Transcription complex disruption caused by a transition in chromatin structure

Developmental Control of Cell Cycle and Signaling

In most species, the earliest stages of embryogenesis are characterized by rapid proliferation, which must be tightly controlled with other cellular processes across the large scale of the embryo. The study of this coordination has recently revealed new mechanisms of regulation of morphogenesis. Here, I discuss progress on how the integration of biochemical and mechanical signals leads to the proper positioning of cellular components, how signaling waves ensure the synchronization of the cell cycle, and how cell cycle transitions are properly timed. Similar concepts are emerging in the control of morphogenesis of other tissues, highlighting both common and unique features of early embryogenesis.

Local nuclear to cytoplasmic ratio regulates H3.3 incorporation via cell cycle state during zygotic genome activation

Early embryos often have unique chromatin states prior to zygotic genome activation (ZGA). In Drosophila, ZGA occurs after 13 reductive nuclear divisions during which the nuclear to cytoplasmic (N/C) ratio grows exponentially. Previous work found that histone H3 chromatin incorporation decreases while its variant H3.3 increases leading up to ZGA. In other cell types, H3.3 is associated with sites of active transcription and heterochromatin, suggesting a link between H3.3 and ZGA. Here, we test what factors regulate H3.3 incorporation at ZGA. We find that H3 nuclear availability falls more rapidly than H3.3 leading up to ZGA. We generate H3/H3.3 chimeric proteins at the endogenous H3.3A locus and observe that chaperone binding, but not gene structure, regulates H3.3 behavior. We identify the N/C ratio as a major determinant of H3.3 incorporation. To isolate how the N/C ratio regulates H3.3 incorporation we test the roles of genomic content, zygotic transcription, and cell cycle state. We determine that cell cycle regulation, but not H3 availability or transcription, controls H3.3 incorporation. Overall, we propose that local N/C ratios control histone variant usage via cell cycle state during ZGA.

The Nuclear-to-Cytoplasmic Ratio: Coupling DNA Content to Cell Size, Cell Cycle, and Biosynthetic Capacity

Though cell size varies between different cells and across species, the nuclear-to-cytoplasmic (N/C) ratio is largely maintained across species and within cell types. A cell maintains a relatively constant N/C ratio by coupling DNA content, nuclear size, and cell size. We explore how cells couple cell division and growth to DNA content. In some cases, cells use DNA as a molecular yardstick to control the availability of cell cycle regulators. In other cases, DNA sets a limit for biosynthetic capacity. Developmentally programmed variations in the N/C ratio for a given cell type suggest that a specific N/C ratio is required to respond to given physiological demands. Recent observations connecting decreased N/C ratios with cellular senescence indicate that maintaining the proper N/C ratio is essential for proper cellular functioning. Together, these findings suggest a causative, not simply correlative, role for the N/C ratio in regulating cell growth and cell cycle progression.

From mother to embryo: A molecular perspective on zygotic genome activation

In animals, the early embryo is mostly transcriptionally silent and development is fueled by maternally supplied mRNAs and proteins. These maternal products are important not only for survival, but also to gear up the zygote's genome for activation. Over the last three decades, research with different model organisms and experimental approaches has identified molecular factors and proposed mechanisms for how the embryo transitions from being transcriptionally silent to transcriptionally competent. In this chapter, we discuss the molecular players that shape the molecular landscape of ZGA and provide insights into their mode of action in activating the transcription program in the developing embryo.

Histone concentration regulates the cell cycle and transcription in early development

The early embryos of many animals, including flies, fish and frogs, have unusually rapid cell cycles and delayed onset of transcription. These divisions are dependent on maternally supplied RNAs and proteins including histones. Previous work suggests that the pool size of maternally provided histones can alter the timing of zygotic genome activation (ZGA) in frogs and fish. Here, we examine the effects of under- and overexpression of maternal histones in Drosophila embryogenesis. Decreasing histone concentration advances zygotic transcription, cell cycle elongation, Chk1 activation and gastrulation. Conversely, increasing histone concentration delays transcription and results in an additional nuclear cycle before gastrulation. Numerous zygotic transcripts are sensitive to histone concentration, and the promoters of histone-sensitive genes are associated with specific chromatin features linked to increased histone turnover. These include enrichment of the pioneer transcription factor Zelda, and lack of SIN3A and associated histone deacetylases. Our findings uncover a crucial regulatory role for histone concentrations in ZGA of Drosophila.

