species in human cells of several different types and in several different physiological states was studied. Up to five H1 and two H1 degrees isoprotein species could be resolved by two-dimensional electrophoresis. All five H1 isoprotein species were synthesized in exponentially growing cultures of IMR-90 human fibroblasts; in quiescent IMR-90 cells the synthesis of three H1 isoprotein species was greatly decreased while the synthesis of two others was much less affected. When DNA synthesis in exponentially growing cultures of IMR-90 was inhibited, the pattern of H1 isoprotein synthesis became similar to that found in quiescent cultures. Other human cells, isolated from blood, yielded similar results. These results suggest that the pattern of H1 synthesis is the same for cells in non-S phases of the cell cycle and in quiescent cells. Thus for histone H1 in human cells the relationship of the variant synthesis pattern to the growth state and DNA replication is similar to that of the core histone H3 but not that of H2A.

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.

Conserved dyad symmetry structures at the 3' end of H5 histone genes. Analysis of the duck H5 gene

The duck H5 histone gene and its flanking DNA have been isolated and sequenced. S1 nuclease mapping reveals that transcription starts 149 nucleotides upstream of the initiation codon and that the site of polyadenylation is located 200 nucleotides downstream of the termination codon. A comparison with the chicken H5 gene demonstrates that the 3' non-translated segment of the polyadenylated H5 mRNA carries two conserved dyad symmetry sequences. The first potential hairpin is located directly after the termination codon of the H5 gene and is highly conserved, whereas the second stem and loop structure maps shortly upstream of the polyadenylation site and shows a homology block at the central part of this inverted DNA repeat.

A survey of H1o-and H5-like protein structure and distribution in higher and lower eukaryotes

A survey of H1o and H5-like proteins has been conducted through a range of higher and lower eukaryotic species. All mammals examined possessed H1o proteins, although in variable amounts, and the protein's structure was well conserved, though not invariant. The testis-specific histone Hlt (from rat) did not have an H1o-like structure and it appears that H1o does not occur in spermatocytes in any form. The results also show that Xenopus laevis contains H1o-like proteins, but lower, non-vertebrate eukaryotes (a crustacean, two fungi and a plant tissue) do not possess H1o or H5 proteins. The evidence suggests that H1o and H5 proteins may be considered as belonging to one family, distinct from H1 types. This H1o/H5 family may well be 'replacement histone' variants of H1. The results do not support suggestions of roles such as repression of DNA synthesis or of transcription for H1o/H5 proteins.

Genomic organization of the genes coding for the six main histones of the chicken: complete sequence of the H5 gene

The organization of the genes coding for histones in the chicken has been examined, with special reference to that coding for the tissue-specific, developmentally regulated histone H5. Two recombinant phages containing sequences complementary to cloned H5 cDNA have been isolated from a genomic chicken library. The clones have been characterized by heteroduplex formation, restriction nuclease analysis, hybridization to cloned homologous histone gene probes, and DNA sequencing. Hybridization to genomic DNA has shown that there is only one copy of the H5 gene per haploid genome, whereas there are six to 11 copies of the genes for the other histones. Examination of 29 X 10(3) base-pairs of DNA sequences flanking the H5 gene has revealed the absence of any other histone genes which, although not tandemly reiterated, for the most part appear to reside in loosely organized clusters. The complete DNA sequence of the H5 gene and flanking regions, as well as the mapping of the 5'-end of its messenger RNA by primer extension with AMV reverse transcriptase, has shown that the gene has no introns and little homology to other histone genes, including those for H1.

