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

Linker histone subtypes and their allelic variants

Members of histone H1 family bind to nucleosomal and linker DNA to assist in stabilization of higher-order chromatin structures. Moreover, histone H1 is involved in regulation of a variety of cellular processes by interactions with cytosolic and nuclear proteins. Histone H1, composed of a series of subtypes encoded by distinct genes, is usually differentially expressed in specialized cells and frequently non-randomly distributed in different chromatin regions. Moreover, a role of specific histone H1 subtype might be also modulated by post-translational modifications and/or presence of polymorphic isoforms. While the significance of covalently modified histone H1 subtypes has been partially recognized, much less is known about the importance of histone H1 polymorphic variants identified in various plant and animal species, and human cells as well. Recent progress in elucidating amino acid composition-dependent functioning and interactions of the histone H1 with a variety of molecular partners indicates a potential role of histone H1 polymorphic variation in adopting specific protein conformations essential for chromatin function. The histone H1 allelic variants might affect chromatin in order to modulate gene expression underlying some physiological traits and, therefore could modify the course of diverse histone H1-dependent biological processes. This review focuses on the histone H1 allelic variability, and biochemical and genetic aspects of linker histone allelic isoforms to emphasize their likely biological relevance.

Poly A-containing histone H4 mRNA variant (H4-v. 1): isolation and sequence determination from bovine adrenal medulla

A histone H4 cDNA variant (H4-v.1) was cloned from a bovine adrenal medullary phage library using PCR as a method of detection. The isolated clones contained a short 5' untranslated region (UTR) followed by the histone H4 coding region and a long atypical 3'UTR. The 3'UTR comprised the palindromic and purine-rich sequences typical of cell-cycle dependent histone mRNAs, and a 1.1 kb extension downstream of the palindromic sequence ending with a poly(A) track typical of cell-cycle independent histone mRNAs. Northern blot and RT-PCR analyses indicate that the transcript is fully expressed in bovine adrenal medulla. Thus, bovine histone H4-v.1 mRNA represents the first example of a histone H4 transcript that contains both 3'UTR characteristics of cell-cycle dependent and cell-cycle independent histone mRNAs.

Association of a human H1 histone gene with an H2A pseudogene and genes encoding H2B.1 and H3.1 histones

A cluster of human histone genes was found on three overlapping clones isolated from cosmid and bacteriophage libraries. These three overlapping segments of the human genome comprise genes coding for H3.1, an H2A pseudogene, and an H2B.1 gene downstream of the previously characterized H1.2 gene. The cosmid clone covers 30 kb upstream of the H1.2 gene and overlaps with two phage clones covering the core histone genes and the pseudogene. The same arrangement of an H3 gene, an H2A pseudogene and an H2B gene downstream of an H1 gene has been described within a mouse histone gene cluster [Yang et al.:J Biol Chem 262:17118-17125, 1987; Gruber et al.:Gene 95:303-304, 1990].

Structure and expression of the human gene encoding testicular H1 histone (H1t)

The gene coding for the human H1t histone, a testis-specific H1 subtype, was isolated from a genomic library using a human somatic H1 gene as a hybridization probe. The corresponding mRNA is not polyadenylated and encodes a 206-amino-acid protein. Sequence analysis and S1 nuclease mapping of the human H1t gene reveals that the 5' flanking region contains several consensus promoter elements, as described for somatic, i.e., S-phase-dependent H1 subtype genes. The 3' region includes the stem-and-loop structure necessary for mRNA processing of most histone mRNAs. Northern blot analysis with RNAs from different human tissues and cell lines revealed that only testicular RNA hybridized with this gene probe.

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.

