Control of RNA polymerase binding to chromatin by variations in linker histone composition

Linker Histones Maintain Genome Stability and Drive the Process of Cellular Ageing

Ageing comprises a cascade of processes that are inherent in all living creatures. There are fourteen general hallmarks of cellular ageing, the majority of which occur at a molecular level. A significant disturbance in the regulation of genome activity is commonly observed during cellular ageing. Overall confusion and disruption in the proper functioning of the genome are also well-known prerogatives of cancerous cells, and it is believed that this genomic instability provides a direct link between aging and cancer. The spatial organization of nuclear DNA in chromatin is the foundation of the fine-tuning and refined regulation of gene activity, and it changes during ageing. Therefore, chromatin is the platform on which genes and the environment meet and interplay. Different protein factors, small molecules and metabolites affect this chromatin organization and, through it, drive cellular deterioration and, finally, ageing. Hence, studying chromatin structural organization and dynamics is crucial for understanding life, presumably the ageing process. The complex interplay among DNA and histone proteins folds, organizes, and adapts chromatin structure. Among histone proteins, the role of the family of linker histones comes to light. Recent data point out that linker histones play a unique role in higher-order chromatin organization, which, in turn, impacts ageing to a prominent degree. Here, we discuss emerging evidence that suggests linker histones have functions that extend beyond their traditional roles in chromatin architecture, highlighting their critical involvement in genome stability, cellular ageing, and cancer development, thereby establishing them as promising targets for therapeutic interventions.

A Robust Method for the Purification and Characterization of Recombinant Human Histone H1 Variants

Higher order compaction of the eukaryotic genome is key to the regulation of all DNA-templated processes, including transcription. This tightly controlled process involves the formation of mononucleosomes, the fundamental unit of chromatin, packaged into higher order architectures in an H1 linker histone-dependent process. While much work has been done to delineate the precise mechanism of this event in vitro and in vivo, major gaps still exist, primarily due to a lack of molecular tools. Specifically, there has never been a successful purification and biochemical characterization of all human H1 variants. Here we present a robust method to purify H1 and illustrate its utility in the purification of all somatic variants and one germline variant. In addition, we performed a first ever side-by-side biochemical comparison, which revealed a gradient of nucleosome binding affinities and compaction capabilities. These data provide new insight into H1 redundancy and lay the groundwork for the mechanistic investigation of disease-driving mutations.

What is the role of histone H1 heterogeneity? A functional model emerges from a 50 year mystery

For the past 50 years, understanding the function of histone H1 heterogeneity has been mired in confusion and contradiction. Part of the reason for this is the lack of a working model that tries to explain the large body of data that has been collected about the H1 subtypes so far. In this review, a global model is described largely based on published data from the author and other researchers over the past 20 years. The intrinsic disorder built into H1 protein structure is discussed to help the reader understand that these histones are multi-conformational and adaptable to interactions with different targets. We discuss the role of each structural section of H1 (as we currently understand it), but we focus on the H1's C-terminal domain and its effect on each subtype's affinity, mobility and compaction of chromatin. We review the multiple ways these characteristics have been measured from circular dichroism to FRAP analysis, which has added to the sometimes contradictory assumptions made about each subtype. Based on a tabulation of these measurements, we then organize the H1 variants according to their ability to condense chromatin and produce nucleosome repeat lengths amenable to that compaction. This subtype variation generates a continuum of different chromatin states allowing for fine regulatory control and some overlap in the event one or two subtypes are lost to mutation. We also review the myriad of disparate observations made about each subtype, both somatic and germline specific ones, that lend support to the proposed model. Finally, to demonstrate its adaptability as new data further refines our understanding of H1 subtypes, we show how the model can be applied to experimental observations of telomeric heterochromatin in aging cells.

The role of linker histone H1 modifications in the regulation of gene expression and chromatin dynamics

BACKGROUND

Linker histone H1 is a structural component of chromatin. It exists as a family of related proteins known as variants and/or subtypes. H1.1, H1.2, H1.3, H1.4 and H1.5 are present in most somatic cells, whereas other subtypes are mainly expressed in more specialized cells.

