Modulation of nucleosome structure by histone subtypes in sea urchin embryos

Histone variants of H2A and H3 families are regulated during in vitro aging in the same manner as during differentiation

In a previous communication, we showed that the H2A.1/H2A.2 histone variant ratio decreases in a linear manner during the in vitro aging of human diploid fibroblasts. This ratio is known to decrease in the same manner in progressive stages of development and in the process of differentiation, and is thus considered to be a biochemical marker for differentiation. A detailed analysis of the synthesis of H2A and H3 histone variants as a function of cumulative population doublings in the same in vitro cell system is presented in this study. Quantitative analysis of these variants in the G0 phase, synchronized fibroblasts has shown that their relative amount in chromatin, as well as their biosynthesis rate, change during in vitro aging of human diploid fibroblasts, revealing both up-and down-regulation of certain variants as a function of cumulative population doublings. Furthermore, we show by morphometric studies employing the seven distinct fibroblast morphotypes, as described by the Bayreuther classification, that this regulation is attributable to the replicative sub-populations. These results reveal that histone variants of the H2A and H3 families are regulated during in vitro aging in the same manner as that during differentiation.

Alteration of nucleosome structure as a mechanism of transcriptional regulation

The nucleosome, which is the primary building block of chromatin, is not a static structure: It can adopt alternative conformations. Changes in solution conditions or changes in histone acetylation state cause nucleosomes and nucleosomal arrays to behave with altered biophysical properties. Distinct subpopulations of nucleosomes isolated from cells have chromatographic properties and nuclease sensitivity different from those of bulk nucleosomes. Recently, proteins that were initially identified as necessary for transcriptional regulation have been shown to alter nucleosomal structure. These proteins are found in three types of multiprotein complexes that can acetylate nucleosomes, deacetylate nucleosomes, or alter nucleosome structure in an ATP-dependent manner. The direct modification of nucleosome structure by these complexes is likely to play a central role in appropriate regulation of eukaryotic genes.

The embryonic enhancer-binding protein SSAP contains a novel DNA-binding domain which has homology to several RNA-binding proteins

Stage-specific activator protein (SSAP) is a 43-kDa polypeptide that binds to an enhancer element of the sea urchin late histone H1 gene. This enhancer element mediates the transcriptional activation of the late histone H1 gene in a temporally specific manner at the mid-blastula stage of embryogenesis. We have cloned cDNAs encoding SSAP by using polyclonal antibodies raised against purified SSAP to screen expression libraries. SSAP is unrelated to previously characterized transcription factors; however, it exhibits striking homology to a large family of proteins involved in RNA processing. The protein is a sequence-specific DNA-binding protein that recognizes both single- and double-stranded DNA. The DNA-binding domain of the protein was localized to the conserved RNA recognition motif (RRM). In addition to tandem copies of this conserved domain, SSAP contains a central domain that is rich in glutamine and glycine and a C-terminal domain that is enriched in serine, threonine, and basic amino acids. Overexpression of SSAP in sea urchin embryos by microinjection of either synthetic mRNA or an SSAP expression vector results in four- to eightfold transactivation of target reporter genes that contain the enhancer sequence. Transactivation occurs beginning only at the mid-blastula stage of development, suggesting that SSAP must be modified in a stage-specific manner in order to activate transcription. In addition, there are a number of other RRM-containing proteins that contain glutamine-rich regions which are postulated to function in the regulation of RNA processing. Instead, we suggest that SSAP is a member of a family of glutamine-rich RRM proteins which constitute a novel class of transcription factors.

Distribution of globin genes and histone variants in micrococcal nuclease-generated subfractions of chromatin from Friend erythroleukemia cells at different malignant states