A genomic clone encoding a novel proliferation-dependent histone H2A.1 mRNA enriched in the poly(A)+ fraction

Replication-dependent histone mRNAs are prime examples of nonpolyadenylated mRNAs. We isolated and characterized cDNAs and a genomic clone for a replication-dependent histone H2A.1 mRNA which segregated into the poly(A)+ fraction during mRNA isolation through an oligo(dT)-cellulose column. However, the results of sequencing of the genomic clone suggested that the mRNA did not contain a poly(A) tail. Instead, the genomic sequence revealed a nonterminal oligo(A) tract directly upstream from the typical 3'-terminal hairpin loop of replication-dependent histone mRNAs. The nonterminal oligo(A) tract consisted of 14 adenylate residues interrupted by one guanylate residue (A4GA10). We concluded that this short oligo(A) stretch mediated binding of the mRNA to oligo(dT) even after stringent washes with 0.1 M NaCl, indicating that rather short oligo(A) sequences can ensure binding to oligo(dT)-cellulose. The cDNA and genomic clones contained an AAATAAG sequence at the end of the coding region. It has been suggested that this sequence contains a polyadenylation signal in some yeast and mouse transcripts, but it does not function as a polyadenylation signal in the histone transcript described in this paper.

A genomic clone encoding a novel proliferation-dependent histone H2A.1 mRNA enriched in the poly(A)+ fraction

Replication-dependent histone mRNAs are prime examples of nonpolyadenylated mRNAs. We isolated and characterized cDNAs and a genomic clone for a replication-dependent histone H2A.1 mRNA which segregated into the poly(A)+ fraction during mRNA isolation through an oligo(dT)-cellulose column. However, the results of sequencing of the genomic clone suggested that the mRNA did not contain a poly(A) tail. Instead, the genomic sequence revealed a nonterminal oligo(A) tract directly upstream from the typical 3'-terminal hairpin loop of replication-dependent histone mRNAs. The nonterminal oligo(A) tract consisted of 14 adenylate residues interrupted by one guanylate residue (A4GA10). We concluded that this short oligo(A) stretch mediated binding of the mRNA to oligo(dT) even after stringent washes with 0.1 M NaCl, indicating that rather short oligo(A) sequences can ensure binding to oligo(dT)-cellulose. The cDNA and genomic clones contained an AAATAAG sequence at the end of the coding region. It has been suggested that this sequence contains a polyadenylation signal in some yeast and mouse transcripts, but it does not function as a polyadenylation signal in the histone transcript described in this paper.

Expression of replication-dependent histone genes in avian spermatids involves an alternate pathway of mRNA 3'-end formation

In somatic cells the expression of replication-dependent histone genes is coupled to the S phase of the cell cycle. However, we have found a number of novel H2a, H2b, and H3 poly(A)+ RNA species in avian haploid round spermatids. The spermatid-specific H2a and H2b 0.8-kilobase RNAs are transcribed from a subset of the replication-dependent H2a and H2b gene families. Two cDNAs derived from the spermatid-specific H2b transcripts were isolated and sequenced. The structures of these cDNAs reveal that the spermatid-specific RNAs are identical to the 0.5-kilobase poly(A)- H2b mRNAs expressed in proliferating somatic cells, except for the addition of poly(A) at the 3' ends. The site of poly(A) addition in the spermatid-specific RNAs is located 26 to 28 nucleotides 3' of the poly(A)- H2b mRNA terminus. Thus, the hairpin structures and purine-rich elements required for the U7 small nuclear ribonucleoprotein-mediated cleavage reaction that generates the 3' ends of poly(A)- H2b mRNAs are not utilized in spermatids and are retained in the poly(A)+ H2b RNAs.

Expression of a novel histone 2B during mouse spermiogenesis

During mammalian spermiogenesis transitional proteins and protamines replace histones on the DNA as the chromatin condenses. While previous studies suggested that histone genes are inactive postmeiotically, we have shown both by steady-state RNA analysis and nuclear run-off transcription assays that histone 2b (H2b) transcription occurs in mouse round spermatids. In addition, a novel H2b cDNA clone has been isolated from an adult mouse testes cDNA library. The sequence of this cDNA clone predicts a protein that is extremely similar to other mouse H2b proteins, except at the carboxyl-terminus where the testes H2b contains an additional 12 amino acids, seven of which are hydrophobic. In contrast to the replication-dependent histone mRNAs, the 3' untranslated region of this cDNA contains the poly(A) addition sequence (AAUAAA) upstream of a poly(A) tract. Furthermore, the conserved hairpin structure immediately upstream of replication-dependent histone mRNA termini is not present. Northern blot analysis of RNA from embryonic, ovarian, spermatogenic, and a variety of somatic tissues reveals that this novel H2b transcript is spermatid specific. The H2b mRNA is in polyribosomes isolated from spermatogenic cells, strongly suggesting that it is translated during spermiogenesis.

