Qualitative Species Differences and Quantitative Tissue Differences in the Distribution of Lysine-rich Histones

Analysis of histones, histone variants, and their post-translationally modified forms

For many years, histones were considered passive structural components of eukaryotic chromatin. Meanwhile it has been proven that histones also participate in gene regulation and repression via post-translational modification. The multitude of these post-translational modifications and the existence of numerous histone variants require particular separation strategies for their analysis, a prerequisite for studying biological processes. The most widely utilized techniques for the separation of histones, namely PAGE, HPCE, RP-HPLC, and hydrophilic Interaction LC, are reviewed here. Problems inherent to the analysis of histones owing to their unique physical and chemical properties along with advantages and shortcomings of particular methods are discussed.

The rat histone H1d gene has intragenic activating sequences that are absent from the testis-specific variant H1t

In some cases core histone genes in the mouse depend on intragenic sequence elements for high level expression [Gene 176 (1996) 1]. Here we report that the highly expressed gene for rat linker histone H1d also contains an intragenic activating region (IAR). Using transient transfection assays in mouse fibroblast NIH3T3 cells, we showed that rat H1d contains a downstream region (+21 to +116) that imparts a two- to threefold up-regulation of fused reporters. This region also activated expression when moved to the promoter region, though the effect was dependent on its distance from other promoter elements. The IAR contains sequence homologies to the core alpha and Omega elements identified as functional protein binding sites within the mouse H3.2 coding region activating sequence (CRAS). A pair of Omega elements (+32 and +66) accounts for the activating effect of the H1d intragenic region as shown by targeted mutations as well as stepwise deletions. The H1d and H3.2 Omega sequences bound similar and perhaps identical proteins by gel shift analysis. The H1d alpha-like sequence at +56 overlaps the translational start codon and was therefore not mutated. Like the mouse H3.2 alpha element, it bound transcription factor YY1 in gel shift assays. H1t, the gene for the testis-specific linker histone, did not demonstrate an IAR. While H1t has a similar alpha sequence and did bind YY1, it lacks the Omega homologies of H1d. Sequence comparison shows that the YY1/alpha site as well as the adjacent Omega site are likely present in genes for other standard H1 variants, but that the +32 Omega site in the 5' untranslated region (UTR) of H1d is unique. We conclude that the +32 and +66 Omega sequences of the rat H1d gene contribute significantly to its high-level expression.

Resolution of allelic and non-allelic variants of histone H1 by cation-exchange-hydrophilic-interaction chromatography

A mixed-mode high-performance liquid chromatography (HPLC) method that resolves the six known non-allelic variants of chicken erythrocyte histone H1 is described. Common, but previously unknown, allelic variants of H1 that comigrate in polyacrylamide gel electrophoresis are also resolved. The resolution of H1 variants achieved by this method should be useful in determining the functional significance of H1 sequence heterogeneity and in analyses of post-translational modification of H1. Furthermore, the principles behind the separation should be applicable to analyses of polymorphism in other proteins.

Capillary electrophoresis of histone H1 variants at neutral pH in dynamically modified fused- silica tubing

Existing methods for the analysis of histone H1 by capillary electrophoresis (CE) employ acidic buffers (pH <3.0) to suppress silanol ionization and minimize the loss of these extremely basic proteins by adsorption to capillary walls. Here we describe the use of Polybrene (PB) as a dynamic modification reagent in a simple procedure that facilitates the analysis of chicken H1 at neutral pH. PB is adsorbed to the inner surfaces of capillaries to render them cationic prior to use and a low concentration of PB is included in the electrolyte to replenish the coating during use. Inclusion of ethylenediaminetetraacetic acid (EDTA) in the electrolyte results in the assembly of a dynamic cation-exchange layer upon the immobilized PB that influences the relative mobilities of H1 variants. The six nonallelic variants of H1 known in this species as well as certain allelic variants are resolved. Because the procedure is effective in preventing the adsorption of proteins as basic as H1 at neutral pH, this strategy should facilitate CE analyses of many basic proteins under conditions that maintain their native conformation.