The distribution of the alpha and beta-globin genes and histone variants was examined in micrococcal nuclease-generated chromatin fractions of three Friend murine erythroleukemia cell types differing in malignant potential and inducibility to erythroid differentiation. A preferential concentration of globin gene sequences, as compared to satellite DNA, was noted in a physiological salt-soluble, histone H1-depleted, mononucleosomal chromatin fraction (Sup 120) in all Friend cell types, even those in which the globin gene was not capable of transcriptional activation by chemical induction. The level of globin gene enrichment in the Sup 120 fraction was highest in the most malignant and inducible cell type. The chemical induction of erythroid differentiation in this cell line did not change the distribution of globin genes in the chromatin fractions. The Sup 120 chromatin fraction prepared from mouse brain nuclei was not enriched in globin genes. Besides the previously reported low H2A. 1/H2A.2 ratio [Blankstein and Levy: Nature 260:638-640, 1976], chromatin from the most tumorigenic cell type showed the lowest H2B.2 to H2B.1 ratio, highest levels of histone H4 acetylation, and the most pronounced change in relative amounts of two major electrophoretic bands of histone H1 variants as compared to the less malignant cell types. The histone variant content of the micrococcal nuclease-generated chromatin fractions from the three Friend cell types reflected the core histone variant differences for the respective intact nuclei. However, the electrophoretic separation of mononucleosomes by size revealed several classes with different H2A variant ratios. The results demonstrate the existence of structural differences in globin gene and histone variants in erythroleukemia cell chromatin associated with distinguishable phenotypes during malignant cell progression.

Trypanosoma brucei brucei: differences in the nuclear chromatin of bloodstream forms and procyclic culture forms

Nucleosome filaments of two stages of the life-cycle of Trypanosoma brucei brucei, namely bloodstream forms and procyclic culture forms, were investigated by electron microscopy. Chromatin of bloodstream forms showed a salt-dependent condensation. The level of condensation was higher than that shown by chromatin from procyclic culture forms, but 30 nm fibres as formed in rat liver chromatin preparations were not found. Analysis of histones provided new evidence for the existence of H1-like proteins, which comigrated in the region of the core histones in SDS-PAGE and in front of the core histones in Triton acid urea gels. Differences were found between the H1-like proteins of the two trypanosome stages as well as between the core histones in their amount, number of bands and banding pattern. It can be concluded that T. b. brucei contains a full set of histones, including H1-like proteins, and that the poor condensation of its chromatin is not due to the absence of H1, but most probably due to histone-DNA interaction being weak. It is obvious that structural and functional differences of the chromatin exist not only between T. b. brucei and higher eukaryotes, but also between various stages of the life-cycle of the parasite. It is therefore not adequate to investigate the chromatin only of the procyclic culture forms as a model for all stages of the life-cycle of T. b. brucei.

The histones of Caenorhabditis elegans: no evidence of stage-specific isoforms. An overview

The nematode Caenorhabditis elegans expresses one species of H2A and one species of H4 molecules, at least two species of H1 (H1.1, H1.2), two species of H2B (H2B.1, H2B.2) and 2-4 species of H3 (H3.1 and H3.3 and an unassigned Ile/Leu microheterogeneity in H3). The study of their primary structures has been completed now and all of them, with the exception of the Ile/Leu microheterogeneity in H3, have been assigned to protein spots on two-dimensional gels. One spot, previously designated H3.2, probably represents C-terminally cleaved H3.1. The relative abundance of the isohistones was essentially the same when derived from either eggs, gravid adults or postreproductive, senescent worms. The degree of post-translational modification, however, particularly acetylation of H2A, H2B and H3 histone species, was reduced at old age.

Histone variants in mouse centromeric chromatin

Highly purified centromeric heterochromatin was isolated from mouse liver nuclei and the pattern of core histone variants was analyzed. In comparison with total chromatin, the centromeric heterochromatin of young animals was characterized by (1) enrichment in the replication-dependent variants H2A1, H2B2 and H3(2), (2) reduced amount of the minor variant H2Az and (3) absence of ubiquitinated molecules of H2A. This specific variant pattern changed upon ageing as a result of accumulation of replacement variants so that in adult animals both chromatin preparations exhibited similar pattern for H2A and H2B, while the difference in the profile of H3 variants was preserved.