Repair of UV induced DNA lesions in ribosomal gene chromatin and the role of "Odd" RNA polymerases (I and III)

In fast growing eukaryotic cells, a subset of rRNA genes are transcribed at very high rates by RNA polymerase I (RNAPI). Nuclease digestion-assays and psoralen crosslinking have shown that they are open; that is, largely devoid of nucleosomes. In the yeast Saccharomyces cerevisae, nucleotide excision repair (NER) and photolyase remove UV photoproducts faster from open rRNA genes than from closed and nucleosome-loaded inactive rRNA genes. After UV irradiation, rRNA transcription declines because RNAPI halt at UV photoproducts and are then displaced from the transcribed strand. When the DNA lesion is quickly recognized by NER, it is the sub-pathway transcription-coupled TC-NER that removes the UV photoproduct. If dislodged RNAPI are replaced by nucleosomes before NER recognizes the lesion, then it is the sub-pathway global genome GG-NER that removes the UV photoproducts from the transcribed strand. Also, GG-NER maneuvers in the non-transcribed strand of open genes and in both strands of closed rRNA genes. After repair, transcription resumes and elongating RNAPI reopen the rRNA gene. In higher eukaryotes, NER in rRNA genes is inefficient and there is no evidence for TC-NER. Moreover, TC-NER does not occur in RNA polymerase III transcribed genes of both, yeast and human fibroblast.

Propagation of histone marks and epigenetic memory during normal and interrupted DNA replication

Although all nucleated cells within a multicellular organism contain a complete copy of the genome, cell identity relies on the expression of a specific subset of genes. Therefore, when cells divide they must not only copy their genome to their daughters, but also ensure that the pattern of gene expression present before division is restored. While the carrier of this epigenetic memory has been a topic of much research and debate, post-translational modifications of histone proteins have emerged in the vanguard of candidates. In this paper we examine the mechanisms by which histone post-translational modifications are propagated through DNA replication and cell division, and we critically examine the evidence that they can also act as vectors of epigenetic memory. Finally, we consider ways in which epigenetic memory might be disrupted by interfering with the mechanisms of DNA replication.

Maintenance of gene expression patterns

In development, cells pass on established gene expression patterns to daughter cells over multiple rounds of cell division. The cellular memory of the gene expression state is termed maintenance, and the proteins required for this process are termed maintenance proteins. The best characterized are proteins of the Polycomb and trithorax Groups that are required for silencing and maintenance of activation of target loci, respectively. These proteins act through DNA elements termed maintenance elements. Here, we re-examine the genetics and molecular biology of maintenance proteins. We discuss molecular models for the maintenance of activation and silencing, and the establishment of epigenetic marks, and suggest that maintenance proteins may play a role in propagating the mark through DNA synthesis.

Xenopus transcription factor IIIA and the 5S nucleosome: development of a useful in vitro system

5S RNA genes in Xenopus are regulated during development via a complex interplay between assembly of repressive chromatin structures and productive transcription complexes. Interestingly, 5S genes have been found to harbor powerful nucleosome positioning elements and therefore have become an important model system for reconstitution of eukaryotic genes into nucleosomes in vitro. Moreover, the structure of the primary factor initiating transcription of 5S DNA, transcription factor IIIA, has been extensively characterized. This has allowed for numerous studies of the effect of nucleosome assembly and histone modifications on the DNA binding activity of a transcription factor in vitro. For example, linker histones bind 5S nucleosomes and repress TFIIIA binding in vitro in a similar manner to that observed in vivo. In addition, TFIIIA binding to nucleosomes assembled with 5S DNA is stimulated by acetylation or removal of the core histone tail domains. Here we review the development of the Xenopus 5S in vitro system and discuss recent results highlighting new aspects of transcription factor - nucleosome interactions,

CAF-1 and the inheritance of chromatin states: at the crossroads of DNA replication and repair

Chromatin is no longer considered to be a static structural framework for packaging DNA within the nucleus but is instead believed to be an interactive component of DNA metabolism. The ordered assembly of chromatin produces a nucleoprotein template capable of epigenetically regulating the expression and maintenance of the genome. Factors have been isolated from cell extracts that stimulate early steps in chromatin assembly in vitro. The function of one such factor, chromatin-assembly factor 1 (CAF-1), might extend beyond simply facilitating the progression through an individual assembly reaction to its active participation in a marking system. This marking system could be exploited at the crossroads of DNA replication and repair to monitor genome integrity and to define particular epigenetic states.