The phosphorylation site of Ca(2+)-dependent protein kinase from alfalfa

A 50 kDa, calcium-dependent protein kinase (CDPK) was purified about 1000-fold from cultured cells of alfalfa (Medicago varia) on the basis of its histone H1 phosphorylation activity. The major polypeptide from bovine histone H1 phosphorylated by either animal protein kinase C (PK-C) or by the alfalfa CDPK gave an identical phosphopeptide pattern. The phosphoamino acid determination showed phosphorylation of serine residues in histone H1 by the plant enzyme. Histone-related oligopeptides known to be substrates for animal histone kinases also served as substrates for the alfalfa kinase. Both of the studied peptides (GKKRKRSRKA; AAASFKAKK) inhibited phosphorylation of H1 histones by bovine and alfalfa kinases. The results of competition studies with the nonapeptide (AAASFKAKK), which is a PK-C specific substrate, suggest common features in target recognition between the plant Ca(2+)-dependent kinase and animal protein kinase C. We also propose that synthetic peptides like AAASFKAKK can be used as a tool to study substrates of plant kinases in crude cell extracts.

Expression of replication-dependent histone genes in avian spermatids involves an alternate pathway of mRNA 3'-end formation

In somatic cells the expression of replication-dependent histone genes is coupled to the S phase of the cell cycle. However, we have found a number of novel H2a, H2b, and H3 poly(A)+ RNA species in avian haploid round spermatids. The spermatid-specific H2a and H2b 0.8-kilobase RNAs are transcribed from a subset of the replication-dependent H2a and H2b gene families. Two cDNAs derived from the spermatid-specific H2b transcripts were isolated and sequenced. The structures of these cDNAs reveal that the spermatid-specific RNAs are identical to the 0.5-kilobase poly(A)- H2b mRNAs expressed in proliferating somatic cells, except for the addition of poly(A) at the 3' ends. The site of poly(A) addition in the spermatid-specific RNAs is located 26 to 28 nucleotides 3' of the poly(A)- H2b mRNA terminus. Thus, the hairpin structures and purine-rich elements required for the U7 small nuclear ribonucleoprotein-mediated cleavage reaction that generates the 3' ends of poly(A)- H2b mRNAs are not utilized in spermatids and are retained in the poly(A)+ H2b RNAs.

Transcription from the intron-containing chicken histone H2A.F gene is not S-phase regulated

The nucleotide sequence of an 8.2 kb BamHI fragment containing the entire chicken histone H2AF gene has been determined. Unlike the majority of histone genes, the coding region is interrupted by four intervening sequences. While sequencing the 8.2 kb BamHI fragment it was found that the promoter and first exon of an unidentified non-histone gene lies immediately downstream of the H2AF gene. Studies of H2AF gene transcription show that, unlike the major core and H1 histone genes, it is not coupled to DNA synthesis.

The stem-loop structure at the 3' end of histone mRNA is necessary and sufficient for regulation of histone mRNA stability

Chimeric genes were made by fusing mouse histone genes with a human alpha-globin gene. The genes were introduced into mouse L cells and the stability of the chimeric mRNAs was measured when DNA synthesis was inhibited. An mRNA containing all the globin coding sequences and the last 30 nucleotides of the histone mRNA was degraded at the same rate as histone mRNA.