Histone gene switching in murine erythroleukemia cells is differentiation specific and occurs without loss of cell cycle regulation

We investigated the expression characteristics of the fully replication-dependent (FRD) and the partially replication-dependent (PRD) histone gene variants by measuring changes in steady-state mRNA levels during hexamethylene bisacetamide (HMBA)-induced differentiation of murine erythroleukemia (MEL) cells. Between 24 and 60 h after induction, there was a dramatic switch in histone gene expression, such that the ratio of PRD to FRD transcripts increased severalfold over that found in uninduced MEL cells. We demonstrated that this gene switching was not simply a partial or complete uncoupling of PRD gene expression from DNA synthesis. PRD and FRD transcript levels were regulated coordinately upon treatment of uninduced or induced MEL cells with inhibitors of DNA synthesis, protein synthesis, or both. Using several criteria, we were unable to detect any difference in PRD and FRD gene expression under any conditions except in cells undergoing differentiation. MEL cells were arrested at a precommitment stage of differentiation by induction with HMBA in the presence of dexamethasone (DEX). If DEX was subsequently removed, DNA synthesis resumed, the cells underwent commitment, and histone gene switching was observed. In contrast, if both DEX and HMBA were removed, DNA synthesis still resumed, but commitment did not occur and no gene switching was observed. These results imply that histone gene switching is intimately related to the differentiation process.

Histone gene switching in murine erythroleukemia cells is differentiation specific and occurs without loss of cell cycle regulation

We investigated the expression characteristics of the fully replication-dependent (FRD) and the partially replication-dependent (PRD) histone gene variants by measuring changes in steady-state mRNA levels during hexamethylene bisacetamide (HMBA)-induced differentiation of murine erythroleukemia (MEL) cells. Between 24 and 60 h after induction, there was a dramatic switch in histone gene expression, such that the ratio of PRD to FRD transcripts increased severalfold over that found in uninduced MEL cells. We demonstrated that this gene switching was not simply a partial or complete uncoupling of PRD gene expression from DNA synthesis. PRD and FRD transcript levels were regulated coordinately upon treatment of uninduced or induced MEL cells with inhibitors of DNA synthesis, protein synthesis, or both. Using several criteria, we were unable to detect any difference in PRD and FRD gene expression under any conditions except in cells undergoing differentiation. MEL cells were arrested at a precommitment stage of differentiation by induction with HMBA in the presence of dexamethasone (DEX). If DEX was subsequently removed, DNA synthesis resumed, the cells underwent commitment, and histone gene switching was observed. In contrast, if both DEX and HMBA were removed, DNA synthesis still resumed, but commitment did not occur and no gene switching was observed. These results imply that histone gene switching is intimately related to the differentiation process.

Chromosomal proteins of Arabidopsis thaliana

In plants with large genomes, each of the classes of the histones (H1, H2A, H2B, H3 and H4) are not unique polypeptides, but rather families of closely related proteins that are called histone variants. The small genome and preponderance of single-copy DNA in Arabidopsis thaliana has led us to ask if this plant has such families of histone variants. We have thus isolated histones from Arabidopsis and analyzed them on four polyacrylamide gel electrophoretic systems: an SDS system; an acetic acid-urea system; a Triton transverse gradient system; and a two-dimensional system combining SDS and Triton-acetic acid-urea systems. This approach has allowed us to identify all four of the nucleosomal core histones in Arabidopsis and to establish the existence of a set of H2A and H2B variants. Arabidopsis has at least four H2A variants and three H2B variants of distinct molecular weights as assessed by electrophoretic mobility on SDS-polyacrylamide gels. Thus, Arabidopsis displays a diversity in these histones similar to the diversity displayed by plants with larger genomes such as wheat.The high mobility group (HMG) non-histone chromatin proteins have attracted considerable attention because of the evidence implicating them as structural proteins of transcriptionally active chromatin. We have isolated a group of non-histone chromatin proteins from Arabidopsis that meet the operational criteria to be classed as HMG proteins and that cross-react with antisera to HMG proteins of wheat.

The effects of in vitro age and culture state on histone variant synthesis in human diploid fibroblasts

Histone variant synthesis patterns from human diploid fibroblast-like cells of different in vitro ages were determined during exponential growth, at confluence, and during low serum arrest. The results are reported as the ratios of H2A variant synthesis (H2A.1 and H2A.2/H2A.x and H2A.z) and H3 variant synthesis (H3.1 and H3.2/H3.3) that have been used to characterize individual cell cycle states. Hydroxyurea was employed in some experiments to reduce S phase cells. The results indicate that high population doubling level (PDL) cells move through the G1 phase of the division cycle during exponential growth and exist in the G0 cell cycle state at confluence and during low serum arrest. Low PDL cells, however, exist in the G1 cell cycle state at confluence and revert to a G0 state only after maintenance as quiescent populations. This would suggest that when stimulated high PDL cells cannot enter into S phase, they revert to a GO cell cycle state.