SCOPE OF REVIEW

H1 subtypes have been shown to have unique functions in chromatin structure and dynamics. This can occur at least in part via specific post-translational modifications of distinct H1 subtypes. However, while core histone modifications have been extensively studied, our knowledge of H1 modifications and their molecular functions has remained for a long time limited to phosphorylation. In this review we discuss the current state of knowledge of linker histone H1 modifications and where possible highlight functional differences in the modifications of distinct H1 subtypes.

MAJOR CONCLUSIONS AND GENERAL SIGNIFICANCE

H1 histones are intensely post-translationally modified. These modifications are located in the N- and C-terminal tails as well as within the globular domain. Recently, advanced mass spectrometrical analysis revealed a large number of novel histone H1 subtype specific modification sites and types. H1 modifications include phosphorylation, acetylation, methylation, ubiquitination, and ADP ribosylation. They are involved in the regulation of all aspects of linker histone functions, however their mechanism of action is often only poorly understood. Therefore systematic functional characterization of H1 modifications will be necessary in order to better understand their role in gene regulation as well as in higher-order chromatin structure and dynamics.

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.

Forces and torques in the nucleus: chromatin under mechanical constraints

Genomic DNA in eukaryotic cells is organized in discrete chromosome territories, each consisting of a single huge hierarchically supercoiled nucleosomal fiber. Through dynamic changes in structure, resulting from chemical modifications and mechanical constraints imposed by numerous factors in vivo, chromatin plays a critical role in the regulation of DNA metabolism processes, including replication and transcription. Indeed, DNA-translocating enzymes, such as polymerases, produce physical constraints that chromatin has to overcome. Recent techniques, in particular single-molecule micromanipulation, have allowed precise quantization of forces and torques at work in the nucleus and have greatly improved our understanding of chromatin behavior under physiological mechanical constraints. These new biophysical approaches should enable us to build realistic mechanistic models and progressively specify the ad hoc and hazy "because of chromatin structure" argument often used to interpret experimental studies of biological function in the context of chromatin.

Saccharomyces cerevisiae linker histone Hho1p functionally interacts with core histone H4 and negatively regulates the establishment of transcriptionally silent chromatin

Saccharomyces cerevisiae linker histone Hho1p is not essential for cell viability, and very little is known about its function in vivo. We show that deletion of HHO1 (hho1Delta) suppresses the defect in transcriptional silencing caused by a mutation in the globular domain of histone H4. hho1Delta also suppresses the reduction in HML silencing by the deletion of SIR1 that is involved in the establishment of silent chromatin at HML. We further show that hho1Delta suppresses changes in silent chromatin structure caused by the histone H4 mutation and sir1Delta. These results suggest that HHO1 plays a negative role in transcriptionally silent chromatin. We also provide evidence that Hho1p hinders the de novo establishment of silent chromatin but does not affect the stability of preexistent silent chromatin. Unlike canonical linker histones in higher eukaryotes that have a single conserved globular domain, Hho1p possesses two globular domains. We show that the carboxyl-terminal globular domain of Hho1p is dispensable for its function, suggesting that the mode of Hho1p action is similar to that of canonical linker histones.

Distribution of somatic H1 subtypes is non-random on active vs. inactive chromatin II: distribution in human adult fibroblasts

For nearly twenty years researchers have observed changes in the histone H1 subtype content of tissues as an organism develops into an adult. To better understand the consequences of such changes, immunofractionation of chromatin using previously characterized antibodies specific for human H1 subtypes was employed in the analysis of a fibroblast cell strain derived from a 37-year-old individual. DNAs isolated from immunoprecipitates were probed for the existence of a variety of DNA sequences. The results presented lend further support to a previously-proposed model (Parseghian et al. [2000] Chromosome Res 8:405-424) in which transcription of a sequence is accompanied by the selective depletion of subtypes. The data also suggest that there is more total H1 on actively transcribed sequences in these cells as compared to fetal fibroblasts and that there is less difference in the subtype compositions of active genes vs. inactive sequences in this strain. Specifically, the consequences of these changes appear to correlate with the attenuation of the heat shock response in aging fibroblasts. In a broader context, these results could explain why there are reductions in transcription in cells from mature tissue that approach senescence.