Fractionation of human H1 subtypes and characterization of a subtype-specific antibody exhibiting non-uniform nuclear staining

Four histone H1 subtypes and H1(0) were fractionated from human placental nuclei and purified to homogeneity by a combination of Bio-Rex 70 chromatography and reverse-phase high-performance liquid chromatography (RP-HPLC). Polyclonal antibodies were generated in rabbits against one of these subtypes designated H1-3. Antibodies reacted only against this subtype in enzyme-linked immunosorbent assays and Western assays; subtype specificity was documented further by Western blotting of cell and nuclear extracts. They crossreacted with monkey H1, but not with H1 from other vertebrates tested. The epitope(s) recognized were mapped by immunoblotting against peptides prepared by cleavage with N-bromosuccinimide (NBS) and alpha-chymotrypsin; it includes the variant amino-terminal tail of the protein as well as a portion of the globular domain. The antibody stains mitotic chromosomes weakly but uniformly and, unlike antibodies that recognize total H1 which show uniform nuclear staining after indirect immunofluorescence localization, anti-H1-3 exhibits preferential labelling of the nuclear periphery. This non-uniform staining suggests compartmentalization of this subtype which may have functional significance with respect to differential chromatin condensation.

Histone H1a subtype presents structural differences compared to other histone H1 subtypes. Evidence for a specific motif in the C-terminal domain

Following a previous isolation by reverse-phase HPLC of five histone H1 subtypes from adult rat liver, purity of three of them, H1a, H1b and H1d (according to Lennox's nomenclature), was achieved. Structural features of these three subtypes were investigated. Partial cleavage of these subtypes by endoproteinase Glu-C showed a different behavior of the H1a subtype when compared to the H1b and H1d subtypes. Under the conditions used in this work, the H1b and H1d subtypes present three major sites accessible to the endoproteinase Glu-C, while the H1a subtype presents only one major site accessible to the proteinase. Partial N-terminal sequence of the different fragments obtained after proteolysis indicated that the two H1b and H1d subtypes were cleaved inside the globular domain (Glu-54,-75) and between the globular domain and the C-terminal one (Glu-116). The H1a subtype was only cleaved between the globular domain and the C-terminal tail (Glu-116), though Glu-54 and Glu-75 sites were present. These results would suggest some differences in the conformation of these proteins. Furthermore, the partial determined sequences of H1b and H1d showed 85% similarity to each other (the main differences were threonine residues instead of alanine residues in the C-terminal domain) while H1a was only 60% similar to H1b and H1d, for the sequences which aligned. The strongest differences between the H1a subtype and the two other subtypes were observed in the first amino acid residues of the C-terminal domain. The 117-126 amino acid residues (SKASTTKVTV) of H1a were quite different from those of H1b and H1d. This sequence, which showed a number of serine and threonine residues, was not found in any other histone sequence, after consultation with data bases. This H1a subtype was a minor component in adult liver (2.4%). As it was described in testis as a major component, testis histone H1 proteins were fractionated onto reverse-phase HPLC under the same conditions as those used for histone H1 proteins from liver. The pure testis H1a fraction was submitted to the endoproteinase Glu-C digestion. The pattern digestion was the same as that observed for liver H1a. The two 44-76 and 117-126 determined amino acid residues of H1a from testis were strictly identical to those of liver H1a. We demonstrate that H1a is the same protein in liver and testis and we give evidence for a specific motif SKASTTKVTV (117-126 residues) in the sequence of the C-terminal domain.(ABSTRACT TRUNCATED AT 400 WORDS)

Isolation of a genomic clone encoding the rat histone variant, H1d

Mammals contain a family of five closely related H1 histone variants (H1a-e) as well as two less closely related forms, H10 and H1t. We have sequenced a rat genomic clone that encodes one of the standard H1 variants. An RNA transcript of the gene was made with bacteriophage SP6 RNA polymerase and translated in a cell-free system. The protein synthesized in vitro was identified as variant H1d by its electrophoretic mobility.

Separation of Friend erythroleukaemic cell histones and high-mobility-group proteins by reversed-phase high-performance liquid chromatography

A procedure for the rapid separation of histones and high-mobility-group (HMG) proteins from Friend erythroleukaemic cells (line F4N) by reversed-phase high-performance liquid chromatography is reported. By using a Nucleosil 300-5 C4 column and a multistep water-acetonitrile gradient containing 0.1% trifluoroacetic acid, the HMG-1 and HMG-2 proteins, several H1 subfractions including H1(0), H4, H2B, two H2A variants and two H3 subfractions were separated. Under changed conditions, by applying a varied acetonitrile gradient system, even two H2B variants were fractionated. The methods described seem to be a real alternative to the time-consuming polyacrylamide gel electrophoresis.