The histone H3 and H4 mRNAs are polyadenylated in maize

Northern blot analysis revealed that the histone H3 and H4 mRNAs are of unusual large size in germinating maize embryos. S1-mapping experiments show that the 3'-untranslated regions of the mRNAs transcribed from 3 H3 and 2 H4 maize genes previously described are much longer than in the non-polyadenylated histone mRNAs which represent a major class in animals. Moreover, oligo d(T) cellulose fractionation of RNAs isolated at different developmental stages indicates that more than 99% of the maize H3 and H4 mRNAs are polyadenylated. A putative polyadenylation signal is present in all five genes 17 to 27 nucleotides before the 3'-ends of the mRNAs.

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.

The stem-loop structure at the 3' end of histone mRNA is necessary and sufficient for regulation of histone mRNA stability

Chimeric genes were made by fusing mouse histone genes with a human alpha-globin gene. The genes were introduced into mouse L cells and the stability of the chimeric mRNAs was measured when DNA synthesis was inhibited. An mRNA containing all the globin coding sequences and the last 30 nucleotides of the histone mRNA was degraded at the same rate as histone mRNA.

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.

Molecular cloning of a pea H1 histone cDNA

A pea (Pisum sativum, var. Little Marvel) H1 histone cDNA has been isolated from a lambda gt11 expression vector library. This cDNA has been sequenced and shown to represent the entire protein-coding region of the mRNA. The deduced protein sequence is 265 amino acids long (28018 Da) and contains 70 lysines and 3 arginines. The structure of the encoded protein is comparable to animal lysine-rich histones. The central region, which has an amino acid composition similar to that found in the globular domains of animal lysine-rich histones, is flanked by an amino-terminal region rich in lysine, glutamic acid and proline and by a carboxyl-terminal region rich in lysine, alanine, valine and proline. Despite the structural similarities, the protein has little sequence homology with animal lysine-rich histones. This H1 protein is unusual because 12 of the first 40 amino acids are glutamic acid.

Expression of mouse histone genes: transcription into 3' intergenic DNA and cryptic processing sites downstream from the 3' end of the H3 gene

Introduction of the mouse histone H3.1 gene into tk- mouse L cells by cotransfection with the herpesvirus thymidine kinase gene resulted in the production of two mRNAs from the transfected gene, one with a normal 3' end and the other one with a longer 3'-untranslated region, ending at site X, which was poly(A)+. In contrast, the endogenous histone H3.1 gene only produced a single mRNA. The cryptic poly(A)+ site was only used when the histone H3.1 gene was transfected. To localize possible downstream cryptic processing sites, the hairpin loop at the end of the histone gene was deleted and the resulting deletions were introduced into L cells. Two major mRNAs were produced from this gene, one ending at site X and the major one ending at site Y, which was located 150 nucleotides before site X. Transcription extended downstream of site X efficiently in the endogenous gene, as judged by the extent of transcription of downstream sequences in isolated nuclei. Transcription extended downstream of site X in the transfected gene because the placement of a normal histone 3' end downstream of site X resulted in transcripts that ended at site X and longer transcripts that ended with the new histone 3' end. These results indicate that transcription may normally proceed a substantial distance past the hairpin loop (greater than 500 bases). The formation of the different 3' ends in these transfected genes was due to competition between different processing mechanisms.

Expression of mouse histone genes: transcription into 3' intergenic DNA and cryptic processing sites downstream from the 3' end of the H3 gene

Introduction of the mouse histone H3.1 gene into tk- mouse L cells by cotransfection with the herpesvirus thymidine kinase gene resulted in the production of two mRNAs from the transfected gene, one with a normal 3' end and the other one with a longer 3'-untranslated region, ending at site X, which was poly(A)+. In contrast, the endogenous histone H3.1 gene only produced a single mRNA. The cryptic poly(A)+ site was only used when the histone H3.1 gene was transfected. To localize possible downstream cryptic processing sites, the hairpin loop at the end of the histone gene was deleted and the resulting deletions were introduced into L cells. Two major mRNAs were produced from this gene, one ending at site X and the major one ending at site Y, which was located 150 nucleotides before site X. Transcription extended downstream of site X efficiently in the endogenous gene, as judged by the extent of transcription of downstream sequences in isolated nuclei. Transcription extended downstream of site X in the transfected gene because the placement of a normal histone 3' end downstream of site X resulted in transcripts that ended at site X and longer transcripts that ended with the new histone 3' end. These results indicate that transcription may normally proceed a substantial distance past the hairpin loop (greater than 500 bases). The formation of the different 3' ends in these transfected genes was due to competition between different processing mechanisms.