Epigenetics: regulation through repression

Epigenetics is the study of heritable changes in gene expression that occur without a change in DNA sequence. Epigenetic phenomena have major economic and medical relevance, and several, such as imprinting and paramutation, violate Mendelian principles. Recent discoveries link the recognition of nucleic acid sequence homology to the targeting of DNA methylation, chromosome remodeling, and RNA turnover. Although epigenetic mechanisms help to protect cells from parasitic elements, this defense can complicate the genetic manipulation of plants and animals. Essential for normal development, epigenetic controls become misdirected in cancer cells and other human disease syndromes.

Hepatocyte nuclear factor 3 relieves chromatin-mediated repression of the alpha-fetoprotein gene

The alpha-fetoprotein gene (AFP) is tightly regulated at the tissue-specific level, with expression confined to endoderm-derived cells. We have reconstituted AFP transcription on chromatin-assembled DNA templates in vitro. Our studies show that chromatin assembly is essential for hepatic-specific expression of the AFP gene. While nucleosome-free AFP DNA is robustly transcribed in vitro by both cervical (HeLa) and hepatocellular (HepG2) carcinoma extracts, the general transcription factors and transactivators present in HeLa extract cannot relieve chromatin-mediated repression of AFP. In contrast, preincubation with either HepG2 extract or HeLa extract supplemented with recombinant hepatocyte nuclear factor 3 alpha (HNF3alpha), a hepatic-enriched factor expressed very early during liver development, is sufficient to confer transcriptional activation on a chromatin-repressed AFP template. Transient transfection studies illustrate that HNF3alpha can activate AFP expression in a non-liver cellular environment, confirming a pivotal role for HNF3alpha in establishing hepatic-specific gene expression. Restriction enzyme accessibility assays reveal that HNF3alpha promotes the assembly of an open chromatin structure at the AFP promoter. Combined, these functional and structural data suggest that chromatin assembly establishes a barrier to block inappropriate expression of AFP in non-hepatic tissues and that tissue-specific factors, such as HNF3alpha, are required to alleviate the chromatin-mediated repression.

The human H2A and H2B histone gene complement

Sequences and expression patterns of newly isolated human histone H2A and H2B genes and the respective proteins were compared with previously isolated human H2A and H2B genes and proteins. Altogether, 15 human H2A genes and 17 human H2B genes have been identified. 14 of these are organized as H2A/H2B gene pairs, while one H2A gene and three H2B genes are solitary genes. Two H2A genes and two H2B genes turned outto be pseudogenes. The 13 H2A genes code for at least 6 different amino acid sequences, and the 15 H2B genes encode 11 different H2B isoforms. Each H2A/H2B gene pair is controlled by a divergent promoter spanning 300 to 330 nucleotides between the coding regions of the two genes. The highly conserved divergent H2A/H2B promoters can be classified in two groups based on the patterns of consensus sequence elements. Group I promoters contain a TATA box for each gene, two Oct-1 factor binding sites, and three CCAAT boxes. Group II promoters contain the same elements as group I promoters and an additional CCAAT box, a binding motif for E2F and adjacent a highly conserved octanucleotide (CACAGCTT) that has not been described so far. Five of the 6 gene pairs and 4 solitary genes with group I promoters are localized in the large histone gene cluster at 6p21.3-6p22, and one gene pair is located at 1q21. All group II promoter associated genes are contained within the histone gene subcluster at D6S105, which is located at a distance of about 2 Mb from the major subcluster at 6p21.3-6p22 containing histone genes with group I promoters. Almost all group II H2A genes encode identical amino acid sequences, whereas group I H2A gene products vary at several positions. Using human cell lines, we have analyzed the expression patterns of functional human H2A/H2B gene pairs organized within the two histone gene clusters on the short arm of chromosome 6. The genes show varying expression patterns in different tumor cell lines.

The nucleosome: a powerful regulator of transcription

Nucleosomes provide the architectural framework for transcription. Histones, DNA elements, and transcription factors are organized into precise regulatory complexes. Positioned nucleosomes can facilitate or impede the transcription process. These structures are dynamic, reflecting the capacity of chromatin to adopt different functional states. Histones are mobile with respect to DNA sequence. Individual histone domains are targeted for posttranslational modifications. Histone acetylation promotes transcription factor access to nucleosomal DNA and relieves inhibitory effects on transcriptional initiation and elongation. The nucleosomal infrastructure emerges as powerful contributor to the regulation of gene activity.

Functionally relevant histone-DNA interactions extend beyond the classically defined nucleosome core region

We demonstrate that core histones can affect the accessibility of a DNA element positioned outside of the classically defined nucleosome core region. The distance between a well positioned nucleosome and the binding site for the 5 S-specific transcription factor TFIIIA was systematically varied and the relative binding affinity for TFIIIA determined. We found that core histone-DNA interactions attenuate the affinity of TFIIIA for its cognate DNA element by a factor of 50-100-fold even when the critical binding region lies well outside of the classically defined nucleosome core region. These results have implications for the validity of parallels drawn between the accessibility of general nucleases to DNA sequences in chromatin and the activity of actual sequence-specific DNA binding factors.