Histone gene expression in early development of Xenopus laevis. Analysis of histone mRNA in oocytes and embryos by blot-hybridization and cell-free translation

This study comprises the hybridization analysis of electrophoretically separated histone mRNAs from oocytes and embryos of Xenopus laevis, and analysis of in vitro translation products of these mRNAs on polyacrylamide gels containing sodium dodecyl sulfate (SDS) or Triton X-100. In oocytes and embryos up to the tailbud stage, four types of mRNAs complementary to histone H2B DNA and two complementary to histone H4 DNA can be discriminated by their different electrophoretic mobilities on polyacrylamide gels. Electrophoretic heterogeneity was not detected for messengers for histones H2A and H3. Histone mRNA, purified by hybridization under stringent conditions with a cloned histone gene cluster, was used to direct histone protein synthesis in a wheat-germ cell free system. The proteins synthesized comigrate with purified marker histones when electrophoresed on SDS-gels or acid-urea gels containing Triton X-100. When hybrid-selected histone mRNAs from oocytes and embryos in different developmental stages are translated, the proteins made by the mRNA from one stage can not be discriminated from those made by the mRNA from another stage after electrophoresis on SDS-gels or acid urea Triton X-100 gels.

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 isoleucine-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 H1(0) 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.

Chicken histone H5 mRNA: the polyadenylated RNA lacks the conserved histone 3' terminator sequence

Using 3 overlapping cDNA clones we have determined the nucleotide sequence of chicken histone H5 mRNA. The mRNA does not contain the 23 base conserved sequence element that is present at the 3' end of cell-cycle regulated histone mRNAs. Although the RNA is polyadenylated it lacks the 3' AAUAAA sequence.

Localization of histone H5 in the subunit organization of chromatin using immunoelectron microscopy

In avian erythroid cells the erythrocyte-specific histone H5 is involved, like H1, in the packing of nucleosomes in the 25-nm chromatin fibers. In this study the distribution of histone H5 along the polynucleosomal chains was visualized by immunoelectron microscopy. Trinucleosomes from chicken erythrocytes and liver were used in order to test the specificity of the reaction with purified rabbit anti-H5 antibodies at various ionic strengths (5-80 mM). Long-chain chromatin was then reacted with anti-H5 antibodies and with sorted monomeric ferritin conjugate under chosen conditions. The antigenic determinants of histone H5 in the 25-nm fiber of long-chain chromatin (at 80 mM NaCl) are as accessible to the specific antibodies as in trinucleosomes. When the immunocomplexes were examined by electron microscopy in a low-ionic-strength buffer, permitting maximum extension of the chromatin structure on the grid, clusters of compacted nucleosomes were seen, separated by short regions of relaxed nucleosomes. Single nucleosomes enlarged by the antibodies are sometimes visible in the extended domains. We conclude that histone H5 is located primarily on series of adjacent nucleosomes but it can also be found on single nucleosomes located in the H1-enriched extended domains.

Nucleosomes containing histones H1 or H5 are closely interspersed in chromatin

The distribution of histones H1 and H5 along chromatin fibers has been examined in the nucleated hen erythrocyte. Nucleosome oligomers, produced by micrococcal nuclease digestion of nuclei, were sequentially reacted with affinity-chromatography purified rabbit anti-H5 and sheep anti-rabbit antibodies. Quantitation of the relative amounts of H1 and H5 in the precipitated and supernatant fractions as a function of the oligomer number was consistent with a close interspersion of both types of histones, probably a random one. This conclusion was supported by the immunoprecipitation of longer chromatin fibers. This pattern of distribution appears to apply both to bulk chromatin and to chromatin inactivated during the maturation of the erythrocyte.

Construction of chimeric plasmids containing histone H5 cDNA from hen erythrocyte. DNA sequence of a fragment derived from the 5' region of H5 mRNA

We report that construction and characterization of chicken erythrocyte histone H5 cDNA recombinant plasmids. cDNA was synthesized from poly(A)+ polysomal RNA enriched in H5 mRNA and inserted into the PstI site of pBR322. Several clones containing H5 cDNA sequences were obtained and one of them (p541), expressing H5 antigenic determinants, was sequenced. The DNA insert of p541 contains 118 nucleotides from the 5' non-translated region of H5 mRNA and sequences coding for up to residue 46 of the N-terminus of the arginine (position 15) H5 variant. There is a strikingly high number of repeated sequences both in the leader and coding region; among these, the octanucleotide 5' GCG GCG GC 3' is found five times along the sequence. Although the H5 mRNA 5' leader is GC-rich (66%), there is an AT-rich region, about 16 nucleotides long, which shares strong homology with the leaders of sea urchin histone H1 mRNAs.