Transcription of the histone H5 gene is not S-phase regulated

Levels of the tissue-specific linker histone H5 are elevated in mature erythroid cells as compared with levels in dividing cells of the same lineage. We examined levels of H5 mRNA in relation to the cell cycle in early erythroid cells transformed by avian erythroblastosis virus to determine whether the gene for this unusual histone is S-phase regulated. Northern blotting analyses revealed that during the cell cycle steady-state levels of H5 mRNA remained relatively constant in contrast to levels of the major core and H1 mRNAs which increased approximately 15-fold during S phase. In vitro pulse-labeling experiments involving nuclei isolated from synchronized cells at various stages of the cell cycle revealed that transcription of the H5 gene was not initiated at any particular stage of the cell cycle but was constitutive. In contrast, transcription of the H2A gene(s) initiated in early S phase, was present throughout the DNA replicative phase, and was essentially absent in G1 and G2 phases.

Butyrate induced accumulation of a 2.3 kb polyadenylated H1(0) histone mRNA in HeLa cells

Sodium butyrate was used to induce the accumulation of human H1(0) mRNA in HeLa cells. The length of this mRNA (2,300 nucleotides) was determined by Northern blot hybridization and S1 nuclease analysis using a human H1(0) gene probe. The mRNA shows long 5' and 3' non coding segments and it is polyadenylated. The signal for this step of mRNA maturation (cleavage and polyadenylation) appears to be the hexanucleotide AAUAAA in analogy to most (other than histone) mRNA species. Thus, the mode of maturation of H1(0) mRNA differs, on one hand, from that of the cell cycle dependent mRNA species, where it is based on a specific stem-and-loop structure. On the other hand, the 3' end of H1(0) mRNA varies from H5 mRNA, which is characterized by two unique dyad symmetry structures at its 3' end.

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.

Transcription of the histone H5 gene is not S-phase regulated

Levels of the tissue-specific linker histone H5 are elevated in mature erythroid cells as compared with levels in dividing cells of the same lineage. We examined levels of H5 mRNA in relation to the cell cycle in early erythroid cells transformed by avian erythroblastosis virus to determine whether the gene for this unusual histone is S-phase regulated. Northern blotting analyses revealed that during the cell cycle steady-state levels of H5 mRNA remained relatively constant in contrast to levels of the major core and H1 mRNAs which increased approximately 15-fold during S phase. In vitro pulse-labeling experiments involving nuclei isolated from synchronized cells at various stages of the cell cycle revealed that transcription of the H5 gene was not initiated at any particular stage of the cell cycle but was constitutive. In contrast, transcription of the H2A gene(s) initiated in early S phase, was present throughout the DNA replicative phase, and was essentially absent in G1 and G2 phases.

Individual Xenopus histone genes are replication-independent in oocytes and replication-dependent in Xenopus or mouse somatic cells

We have assessed the response of many histone H3 mRNAs and an H1C mRNA in Xenopus tissue culture cells after treatment with the DNA synthesis inhibitor hydroxyurea. The amount of the histone mRNAs falls rapidly in response to the inhibitor. This response is prevented by cycloheximide. Cloned Xenopus histone genes were transfected into mouse cells and a cell line was obtained in which the Xenopus genes were actively expressed giving rise to mRNA with correct 5'-termini. The Xenopus genes were correctly regulated at the level of mRNA amounts in the mouse cell line. Nuclear microinjection experiments with Xenopus oocytes and S1 nuclease analysis of normal ovary RNA showed that the H1C gene, and probably also two H3 genes, which are replication-dependent in somatic cells are expressed in oocytes and are therefore replication-independent in this cell type. The same promoters are used in both replication-dependent and independent expression.