Histone H1 heterogeneity in the midge, Chironomus thummi. Structural comparison of the H1 variants in an organism where their intrachromosomal localization is possible

1. Seven subfractions of histone H1 have been isolated and purified from larvae of Chironomus thummi (Diptera). They have been denominated I-1, II-1, II-2, II-3, III-1, III-2, and III-3, according to the order of migration in two steps of preparative electrophoresis. 2. The amino acid compositions are similar to those of other H1 histones. Subfractions I-1 and II-1 were found to contain one methionine and two tyrosine residues, II-2 contained two methionine and three tyrosine residues, and III-1 one methionine and three tyrosine residues. The other subfractions contained one or two methionine and two or three tyrosine residues. For subfractions I-1 and II-1 a chain length of about 252 amino acids was estimated. 3. Peptide pattern analyses after chemical cleavage at the methionine and tyrosine residues, and enzymatic cleavage with thrombin and chymotrypsin, respectively, showed that all subfractions have different individual primary structures. A comparison of peptide sizes and of the positions in the peptide patterns of epitopes recognized by monoclonal antibodies was made to check whether some of the subfractions could arise by proteolytic degradation of others. This possibility can be excluded for five of the subfractions and is very improbable for the two others. Treatment of C. thummi H1 with alkaline phosphatase did not change the pattern of subfractions, while the phosphorylated subfraction of histone H2A disappeared after this treatment. Most and very probably all subfractions are thus H1 sequence variants. 4. Inbred strains and individual larvae of C. thummi were found to comprise all seven variants. The H1 heterogeneity can therefore not be due to allelic polymorphism. Salivary gland nuclei were found to contain variant I-1 and at least some of the other variants. 5. H1 from Drosophila melanogaster and from calf thymus were used as reference molecules in all cleavage experiments and yielded the peptide patterns expected from the sequence. The comparison discriminates the group of C. thummi H1 histones clearly from Drosophila and calf thymus H1. Limited trypsin digestion yielded a protected peptide of uniform size in six of the seven variants which was considerably smaller than the protected central domain of calf thymus H1. 6. Two other species of Chironomidae, C. pallidivittatus and Glyptotendipes barbipes were found to contain five and three H1 subfractions, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Genes for chromosomal proteins expressed before and after meiosis

Cloned gene sequences have been isolated for two testis-specific chromosomal proteins, one of which, histone (H1t), appears during meiosis, whereas the other, transition protein 1 (TP1), appears only during the later steps of spermatid development. Aspects of the regulation of each gene have been examined. In the case of H1t, analysis of its promoter region shows that it contains excellent matches to each of the four sequence homologies identified for the usual somatic H1 variants, so that the factor(s) that restrict H1t expression to spermatocytes remain a mystery. In the case of TP1, a cDNA clone allowed identification of its message by Northern blots as well as by in situ hybridization. The message appears postmeiotically in late round spermatids but is translationally repressed until the spermatid nucleus begins to condense.

H1 histone subtypes and subtype synthesis switches of normal and delobed embryos of Ilyanassa obsoleta

Histone H1 subtype complexity and H1 histone subtype synthesis switches were characterized during the development of normal embryos of the mud snail Ilyanassa obsoleta. The effect of the removal of the third polar lobe on the normal H1 pattern of synthesis was then investigated in the delobed embryo to determine if classical polar lobe effects are accompanied by a perturbation of these patterns. SDS-gel electrophoresis and fluorography of radiolabeled 5% perchloric acid-soluble nuclear extracts resolved six H1 proteins designated bands 1-6. Bands 1-5 migrate as a cluster of individual bands with similar mobilities. Band 6 has a substantially slower mobility. The synthesis of band 6 is predominant during the first 6 hr post-trefoil. During cleavage and gastrulation bands 1 and 2 are predominant while band 3, 4, and 5 become predominant during organogenesis. In addition, it has been found that removal of the polar lobe delays the off-switch of the early bands 6, 1, and 2 and the on-switch of the late bands 3, 4, and 5. This must result in a different H1 composition in the chromatin of the two embryo types. Cell number data of normal and delobed embryos reveal that the delay in subtype synthesis switching is not caused by an overall delay of cell division in the delobed embryo. However, the data indicate that a subpopulation of cells may not divide, or may divide late, in the delayed embryo. The data also suggest that the D cell lineage may be involved in the control of histone synthesis switching in the A, B, and C cell lineages.