A compendium of the histone H1 family of somatic subtypes: An elusive cast of characters and their characteristics

The last 35 years has seen a substantial amount of information collected about the somatic H1 subtypes, yet much of this work has been overshadowed by research into highly divergent isoforms of H1, such as H5. Reports from several laboratories in the past few years have begun to call into question some of the traditional views regarding the general function of linker histones and their heterogeneity. Hence, the impression in some circles is that less is known about these ubiquitous nuclear proteins as compared with the core histones. The goal of the following review is to acquaint the reader with the ubiquitous somatic H1s by categorizing them and their characteristics into several classes. The reasons for our current state of misunderstanding is put into a historical context along with recent controversies centering on the role of H1 in the nucleus. Finally, we propose a model that may explain the functional role of H1 heterogeneity in chromatin compaction.Key words: histone H1, linker histones, chromatin organization, chromatin compaction, heat shock.

The distribution of somatic H1 subtypes is non-random on active vs. inactive chromatin: distribution in human fetal fibroblasts

Chromatin immunoprecipitation was employed to determine whether or not the previously reported depletion of histone H1 on actively transcribed sequences was selective with respect to H1 subtypes. DNA of immunofractionated chromatin was analyzed by slot-blots for repetitive sequences and PCR for single and low-copy sequences. Based on the analysis of a diverse set of sequences, we report distinct differences in subtype distributions. Actively transcribed chromatin, as well as chromatin poised for transcription, is characterized by a relative depletion of somatic H1 subtypes 2 and 4 (H1s-2 and H1s-4),whereas facultative and constitutive heterochromatin contain all four somatic subtypes. These results support a model in which subtypes are selectively depleted upon gene expression. In turn, the data also support the possibility that the somatic subtypes have different functional roles based on their selective depletion from different classes of DNA sequences.

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.

The biochemical and phenotypic characterization of Hho1p, the putative linker histone H1 of Saccharomyces cerevisiae

There is currently no published report on the isolation and definitive identification of histone H1 in Saccharomyces cerevisiae. It was, however, recently shown that the yeast HHO1 gene codes for a predicted protein homologous to H1 of higher eukaryotes (Landsman, D. (1996) Trends Biochem. Sci. 21, 287-288; Ushinsky, S. C., Bussey, H. , Ahmed, A. A., Wang, Y., Friesen, J., Williams, B. A., and Storms, R. K. (1997) Yeast 13, 151-161), although there is no biochemical evidence that shows that Hho1p is, indeed, yeast histone H1. We showed that purified recombinant Hho1p (rHho1p) has electrophoretic and chromatographic properties similar to linker histones. The protein forms a stable ternary complex with a reconstituted core di-nucleosome in vitro at molar rHho1p:core ratios up to 1. Reconstitution of rHho1p with H1-stripped chromatin confers a kinetic pause at approximately 168 base pairs in the micrococcal nuclease digestion pattern of the chromatin. These results strongly suggest that Hho1p is a bona fide linker histone. We deleted the HHO1 gene and showed that the strain is viable and has no growth or mating defects. Hho1p is not required for telomeric silencing, basal transcriptional repression, or efficient sporulation. Unlike core histone mutations, a hho1Delta strain does not exhibit a Sin or Spt phenotype. The absence of Hho1p does not lead to a change in the nucleosome repeat length of bulk chromatin nor to differences in the in vivo micrococcal nuclease cleavage sites in individual genes as detected by primer extension mapping.