Histone hyperacetylation. Its effects on nucleosome core particle transitions

Effects of histone hyperacetylation on transitions of HeLa cell nucleosome core particles were studied. The transitions examined were induced by low salt concentrations, pH, temperature, and nondissociating high salt. Effects of salt dissociation were also examined. The low-salt transition was found to shift to higher ionic strength by approximately three fold for hyperacetylated particles, a change which may be due simply to the increased overall negative charge on the particles caused by acetylation of lysine residues. Some differences were also seen in the way in which core particles refold after exposure to very low salt (which induces a nonreversible change in the particles). Otherwise no significant effects of hyperacetylation were observed.

Reversibility of the low-salt transition of chromatin core particles

The low-salt transition of chromatin core particles is reversible if the monovalent cation concentration is kept above 0.2 mM. Exposure of the particles to salt concentrations below this value results in a nonreversible secondary transition. The nonreversible changes are relatively slow with a half-time of about 15 minutes. Once exposed to such low ionic strength, the particles then begin to refold with increasing salt in at least two steps over a much higher ionic strength range than is required for the usual low-salt transition. The refolding is very fast, with a half-time less than a minute. Small differences between particles which had or had not been exposed to very low salt persist even when the particles are returned to near physiological ionic strengths.

Isolation and characterization of the gene encoding the testis specific histone protein H2B-2 from the sea urchin Lytechinus pictus

We have cloned and characterized the gene encoding a sperm specific H2B-2 histone subtype from the sea urchin L. pictus. The gene is not clustered with any other histone genes. However, it resembles other histone genes in many respects. The gene contains no intervening sequences or polyadenylation signals. Like other histone genes it contains the conserved 3' hairpin loop sequence and CAAGAAAGA box necessary for the processing of the 3' end of the histone transcript. Upstream of the gene in addition to the TATA box, and two copies of inverted CCAAT Boxes is a conserved sequence element found in many H2B genes. Unlike most histone genes, the H2B-2 gene is expressed in a sex and tissue specific manner only in testis. The sperm H2B-2 subtype encoded by this gene is 142 amino acids in length. The larger size of this H2B protein is accounted for by residues in the N-terminus of the protein consisting of a series of pentapeptide repeats typical of this histone subtype.

Drosophila histone H2A.2 is associated with the interbands of polytene chromosomes

Drosophila chromatin contains two antigenically distinct H2A histones, H2A.1 and H2A.2. Indirect immunofluorescence analyses revealed that anti-H2A.1 binding was distributed throughout polytene chromosomes, whereas anti-H2A.2 binding was interband-specific. Thus, H2A.2 probably contributes to the less compacted structure of interbands. Since each band-interband region is thought to contain a single gene, our results suggest that the distribution of H2A.2 echoes the functional organization of the Drosophila genome. Similar H2A histones occur in eukaryotes ranging from protozoa to mammals. Their placement might be an important determinant of chromatin structure.

Histones and their modifications

Histones constitute the protein core around which DNA is coiled to form the basic structural unit of the chromosome known as the nucleosome. Because of the large amount of new histone needed during chromosome replication, the synthesis of histone and DNA is regulated in a complex manner. During RNA transcription and DNA replication, the basic nucleosomal structure as well as interactions between nucleosomes must be greatly altered to allow access to the appropriate enzymes and factors. The presence of extensive and varied post-translational modifications to the otherwise highly conserved histone primary sequences provides obvious opportunities for such structural alterations, but despite concentrated and sustained effort, causal connections between histone modifications and nucleosomal functions are not yet elucidated.