Use of protein blotting to study the DNA-binding properties of histone H1 and H1 variants

Sub-types of histone H1 have been observed in a variety of tissues from several organisms. One of the best characterized H1 variants is H5 from avian erythrocytes. Several lines of evidence suggest that H5 has a greater affinity for DNA than H1 and is thus thought to account, in part, for the highly condensed and transcriptionally repressed state of avian erythrocyte chromatin. In trout there is an analogous erythrocyte-specific H1 variant, previously termed 'H5' [B.L.A. Miki and J.M. Neelin (1975) Can. J. Biochem. 53, 1158-1169). Using a sensitive and rapid protein-blotting procedure which is specific amongst the histones for histone H1 and its variants, we compared DNA-binding properties of the trout erythrocyte histone 'H5' and chicken H5. By increasing the NaCl concentration of the binding buffer, a gradual decrease in the amount of DNA that bound to chicken H1, trout H1 and trout erythrocyte 'H5' variant was observed, such that at concentrations above 0.37 M, negligible amounts of DNA were bound. By contrast, chicken H5 bound a significantly greater amount of DNA even at a concentration of 0.4 M NaCl. Based on the DNA-binding, properties, we conclude that the trout erythrocyte variant 'H5' is more closely related to H1 than to H5. By assaying the DNA-binding affinity of calf thymus H1 DNA-binding affinity of calf thymus H1 peptide fragments, generated by protease and chemical cleavage, and the sperm-specific H1 variants of the annelid, Platynereis dumerilii, which possess greatly shortened C-terminal tails, we conclude that a domain that includes a very small portion of the C-terminal tail and part of the globular domain is sufficient for the binding of H1 to DNA.

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.

A study on the heterogeneity of histone H1 from dry maize embryos

Maize dry embryo cells have been chosen as model for the study of H1 histone complement of metabolically inactive plant chromatin. H1 has been fractionated into two distinct bands in acetic acid/urea gels. Analysis of each of these bands in SDS-containing gels has shown that maize embryo H1 is heterogeneous, consisting of at least six proteins. Cross-reactivity of maize H1 species with antibodies against mouse liver total H1 and against its individual variants indicates that they share common immunological determinants but differ substantially from each other when compared by peptide mapping. It is concluded that a couple of the plant H1 subfractions are related to animal H1A histone and that another one is related to H1B histone. The other three maize H1 variants are closely related to each other and they also share common immunological determinants with mouse H1A, and possibly with H1B. All maize H1 species slightly cross-react with antibodies against H1 degree, suggesting that no one of the plant subfractions could be characterized as H1 degree in particular. One of the proteins co-extracted and comigrating with H1 is most probably an embryo storage protein.

Histone separation by high-performance liquid chromatography on C4 reverse-phase columns

Previous work in our laboratory (Lindner, H., Helliger, W., and Puschendorf, B. (1986) J. Chromatogr. 357, 301-310) described a rapid separation of H1 and core histones by reverse-phase high-performance liquid chromatography using a Bio-Rad Hi-Pore butyl (C4) silica-based column. Despite the short elution time, a high resolution of the different histone fractions, except H4 and H2A (MHP), could be obtained. In this report we present a method for the separation of H4 and H2A (MHP) as well, while maintaining a similar analysis time. By varying the gradient, trifluoroacetic acid concentration (0.05%), and flow rate (1.3 ml/min) the histones were eluted from the C4 column in the following order: H1 (MHP), H1 (LHP), H2B, H2A (LHP), H4, H2A (MHP), H3 (LHP), and H3 (MHP). LHP and MHP refer to less and more hydrophobic histone variants. The identification of the individual protein fractions was performed by comparison the retention times with pure histone markers as well as by gel electrophoresis.