Dynamics of unfolded nucleosomal fiber

The "rigidity" of chromatin fiber solenoidal structure in different states of condensation was evaluated with the help of gel-electrophoresis. A new property of the unfolded nucleosomal fiber-the capacity to condense with temperature-was demonstrated. These results together with our previously obtained data (W.A. Krajewski et al., Mol. Gen. Genet. 230, pp. 442-448, 1991; W.A. Krajewski et al., Ibid. 231, pp. 17-22, 1991) testify that changes in DNA linking number of transcriptionally active minichromosomes arise in vivo from alteration of nucleosomal solenoid parameters (i.e. from supernucleosomal level of chromatin organization), rather than from core histone modifications only or from increased flexibility of DNA within nucleosomes.

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)

Expression of Xenopus laevis histone H5 gene in yeast

As an approach to assess linker histone function, we engineered a cDNA encoding Xenopus laevis histone H5 (XLH5), into the yeast Saccharomyces cerevisiae, which lacks any known proteins homologous to linker histones. XLH5 cDNA when fused to the yeast GAL10 promoter and 5' untranslated region (UTR) was shown to be accurately transcribed at relatively high levels in cells harvested at mid to late log after exposure to at least 22 mM galactose. The resultant 0.95 kb XLH5 transcript reached steady state levels by approx. 2 h after galactose induction. In contrast, the product, detected by anti-XLH5 antibody, was not stably expressed until 4 h or more after induction, when no apparent growth takes place. The expression product was 27% smaller than native H5 and may have been proteolytically processed. Constitutive transcription and loss of XLH5 expression product occurred using a plasmid construct containing a 275 bp fragment of the pBR322 tetr gene inserted downstream of the GAL10 promoter. This fragment carries a putative yeast cell-type-specific upstream activation sequence.

Loosened nucleosome linker folding in transcriptionally active chromatin of chicken embryo erythrocyte nuclei

We have investigated the mechanism of the electrophoresis-driven chromatin aggregation which had been described by Weintraub (1984, Cell 38, 17-27) as a putative mean for propagation of genetic repression in eukaryotes. We show that the oligonucleosome aggregates are assembled de novo at the starting zone of DNP electrophoresis. A new system of native two-dimensional DNP electrophoresis has been worked out to separate the oligonucleosome aggregates ('A' particles) and the freely-migrating oligonucleosomes ('B' particles). The 'B' particle fraction which is derived from transcriptionally-active chromatin regions undergoes an extensive nuclease degradation of its DNA termini during the nuclease digestion. This fraction is partially depleted of histones H1 and H5 and is enriched in HMG nonhistone proteins. 'A' particles comprise the repressed chromatin DNA fragments which are about 60 b.p. longer than the corresponding DNA oligomers of 'B' particles. An oligonucleosome preparation containing the elongated DNA oligomers has been also isolated by means of sucrose gradient ultracentrifugation. Exonuclease III mapping reveals that the two chromatin fractions differ by an extent of terminal linker DNA trimming during the Micrococcal nuclease digestion rather than by the nucleosome repeat length. The complex character of nuclease digestion is not observed when the chromatin is digested in solution after the nuclear lysis. We argue that the protection of terminal oligonucleosome linkers is due to selective condensation of inactive chromatin in chicken erythrocyte nuclei and that the terminal DNA tails together with linker histones bound to them mediate the aggregation of repressed chromatin fragments.

Histone H1 represses transcription from minichromosomes assembled in vitro

We have previously shown that transcription from a Xenopus 5S rRNA gene assembled into chromatin in vitro can be repressed in the absence of histone H1 at high nucleosome densities (one nucleosome per 160 base pairs of DNA) (A. Shimamura, D. Tremethick, and A. Worcel, Mol. Cell. Biol. 8:4257-4269, 1988). We report here that transcriptional repression may also be achieved at lower nucleosome densities (one nucleosome per 215 base pairs of DNA) when histone H1 is present. Removal of histone H1 from the minichromosomes with Biorex under conditions in which no nucleosome disruption was observed led to transcriptional activation. Transcriptional repression could be restored by adding histone H1 back to the H1-depleted minichromosomes. The levels of histone H1 that repressed the H1-depleted minichromosomes failed to repress transcription from free DNA templates present in trans. The assembly of transcription complexes onto the H1-depleted minichromosomes protected the 5S RNA gene from inactivation by histone H1.