Distribution of histone variants in the sea urchin chromatin fractions obtained by selective micrococcal nuclease digestion

Chromatin fractions differing in their transcriptional activity were isolated by selective micrococcal nuclease digestion of nuclei from sea urchin embryos (Strongylocentrotus droebachiensis) at the gastrula and pluteus stage. The electrophoretic analysis of the chromatin proteins at the gastrula stage showed that a soluble, transcriptionally active fraction of chromatin was enriched with early variants of histones H1 and H2A. The early and late variants of histone H2A at the pluteus stage were distributed randomly between chromatin fractions. However, the content of both variants of histone H1 was essentially decreased in the soluble transcriptionally active fraction of chromatin.

Primary organization of nucleosomal core particles is invariable in repressed and active nuclei from animal, plant and yeast cells

A refined map for the linear arrangement of histones along DNA in nucleosomal core particles has been determined by DNA-protein crosslinking. On one strand of 145-bp core DNA, histones are aligned in the following order: (5') H2B25,35-H455,65-H375,85,95/H488-H2B105,11 5-H2A118-H3135,145/H2A145 (3') (the subscripts give approximate distance in nucleotides of the main histone contacts from the 5'-end). Hence, the histone tetramer (H3,H4)2 and two dimers (H2A-H2B) are arranged on double-stranded core DNA in a symmetrical and rather autonomous way: H2A/H3-(H2A-H2B)-(H3,H4)2-(H2B-H2A)-H3/H2A. The primary organization was found to be very similar in core particles isolated from repressed nuclei of sea urchin sperm and chicken erythrocytes, from active in replication and transcription nuclei of Drosophila embryos and yeast and from somatic cells of lily. These data show that (i) the core structure is highly conserved in evolution and (ii) the overall inactivation of chromatin does not affect the arrangement of histones along DNA and thus does not seem to be regulated on this level of the core structure.

N-Butyrate incubation of immature chicken erythrocytes preferentially enhances the solubility of beta A chromatin

The solubility of adult beta-globin chromatin (beta A chromatin) from immature chicken red blood cells can be controlled by the presence or absence of n-butyrate in a cell incubation medium. In the absence of n-butyrate, only a small percentage (approximately 4%) of the total beta A chromatin is in a soluble chromatin fraction following micrococcal nuclease digestion and centrifugation. This percentage increases to approximately 40-45% of the beta A chromatin if cells are incubated 1 hour in the presence of 10 mM sodium n-butyrate. The highest yield and enrichment of solubilized beta A chromatin is attained when 1-4% of the DNA is rendered acid soluble, and in buffers containing 1.5 - 5 mM MgCl2. The soluble beta A nucleohistone is nucleosome oligomer size (contains DNA 250-600 bases in length) and can be separated from soluble, transcriptionally inert mononucleosomes by agarose A-5m exclusion chromatography. The enhanced solubility appears to be specific for transcriptionally active chromatin. Whereas 40-45% of the beta A chromatin is recovered in the supernatant fraction from n-butyrate incubated immature erythrocytes, nucleohistone containing ovalbumin DNA sequences remains insoluble.

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.

Differences of supranucleosomal organization in different kinds of chromatin: cell type-specific globular subunits containing different numbers of nucleosomes

Fractions of homogeneously-sized supranucleosomal particles can be obtained in high yield and purity from various types of cells by brief micrococcal nuclease digestion (10 or 20 s) of condensed chromatin in 100 mM NaCl followed by sucrose gradient centrifugation and agarose gel electrophoresis. These chromatin particles, which contain only DNA and histones, differed according to cell type. Sea urchin spermatozoa (Paracentrotus lividus) gave rise to heavy particles (ca. 260 S) with a mean diameter (48 nm). These resembled the unit chromatin fibrils fixed in situ, which contain an average of 48 nucleosomes, as determined both by electron microscopy after unraveling in low salt buffer and gel electrophoresis. In contrast, higher order particles from chicken erythrocyte chromatin were smaller (105 S; 36-nm diam) and contained approximately 20 nucleosomes. The smallest type of supranucleosomal particle was obtained from chicken and rat liver (39 S; 32-nm diam; eight nucleosomes). Oligomeric chains of such granular particles could be recognized in regions of higher sucrose density, indicating that distinct supranucleosomal particles of globular shape are not an artifact of exposure to low salt concentrations but can be obtained at near-physiological ionic strength. The demonstration of different particle sizes in chromatin from different types of nuclei is contrary to the view that such granular particles are produced by artificial breakdown into "detached turns" from a uniform and general solenoid structure of approximately six nucleosomes per turn. Our observations indicate that the higher order packing of the nucleosomal chain can differ greatly in different types of nuclei and the supranucleosomal organization of chromatin differs between cell types and is related to the specific state of cell differentiation.