Immunofractionation of chromatin regions associated with histone H1o

Two monoclonal antibodies, which were elicited against histone H5, bind to purified rat liver chromatin and to rat liver H1o but not to rat liver H1. The monoclonal antibodies were immobilized on CNBr-Sepharose and the resulting immunoaffinity column was used to fractionate rat liver oligonucleosomes. Enzyme-linked immunoabsorbant assay (ELISA) and immunoblotting experiments indicate that the nucleosomes bound to the column were tenfold enriched in their content of H1o. Oligonucleosomes, prepared from the livers of either untreated or 3-methylcholanthrene-treated adult rats, were fractionated on the anti-H1o affinity column. The DNA purified from the unfractionated nucleosomes, from the unbound nucleosomes and from the nucleosomes which were bound to the column was examined with various 32P-labeled probes. A slight enrichment in H1o was detected in the coding region of the rat albumin gene. In contrast DNA which was bound to the column was significantly depleted in sequences hybridizing with total cellular RNA (which contains mostly ribosomal RNA) and with sequences hybridizing to the 3'-terminal region of a cytochrome P-450 gene, which is inducible by the chemical carcinogen 3-methylcholanthrene, regardless of whether isolated from control or from carcinogen-treated rat livers. Our experiments clearly demonstrate that chromatin can be efficiently immunofractionated. The results suggest that the H1o content of chromatin regions containing genes which are constitutively transcribed is not necessarily different from that of regions containing non-transcribed genes and that highly inducible genes may be segregated into chromatin regions which are depleted of H1o.

Histone phosphorylation in explants of mouse mammary glands and tumors

Rates of histone phosphorylation were measured in explants of mammary glands from mouse strains with high and low tumor incidence. Explants of hormone dependent and independent mouse mammary tumors were also investigated. All mouse strains studied showed predominant phosphorylation of H2A histone at serine and threonine residues. No differences in rates of H2A phosphorylation in glands were found between strains having different mammary tumor susceptibility. Hormone-dependent GR mouse mammary tumors also showed high H2A phosphorylation, but in some tumors also H1 and H3 were phosphorylated. Hormone-dependent GR tumors had 2-5 times higher histone phosphorylation at serine and threonine than hormone-independent tumors.

Structural properties of the methionine-containing subfraction of rat liver histone H1

Cyanogen bromide (CNBr) cleavage of total rat liver histone H1 generates a C-terminal peptide which originates from a methionine-containing subfraction. This subfraction comprises approx. 20% of the whole rat liver H1 population, resembles calf thymus CTL-1 in size but contains methionine and histidine, higher proportions of serine and less alanine and proline. Edman degradation established the N-terminal sequence of the CNBr peptide as Arg-Arg-Lys-Ala-Ser-Gly-Pro-Pro-Val-Glu. By alignment with calf thymus CTL-1, methionine was identified as residue 30 replacing alanine in a non-conservative replacement. Residue 40 is deleted but sequence homology near the double proline sequence in the G-domain is retained. The CNBr peptide is estimated at 177-181 residues and comprises the complete G- and C-domain and two arginines from the basic cluster in the N-domain. Removal from H1 of all but two residues of the N-domain does not abolish secondary and tertiary folding. This GC-peptide opens new approaches to the study of the function of H1 in chromatin.

Events in glucocorticoid hormone action. A correlation of histone H1 variant pattern changes, hormone binding to cell nuclei, and induction of mouse mammary tumor virus RNA

To approach experimentally changes of chromatin structure introduced by glucocorticoids, the histone H1 compositions of hormone-treated and non-treated mouse mammary tumor cells of the GR line [Ringold, G., Lasfargues, E. Y., Bishop, J. M. and Varmus, H. E. (1975) Virology 65, 135-147] were compared. To define the biologically important hormone concentration range, the cells were exposed to different concentrations of triamcinolone, a synthetic glucocorticoid. The induction of mouse mammary tumor virus (MMTV) RNA was measured by cDNA excess hybridization, and the amount of hormone bound to nuclei was determined by a filter-binding assay. Between 0.3 nM and 30 nM triamcinolone the relative increase in nuclear bound hormone corresponded well with the relative induction of MMTV RNA. The half-life of triamcinolone in nuclei of growing cells was 1 h, as measured by a pulse-chase experiment. Reversed-phase high-performance liquid chromatography of histone H1 resulted in its separation into four subfractions. The treatment of cells with biologically active glucocorticoid, 3 nM or 30 nM triamcinolone or 1 microM dexamethasone, resulted in changes in the relative amounts of two subfractions and to a positional shift of two subfractions as compared to untreated cells. No changes were observed after exposure to 3 nM dexamethasone, a concentration which does not induce MMTV RNA [Ringold, G. M., Yamamoto, K. R., Tomkins, G. M., Bishop, J. M. and Varmus, H. E. (1975) Cell 6, 299-305].