Chicken histone H5: selection of a cDNA recombinant using an extended synthetic primer

We describe the use of a synthetic primer to select a cDNA recombinant clone containing H5 coding sequences. The strategy used was as follows: 1. Prepare oligo(dT) cellulose-bound mRNA from chicken reticulocytes and select 11S-18S material from sucrose gradients. 2. Use this RNA fraction both to prepare a cDNA library and as a template for H5-specific cDNA synthesis using a synthetic primer. 3. Screen out most globin cDNA recombinants with oligo(dT)-primed globin cDNA. 4. Search for H5 recombinants using H5 specific cDNA and verify the identity by DNA sequencing. Our screening suggests an H5 mRNA abundance of about two parts per thousand in chicken reticulocyte poly(A)-containing RNA. The isolation of an H5 cDNA recombinant clone is an initial step in the study of H5 genes and their relationship to H1 and core histone genes.

Nucleosome repeat lengths in the definitive erythroid series of the adult chicken

Morris [1] has suggested that the difference in nucleosome repeat length between chicken liver (200 base pairs) and mature chicken erythrocytes (212 base pairs) may be due to the presence of histone H5 which is found in chicken erythroid cells but not in other tissues. Levels of H5 increase during erythroid maturation in the adult chicken. To determine what influence H5 might have on repeat length, erythroid populations at various stages of maturation were isolated, and repeat lengths and levels of H5 were determined. Bone marrow cells from anemic chickens were cultured in vitro to permit non-cycling erythroblasts to mature and thus increase in density. Less dense cycling basophilic erythroblasts were then isolated by buoyant density centrifugation. This erythroblasts were then isolated by buoyant density centrifugation. This population has a repeat length of 205 base pairs and an H5 content roughly two-thirds that of mature erythrocytes, which have a repeat length of 212 base pairs. A population intermediate in maturation, consisting of cells of the anemic pheripheral blood, has a repeat length of 218 base pairs, and the predominant cell type in this population has an H5 content greater than that of mature erythrocytes. Therefore, changes in histone H5 content are reflected by the nucleosome repeat length during erythroid maturation.

Histone H5 messenger RNA is polyadenylated

In most known systems, histone mRNA lacks the poly(A) sequence at the 3' end of the molecule typical of most mRNAs. Furthermore, the synthesis of histones, unlike that of most proteins, is tightly coupled to DNA synthesis. Nevertheless, histone synthesis occurs in amphibian oocytes in the absence of DNA synthesis. Moreover, it has recently been found that in amphibian oocytes most of the histone mRNA is polyadenylated, and the polyadenylate is probably removed during maturation of the oocyte. Histone H5, an H1-like tissue-specific histone occurring only in nucleated erythrocytes, is also atypical in that it is synthesised in the absence of DNA synthesis during maturation of the red blood cells. We report here that H5 mRNA is polyadenylated.

Histones of Xenopus laevis erythrocytes. Purification and characterization of the lysine-rich fractions

Lysine-rich histones have been isolated from the terminally differentiated erythrocytes of Xenopus laevis. Three major proteins have been separated by ion-exchange chromatography. These proteins have been characterized by electrophoresis, amino acid analysis and immunochemical techniques. It is concluded that two 'typical' lysine-rich subfractions are present in Xenopus erythrocytes and, in addition, a serine-rich histone, that shares no common antigenic determinants with the other lysine-rich histones.

Synthesis of carbonic anhydrase in rabbit and chicken reticulocyte lysates

The synthesis of carbonic anhydrase, the second most abundant soluble protein of red blood cells, is shown to occur in rabbit and chicken reticulocyte lysates. This translation product was identified by chloroform/ethanol extraction, polyacrylamide gel electrophoresis in sodium dodecylsulphate and peptide mapping. In rabbit retic-locyte lysates, predominantly one of the two red cell isozymes, carbonic anhydrase I, is synthesised. The proportion of carbonic anhydrase synthesis (0.2-0.8% of total protein synthesis) in vitro is comparable to that (0.2-1.0%) in vivo for both rabbit and chicken reticulocytes.