Timing of the appearance of macronuclear-specific histone variant hv1 and gene expression in developing new macronuclei of Tetrahymena thermophila

Vegetative cells of the ciliated protozoan Tetrahymena thermophila contain a transcriptionally active macronucleus and a transcriptionally inactive micronucleus. Earlier studies ( Allis , C. D., C. V. C. Glover , J. K. Bowen, and M. A. Gorovsky , 1980, Cell, 20:609-617; and Allis , C. D., Y. S. Ziegler , M. A. Gorovsky , and J. B. Olmsted, 1982, Cell, 31:131-136) demonstrated the existence of a macronuclear-specific histone variant, hv1 , which is enriched in small punctate regions in nucleoli of several mammalian cell lines. These observations suggest that this histone variant is highly conserved in evolution and may be associated with actively transcribed sequences. Despite large differences in structure and function during vegetative growth, macro- and micronuclei are related. During conjugation, the sexual phase of the life cycle in Tetrahymena, postzygotic division products of micronuclei give rise to new micro- and macronuclei, while the old macronucleus moves to the posterior of each cell and is eliminated. In this study using antiserum specific for hv1 , we determined by indirect immunofluorescence the time during conjugation at which hv1 first appears in the developing new macronuclei. In growing, starved, and young mating cells (2-5 h after mixing opposite mating types), only macronuclei are detected with affinity-purified antibodies against hv1 . Newly formed macronuclei are either not stained or only weakly stained in cells in which the old macronucleus is located in the center of the cell. However, new macronuclei are clearly observed in cells in which the old macronucleus has moved to the posterior of the cell (approximately 8 h). During later stages of conjugation (10-16 h), the intensity of hv1 staining in new macronuclei increases with time corresponding to the increasing DNA content of these nuclei. Disappearance of detectable hv1 from old macronuclei begins nearly 1 h after these nuclei reach the posterior cytoplasm (approximately 9-10 h) and is sometimes complete before these nuclei are eliminated from the cells. Autoradiography of cells labeled for brief periods with [3H]uridine shows that new macronuclei begin to synthesize RNA very soon after the second postzygotic division (approximately 8 h). During stages when hv1 is clearly detected in new macronuclei, anlagen are active in RNA synthesis. RNA synthesis in old macronuclei ceases very close to the time when RNA synthesis begins in new macronuclei. Thus, the addition of hv1 coincides closely with the transformation of a transcriptionally inactive germinal nucleus into that of a transcriptionally active somatic nucleus. We suspect that addition of hv1 plays a fundamental role in

H2A.F: an extremely variant histone H2A sequence expressed in the chicken embryo

A cDNA clone bank has been constructed from chicken embryonic RNA. Clones hybridizing poorly to embryonic histone gene probes were selected as possible variant gene transcripts. The DNA sequence of one cDNA predicts an extremely variant H2A protein (H2A.F), which is 40% divergent from the most abundant H2A protein in chicken erythrocyte chromatin. The H2A.F gene is not highly conserved across large species barriers, but in the chicken there may be a family of linked genes. The H2A.F mRNA is approximately equal to 820 base pairs in length and, unlike most other histone mRNAs, is polyadenylylated. Significantly, the H2A.F transcript shows a limited tissue distribution in the chicken embryo.

Molecular drive: a cohesive mode of species evolution

It is generally accepted that mutations may become fixed in a population by natural selection and genetic drift. In the case of many families of genes and noncoding sequences, however, fixation of mutations within a population may proceed as a consequence of molecular mechanisms of turnover within the genome. These mechanisms can be both random and directional in activity. There are circumstances in which the unusual concerted pattern of fixation permits the establishment of biological novelty and species discontinuities in a manner not predicted by the classical genetics of natural selection and genetic drift.