Immunohistochemical distribution of the histone H1(0)/H5 variant in various tissues of adult Xenopus laevis

The cellular distribution of the histone H1(0)/H5 variant has been examined immunohistochemically in various tissues of adult Xenopus laevis, using monoclonal antibodies against this variant that was isolated from erythrocyte nuclei. The H1(0)/H5 variant appears not to be erythrocyte-specific and appears to be present in all cell types of liver, stomach, and skin. In contrast, in oocyte nuclei the H1(0)/H5 variant cannot be detected, whereas they do contain H1; the nuclei of spermatogenic cells contain the H1(0)/H5 variant, but probably less than the somatic cells. In Xenopus no H1(0) variant distinct from H5 seems to occur and the H1(0)/H5 variant apparently may perform a functional role related to mammalian H1(0).

An H1 histone gene from rainbow trout (Salmo gairdnerii)

A 1.7-kbp DNA region from the 10.2-kb cluster containing the five rainbow trout histone genes has been subcloned in pBR322 and completely sequenced. It contains a trout histone H1 gene together with its 5' and 3' flanking sequences. This H1 gene codes for a H1 variant different from the major trout testis H1 previously sequenced by Macleod et al. (1977). Northern blots of total RNA from trout testis, kidney, and liver indicate that this H1 gene is expressed in all three tissues but that the level of H1 mRNA is much higher in testis than in other tissues. The lack of heterogeneity in the sizes and 5' initiation sites of trout H1 mRNAs is surprising in view of the substantial heterogeneity of H1 variant proteins observed previously. The coding sequence of the H1 gene shows strong evidence of repeated partial duplications of a hexapeptide motif of the form Ala.Ala.Ala.Lys.Lys.Pro and of a pentapeptide phosphorylation-site sequence, Lys.Ser.Pro.Lys.Lys, during its evolution. Comparisons are drawn between this gene and the coding sequences of other vertebrate H1 genes from chicken and Xenopus, and a strong homology is seen in the region of amino acids 22-101, which form the hydrophobic "head" of the H1 molecule. The 5' and 3' regulatory signals in the trout H1 are also compared with those of H1 genes from other sequences.

Nucleosome disruption precedes transcription and is largely limited to the transcribed domain of globin genes in murine erythroleukemia cells

We used micrococcal nuclease to separate murine erythroleukemia cell (MELC) chromatin into soluble and insoluble fractions which differ in gene content and chromatin structure. Genes that are not expressed in the erythroid lineage, such as the Ig alpha and albumin genes, distribute preferentially into the soluble rather than the insoluble fraction, and are organized into nucleosomes in both fractions. Both alpha 1- and beta maj-globin genes are enriched in the insoluble fraction and are organized into structures that are partially devoid of nucleosomes in uninduced MELC, when the genes are transcriptionally inactive. Following chemical induction of MELC and the onset of globin gene transcription, globin gene enrichment and nucleosome disruption in the insoluble chromatin fraction increase. Using seven DNA subclones that span the beta maj-globin gene we show that insolubility and nucleosome disruption are largely limited to DNA sequences lying within the transcribed domain. Non-transcribed, flanking sequences are soluble and organized into nucleosomes. In addition, the globin genes found in insoluble, non-nucleosomal chromatin contain previously engaged RNA polymerases which can elongate globin RNA chains in vitro in a pattern qualitatively and quantitatively similar to intact nuclei. These results are discussed in terms of a model for globin gene activation during erythropoeisis.

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.