Histone H1 represses transcription from minichromosomes assembled in vitro

We have previously shown that transcription from a Xenopus 5S rRNA gene assembled into chromatin in vitro can be repressed in the absence of histone H1 at high nucleosome densities (one nucleosome per 160 base pairs of DNA) (A. Shimamura, D. Tremethick, and A. Worcel, Mol. Cell. Biol. 8:4257-4269, 1988). We report here that transcriptional repression may also be achieved at lower nucleosome densities (one nucleosome per 215 base pairs of DNA) when histone H1 is present. Removal of histone H1 from the minichromosomes with Biorex under conditions in which no nucleosome disruption was observed led to transcriptional activation. Transcriptional repression could be restored by adding histone H1 back to the H1-depleted minichromosomes. The levels of histone H1 that repressed the H1-depleted minichromosomes failed to repress transcription from free DNA templates present in trans. The assembly of transcription complexes onto the H1-depleted minichromosomes protected the 5S RNA gene from inactivation by histone H1.

Initiation of transcription on nucleosomal templates

We describe a simple method that uses curved DNAs to move nucleosomes relative to a sequence of interest. With this method, small changes in the association of a T7 RNA polymerase promoter with a nucleosome are shown to lead to significant changes in transcription efficiency.

Isolation and characterisation of five histone H1 subtypes from adult rat liver

A procedure is described for quantitative purification of H10 and five H1-1 subtypes--named H1-1a to e--from adult rat liver by reverse-phase high-pressure liquid chromatography. Milligram amounts of each fraction have been obtained. The H1-1a subtype shows a very high lysine content (34%) and H1-1d subtype has an amino-acid composition close to that of H10, but its electrophoretic mobility is different. Salt dependent folding of these subtypes has been studied by circular dichroism. In the presence of 2 or 10 mM sodium phosphate buffers at pH 7.5, H1-1a shows the lowest alpha-helix content. In phosphate-buffer containing 1 M NaCl the number of residues in alpha-helix for all the subtypes rises to 9-10%. Partial cleavage of these subtypes by endoproteinase Glu-C produce three main peptides arising from C-terminal domains. The interaction of the H1-1 subtypes with 196 basepairs linear DNA, purified from rat liver chromatin by high-pressure ion-exchange liquid chromatography, has for consequences a modification of the patterns of digestion: partial proteolysis of the H1-1a and H1-1b subtypes shows differences in the presence or in absence of DNA; on the contrary, H1-1c and H1-1d seem to have the same organization. So these subtypes may play a role in the differential packing of specific region of chromatin.

Proportions of H1 histone subspecies in human fibroblasts shift during density-dependent growth arrest independent of replicative senescence

H1 histone subspecies have been reported to vary during tissue differentiation, during aging of mammalian tissues, and as a function of DNA replicative activity. Since cultured human fibroblasts have a limited replicative life span which features arrest in the G1 phase of the cell cycle, we sought to distinguish whether any changes in the proportions of the principal H1 histone subspecies (H1A, H1B, and H1o) in late-passage fibroblasts were specific for senescent loss of replicative potential, or rather ensued as a result of prolonged inhibition of cell division. We observed an identical shift in the proportions of H1 histone subspecies during prolonged density-dependent inhibition of growth in both early-passage and late-passage cells. Since under these conditions there were no passage-specific changes, replicative senescence of human fibroblasts does not appear to involve a defect in the control of H1 histone proportions.

Kinetic analysis of psi-DNA structure formation induced by histone H1 and its C-terminal domain

In this paper we have studied the kinetics of psi-DNA structure formation induced by H1 and H1 peptides containing the C-terminal domain, namely the CTB peptide, obtained by thrombin digestion, and the CNBS peptide, derived from N-bromosuccinimide treatment of H1. The time course for the formation of the psi structure has been followed by measuring the changes in ellipticity at 270 nm as a function of time under different experimental conditions. In all cases studied here, we have observed the existence of two elementary processes: one fast, the other slow. Kinetic experiments performed with high molecular weight DNA showed that the greater the salt concentration, the higher was the apparent rate of psi structure formation. In complexes formed with sonicated DNA and H1, CNBS and CTB, we observed that the greater the content of the C-terminal domain, the higher was the apparent rate at which the final psi structure was reached. Thus, the presence of increasing amounts of either salt or C-terminal domain facilitates the formation of the psi structure. The molecular basis for these phenomena is discussed. The influence of the order of addition of the different components of the complex on the kinetics of psi structure induction is also studied.