A unique subspecies of histone H4 mRNA from rat myoblasts contains poly(A)

Fractionation of rat L6 myoblast histone H4 mRNA into its three component subspecies revealed that one of the major subspecies (H4-1) contained poly(A). The unique poly(A)+ H4 mRNA makes up about 8% of the total polysomal H4 mRNA population detected. Unlike the poly(A)- histone mRNAs, whose levels are reduced by greater than 95% when myoblasts differentiate into myotubes, the poly(A)+ subspecies is reduced by only 70%. The poly(A)+ H4 mRNA from myotubes incubated with actinomycin D decays with a half-life of 37-42 min, which is similar to that obtained for the poly(A)- H4 mRNAs in myoblasts. Both the poly(A)+ and poly(A)- subspecies decay at an increased rate after inhibition of DNA synthesis. In myoblasts the poly(A)+ H4 mRNA exists almost exclusively in the polysomal compartment (greater than 95%) with little (less than 5%) in the free ribonucleoprotein (mRNA-protein or mRNP) complex compartment of the cell. Poly(A)- histone H4 mRNA subspecies, on the other hand, are distributed with approximately 80% in the polysomal compartment and 20% in the free mRNP complex compartment. The unique poly(A)+ H4 mRNA is unusual, not only in that it contains poly(A) but also in its behavior compared to poly(A)- H4 mRNAs during terminal differentiation.

The tissue-specific chicken histone H5 gene is transcribed with fidelity in Xenopus laevis oocytes

Vectors containing the chicken H5 gene were micro-injected into the nucleus of Xenopus laevis oocytes. Transcription of the H5 gene was accurately initiated and H5 transcripts with discrete 3' termini were produced. H5 transcripts were not polyadenylated in oocyte nuclei.

Polyadenylation of histone mRNA in Xenopus oocytes and embryos

Oogenesis of amphibians is an atypical situation in which histone mRNA is polyadenylated. The poly(A) tract on H4 mRNA has been examined by S1 nuclease analysis. Throughout oogenesis the poly(A) tract is very short, and nonexistent on some mRNA molecules. The poly(A) tract is completely removed during maturation of the oocyte, and is absent in embryos and cultured cells.

An H1 histone gene-specific 5' element and evolution of H1 and H5 genes

In previous studies we have shown that the H5 gene is not closely linked to the dispersed clusters of core and H1 histone genes. Here we emphasise features of H1 and H5 genes relevant to their expression in the chicken genome. Of particular note is an H1 gene-specific 5' element, 5' AAACACA 3' found upstream of all H1 genes studied to date. This "H1-box" is not found in the related H5 gene, which is expressed only in erythroid cells. A second aspect relates to generation of histone mRNA 3' termini. The H5 gene is shown to contain a remnant of the dyad symmetry element (as well as other conserved sequences) associated with core and H1-histone gene transcript 3' processing. However, it appears as if H5 has evolved a different mechanism in which the mRNA terminus (which is polyadenylated) is displaced downstream from the dyad element. The two clear differences noted here have the potential to affect transcriptional (H1-box) and post-transcriptional (3' terminus processing) regulation of H1 and H5 gene expression.

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.

The histone H5 gene is flanked by S1 hypersensitive structures

The potential of the cloned histone H5 gene to form altered DNA structures has been examined by S1 nuclease digestion of supercoiled recombinant plasmids containing up to 8.8 kbp of chicken DNa. The three main nicking sites map at the upstream and downstream sequences flanking the structural gene. The cleavage sites share sequence homology, strand specificity, and do not seem to be single-stranded. The sequence of the S1-sensitive sites does not suggest that the fragments can adopt any of the known DNA secondary structures.