Regulation of nucleosomal core histone variant levels in differentiating murine erythroleukemia cells

During hexamethylenebis(acetamide)-induced terminal differentiation of murine erythroleukemia (MEL) cells in vitro, the histone variant proportions undergo changes similar to those observed in vivo in terminally differentiating cells of the young mouse. Thus, there is a rapid increase in the relative amounts of the variants H2A.1 and H2B.2 in parallel with the increase in the number of hemoglobin-producing cells and the sharp decrease in the growth rate. We show that the changes in variant proportions are not associated with slower growth per se but are most likely due to differential changes in the rates of variant synthesis as a result of commitment to terminal differentiation. In addition, we observed an inducer-specific increase in the rate of synthesis and the relative amount of the minor H2A variant 4, well before hemoglobin accumulation. We also present evidence that H2A and H2B histones are synthesized and incorporated into chromatin at a significant rate even when DNA synthesis is inhibited, suggesting turnover of these histones. H2A and H2B turnover can be detected directly even in exponentially growing cells. H2A.1 and H2B.2 have higher turnover rates than H2A.2 and H2B.1, respectively, in exponentially growing cells, a difference which is even more pronounced in induced cells. The magnitude of the differential turnover is not sufficient to account for the changes in the histone variant proportions in the short life of induced MEL cells but could explain the slow accumulation of H2A.2, H2B.1, and H3.3 in nondividing adult tissues of the mouse.

The histone H5 variant in Xenopus laevis

The presumptive histone H5 of Xenopus laevis has been characterized by SDS and acid-urea-Triton polyacrylamide gel electrophoresis and compared with chicken histone H5. Chicken H5 has a lower electrophoretic mobility compared to that of Xenopus H5 in both gel systems. It is shown, using a polyclonal antiserum against chicken H5, that the Xenopus histone H5 is immunologically related to chicken histone H5. Monoclonal antibodies have been prepared to the Xenopus histone types H5 and H1A, that do not cross-react, as determined by their reactivity in an enzyme linked immunosorbent assay and by their ability to react with either H1A or H5 in an immunochemical test on total erythrocyte histones that are transferred to nitrocellulose after fractionation by SDS- or acid-urea polyacrylamide gel electrophoresis. As all nuclei of erythrocytes from adult Xenopus laevis can be shown to contain histone H1A and H5, these monoclonal antibodies can be used to further delineate the role of H5 in tissue differentiation.

A minireview of microheterogeneity in H1 histone and its possible significance

Subtypes of H1 histone vary in primary structure, and the higher organisms that have been studied each seem to have about a half-dozen subtypes. The proportions of these subtypes vary with the progress of differentiation as seen in embryonic development, hormonally induced changes, spermatogenesis, and terminal differentiation. The H1 subtypes differ among themselves in their ability to condense DNA and small chromatin fragments. They have the potential, therefore, of causing different parts of the chromatin to be condensed to different degrees.

Macromolecular relationships of liver nuclei and amino acid composition of total histones during bovine ontogeny

The weight of the liver and the macromolecular composition of the liver and its nuclei during bovine ontogeny were compared. The findings suggest that bovine liver growth proceeds in three phases. The macromolecular composition of the liver nuclei was found to remain fairly constant throughout bovine ontogeny.

Properties of a purified nuclear topoisomerase from L1210 cells

A nuclear type I topoisomerase from mouse leukemia L1210 cells has been partially purified and characterized. The sedimentation coefficient of the enzyme by velocity sedimentation is 4.3 S, consistent with a globular protein of 68 kDa. Enzyme activity is stimulated 20-fold in the presence of magnesium over that achieved in KCl alone. The enzyme is completely inhibited in the presence of the berenil congeners HOE 13548 and 15030 while berenil itself caused only partial inhibition at concentrations below 200 micrograms/ml. An acid soluble protein of 30 kDa (by SDS-polyacrylamide gel electrophoresis) co-purified with the topoisomerase but could be separated by precipitation in a low salt buffer. This protein, as well as a protein of similar characteristics, histone H1, stimulated topoisomerase activity over a narrow concentration range. The role of topoisomerase in the DNA strand scission observed in L1210 cells following exposure to intercalating agents remains conjectural as the purified enzyme did not produce nicks in plasmid DNA in the presence of adriamycin.

Age changes in the H1 group of histones from rat liver

Histones were obtained from young and old rat livers by extracting them in 0.25 N HCl. They were fractionated on 15% acid urea polyacrylamide gels containing 6.25 M urea and the changes in the ratio of the major histone fractions as a function of age were calculated. Data presented show a significant increase in the amount of H1 degree fraction in the liver of old rats as compared to young rats. This data is discussed and the possible involvement of H1 degree fraction in an increased resistance of old rat liver chromatin to micrococcal nuclease digestion of linker DNA is suggested. Finally, in connection with this increased resistance, some possible consequences in chromatin structure are discussed.