Developmental regulation of two 5S ribosomal RNA genes

The developmental regulation of two kinds of Xenopus 5S RNA genes (oocyte and somatic types) can be explained by differences in the stability of protein-protein and protein-DNA interactions in a transcription complex that directs transcription initiation by RNA polymerase III. Dissociation of transcription factors from oocyte 5S RNA genes during development allows them to be repressed by chromatin assembly. In the same cells, somatic 5S RNA genes remain active because their transcription complexes are stable.

Developmental regulation of two 5S ribosomal RNA genes

The developmental regulation of two kinds of Xenopus 5S RNA genes (oocyte and somatic types) can be explained by differences in the stability of protein-protein and protein-DNA interactions in a transcription complex that directs transcription initiation by RNA polymerase III. Dissociation of transcription factors from oocyte 5S RNA genes during development allows them to be repressed by chromatin assembly. In the same cells, somatic 5S RNA genes remain active because their transcription complexes are stable.

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.

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.

Perturbation of chromatin structure in the region of the adult beta-globin gene in chicken erythrocyte chromatin

An EcoRI chromatin fragment containing the adult beta-globin gene and flanking sequences, isolated from chicken erythrocyte nuclei, sediments at a reduced rate relative to bulk chromatin fragments of the same size. We show that the specific retardation cannot be reversed by adding extra linker histones to native chromatin. When the chromatin fragments are unfolded either by removing linker histones or lowering the ionic strength, the difference between globin and bulk chromatin fragments is no longer seen. The refolded chromatin obtained by restoring the linker histones to the depleted chromatin, however, exhibits the original sedimentation difference. This difference is therefore due to a special property of the histone octamers on the active gene that determines the extent of its folding into higher-order structure. That it is not due to the differential binding of linker histones in vitro is shown by measurements of the protein to DNA ratios using CsCl density-gradients. Both before and after selective removal of the linker histones, the globin gene fragment and bulk chromatin fragments exhibit only a marginal difference in buoyant density. In addition, we show that cleavage of the EcoRI fragment by digestion at the 5' and 3' nuclease hypersensitive sites flanking the globin gene liberates a fragment from between these sites that sediments normally. We conclude that the hypersensitive sites per se are responsible for the reduction in sedimentation rate. The non-nucleosomal DNA segments appear to be too long to be incorporated into the chromatin solenoid and thus create spacers between separate solenoidal elements in the chromatin, which can account for its hydrodynamic behaviour.

An intervening sequence in an unusual histone H1 gene of Tetrahymena thermophila

An intervening sequence of 254 base pairs interrupts the coding region of the single gene for macronuclear histone H1 of the ciliated protozoan, Tetrahymena thermophila. The intervening sequence has splice junctions similar to those found in RNA polymerase II genes of other organisms. No obvious similarities are observed between this intron and the self-splicing intervening sequence of the Tetrahymena ribosomal gene. The derived amino acid sequence describes a small extremely basic H1 protein missing most of the central hydrophobic domain that is conserved in all other H1 proteins. Macronuclei divide amitotically, without chromosome condensation, suggesting the conserved globular domain of H1 plays a role in higher-order chromatin structure.