The terminal RNA stem-loop structure and 80 bp of spacer DNA are required for the formation of 3' termini of sea urchin H2A mRNA

We have studied the exact sequence requirement for the formation of 3' termini of the sea urchin H2A mRNA in frog oocyte injection experiments. Point mutations destroying the symmetry of the inverted DNA repeat in the mRNA trailer coding sequences prevent the generation of genuine 3' termini. Mutants containing complementary base changes are pseudorevertants and allow the production of H2A mRNA with faithful 3' termini at wild-type levels. Our transcription analyses show that it is primarily the sequence of the transcribed strand that decides whether or not true 3' mRNA termini are produced. This is evidence that an RNA stem-loop structure, rather than a DNA cruciform, is essential for this process. Spacer sequences are absolutely required, because in their absence only H2A mRNA with spacer transcript extensions are found. Once the canonical CAAGAAAGA and flanking sequences are linked to the H2A gene, H2A messenger is synthesized at a suboptimal rate, which can be increased to wild-type levels by the addition of 80 bp of the spacer immediately adjacent to the H2A gene.

Nucleotide sequences of H1 histone genes from Xenopus laevis. A recently diverged pair of H1 genes and an unusual H1 pseudogene

Four clones containing H1 histone gene sequences were previously isolated from a Xenopus laevis genomic library (1) and we now present the complete nucleotide sequences of these H1 genes and their flanking regions. Two of these genes code for minor H1 proteins, probably H1C, when expressed in the oocyte transcription/translation system and are present on clones with almost identical overall organization. However, at the nucleotide level these genes differ in showing base insertions and deletions, as well as substitutions. A third gene sequence which is more related to the major X. laevis H1A, corresponds to the 3' two thirds of an H1 gene. This gene has in place of a 5' coding region at least 1800 bp of apparently noncoding sequence, some of which is A-T rich. The junction does not correspond to the consensus sequence of an intron/exon boundary and therefore this H1 sequence is more likely to represent a pseudogene. Comparisons of the coding and flanking regions of these X. laevis H1 genes indicate the kind of differences which can occur among H1 subtypes within a species. A region of homology noted in the 3' noncoding portion of vertebrate histone genes is discussed in relation to the mechanism of termination of transcription.

A qualitative and quantitative study of subfractions of the histone H10 in various mammalian tissues

The histone H10 was examined from seven mammalian species. All tissues were shown to contain two subfractions of H10, except for those of rabbit, in which little or no H10 was found. The subfraction composition was compared quantitatively in different mouse and hamster tissues, with the conclusion that this composition is tissue-specific. It is proposed that the wide occurrence of H10, together with the evidence of no more or less than two subfractions wherever it occurs, and the tissue-specific nature of the ratio of subfractions, signify that these two subfractions have specific individual functions.

The chicken H5 gene is unlinked to core and H1 histone genes

An H5 cDNA clone was used to select H5 genomal recombinants from a chicken Charon 4A library. DNA sequence analysis shows that the H5 gene contains no introns. Putative 5' promoter elements and a 3' polyadenylation site are present within the 1.8 kb of DNA examined. Analysis of 41 kb of DNA surrounding the H5 gene shows that it is not closely linked to either H1 or core histone genes.Images

Variation in the DNA methylation pattern of expressed and nonexpressed genes in chicken

Using methyl-sensitive and -insensitive restriction enzymes, Hpa II and Msp I, the methylation status of various chicken genes was examined in different tissues and developmental stages. Tissue-specific differences in methylation were found for the delta-crystallin, beta-tubulin, G3PDH, rDNA, and actin genes but not for the histone genes. Developmental decreases in methylation were noted for the delta-crystallin and actin genes in chicken kidney between embryo and adult. Since most of the sequences examined were housekeeping genes, transcriptional differences are apparently not a necessary accompaniment to changes in DNA methylation at the CpG sites examined. The only exception is sperm DNA where the delta-crystallin, beta-tubulin, and actin genes are highly methylated and almost certainly not transcribed. However the G3PDH genes are no more highly methylated in sperm than in other somatic tissues. Many sequences homologous to the rDNA and histone probes used are unmethylated in all tissues examined including sperm, but a methylated rDNA subfraction is more heavily methylated in sperm than in other tissues. We speculate as to the significance of these differences in sperm DNA methylation in the light of possible requirements for early gene activation and the probable deleterious mutagenic effects of heavy methylation within coding sequences.