Dynamic behavior of histone H1 microinjected into HeLa cells

Histone H1 was purified from bovine thymus and radiolabeled with tritium by reductive methylation or with 125I using chloramine-T. Red blood cell-mediated microinjection was then used to introduce the labeled H1 molecules into HeLa cells synchronized in S phase. The injected H1 molecules rapidly entered HeLa nuclei, and a number of tests indicate that their association with chromatin was equivalent to that endogenous histone H1. The injected molecules copurified with HeLa cell nucleosomes, exhibited a half-life of approximately 100 h, and were hyperphosphorylated at mitosis. When injected HeLa cells were fused with mouse 3T3 fibroblasts less than 10% of the labeled H1 molecules migrated to mouse nuclei during the next 48 h. Thus, the intracellular behavior of histone H1 differs markedly from that of high mobility group proteins 1 and 2 (HMG1 and HMG2), which rapidly equilibrate between human and mouse nuclei after heterokaryon formation (Rechsteiner, M., and L. Kuehl, 1979, Cell, 16:901-908; Wu, L., M. Rechsteiner, and L. Kuehl, 1981, J. Cell Biol, 91: 488-496). Despite their slow rate of migration between nuclei, the injected H1 molecules were evenly distributed on mouse and human genomes soon after mitosis of HeLa-3T3 heterokaryons. These results suggest that although most histone H1 molecules are stably associated with interphase chromatin, they undergo extensive redistribution after mitosis.

Cooperative interaction of histone H1 with DNA

The cooperative binding of histone H1 with DNA was studied using a fluorescently labelled histone H1. The titration data were analysed in terms of the large ligand model. The stoichiometric number, n = 65 +/- 10 bases/H1, was independent of NaCl concentration (0.02 - 0.35 M). The nucleation and the cooperative binding constants, K' and K, and the cooperativity parameter q were sensitive to salt concentration; K = 3.6 +/- 0.8 X 10(7) M-1 and q = 1.1 +/- 0.4 X 10(3) at 0.2 M NaCl. The dependence of K' on NaCl concentration revealed that 6 Na+ ions were released from DNA upon complex formation. An extrapolation of K' to 1M NaCl yielded a small value, K' = 5 +/- 2 M-1. Thus the binding of H1 is essentially electrostatic, being compatible with its independence of temperature. A calculation of K' based on the counterion release reproduced the salt concentration dependence of K'. Therefore, the binding of H1 is of an electrostatic territorial type. Thus, H1 may move along the DNA chain to a certain extent, when both salt concentration and the degree of saturation are sufficiently low. The condition is so restricted that the sliding would not play an important role in vivo. It was concluded from the DNA concentration independent binding isotherm that H1 can cooperatively bind onto a single DNA molecule. A simple power law dependence of the cooperativity parameter q upon NaCl concentration was found; q oc[NaCl]h with h = 0.72, though the physical basis of this dependence remains unknown.

Effect of trypsinization and histone H5 addition on DNA twist and topology in reconstituted minichromosomes

Free DNA in solution exhibits an untwisting of the double helix with increasing temperature. We have shown previously that when DNA is reconstituted with histones to form nucleosome core particles, both the core DNA and the adjacent linker DNA are constrained from thermal untwisting. The origin of this constraint is unknown. Here we examine the effect of two modifications of nucleosome structure on the constraint against thermal untwisting, and also on DNA topology. In one experiment, we removed the highly positively charged histone amino and carboxy termini by trypsinization. Alternatively, we added histone H5, a histone H1 variant from chick erythrocytes. Neither of these modifications had any major effect on DNA topology or twist in the nucleosome.

Roles of H1 domains in determining higher order chromatin structure and H1 location

Peptides derived from calf thymus H1 and rat liver H1, comprising only the globular and COOH-terminal domains of the intact molecule and therefore lacking NH2-terminal domains, have been shown by reconstitution to be as effective as the complete H1 molecule in inducing higher-order-chromatin structure. As the globular domain of H1 alone cannot induce chromatin folding, our results demonstrate that this function is primarily controlled by the COOH-terminal domain of the molecule. Surprisingly, these peptides do not locate correctly with respect to the nucleosome. This is demonstrated by their failure to confer upon reconstitutes the ability to protect DNA fragments of chromatosome length when digested with micrococcal nuclease. The precise placement of the H1 molecule (globular domain) with respect to the nucleosome is shown to be influenced by the "tail" domains of both H1 and the core histones.