Transduction of the human immunodeficiency virus type 1 promoter into human chromosomal DNA by adeno-associated virus: effects on promoter activity

Transcription of the human immunodeficiency virus type 1 (HIV-1) genome takes place after integration of the provirus into human chromosomal DNA. HIV transcription is known to be modulated by viral and cellular factors but the influence of flanking chromosomal sequences on proviral gene expression has not been well defined. To investigate the activity of the integrated HIV promoter, we exploited the ability of recombinant adeno-associated virus (AAV-2) to transfer and stably integrate genes into the human genome at random or site-specifically. Chimeric AAV vectors were constructed containing an HIV-CAT reporter cassette; some vectors also contained the neomycin resistance gene to facilitate the isolation of positive clones. HeLa cells were infected with recombinant AAV, in some instances together with wild-type virus as a source of AAV rep function. We isolated 25 clones of G418-resistant cells which carried the integrated HIV-CAT cassette, generally occupying unique sites that did not correspond to the AAV-specific region of chromosome 19. The HIV promoter was transcriptionally active in most of the clones. Basal promoter activity varied substantially among the clones, and its responsivity to the HIV transactivator Tat was also variable. The integrated HIV promoter was transactivated to comparable degrees by the one-exon form and two-exon form of Tat. These findings provide evidence that the transcriptional activity of the HIV promoter can be greatly influenced by the site of proviral insertion.

Chromatin assembly coupled to DNA repair: a new role for chromatin assembly factor I

DNA repair in the eukaryotic cell disrupts local chromatin organization. To investigate whether the resetting of nucleosomal arrays can be linked to the repair process, we developed model systems, with both Xenopus egg extract and human cell extracts, to follow repair and chromatin assembly in parallel on circular DNA templates. Both systems were able to carry out nucleotide excision repair of DNA lesions. We observed that UV-dependent DNA synthesis occurs simultaneously with chromatin assembly, strongly indicating a mechanistic coupling between the two processes. A complementation assay established that chromatin assembly factor I (CAF1) is necessary for this repair associated chromatin formation.

The interaction of transcription factors with nucleosomal DNA

Nucleosome positioning is proposed to have an essential role in facilitating the regulated transcription of eukaryotic genes. Some transcription factors can bind to DNA when it is appropriately wrapped around the histone core, others cannot bind due to the severe deformation of DNA structure. The staged assembly of nucleosomes and positioning of histone-DNA contacts away from promoter elements can facilitate the access of transcription factors to DNA. Positioned nucleosomes can also facilitate transcription through providing the appropriate scaffolding to bring regulatory factors bound at dispersed sites into juxtaposition.

Effects of the simian virus 40 origin of replication on transcription from the human immunodeficiency virus type 1 promoter

Positive and negative effects of DNA replication on gene transcription have been documented in a variety of systems. We examined the effects of the simian virus 40 (SV40) origin of replication on transcription from the human immunodeficiency virus type 1 (HIV-1) promoter, using a transient expression assay in COS-1 cells. The basal activity and Tat transactivation of the HIV promoter were greatly stimulated by the SV40 origin of replication independent of its position relative to the long terminal repeat. These effects were abolished by mutational inactivation of the SV40 origin and were reduced by a DNA replication inhibitor. The magnitude of promoter activation exceeded the increment expected from the increase in template number resulting from DNA replication. The SV40 T-antigen-induced DNA replication augmented the generation of both processive and nonprocessive HIV long terminal repeat-directed transcripts, and Tat primarily enhanced the initiation of those transcripts that were destined to be efficiently elongated. Our data suggest that the HIV promoter displays greater transcriptional activity on replicative DNA templates. This property may influence the activity of integrated HIV provirus and its transition from latency to productive infection.

The role of transcription factors, chromatin structure and DNA replication in 5 S RNA gene regulation

Differential expression of the oocyte and somatic 5 S RNA genes during Xenopus development can be explained by changes in transcription factor and histone interactions with the two types of gene. Both factors and histones bind 5 S RNA genes with specificity. Protein-protein interactions determine the stability of potentially transcriptionally active or repressed nucleoprotein complexes. A decline in transcription factor abundance, differential binding of transcription factors to oocyte and somatic 5 S genes, and increased competition with the histones for association with DNA during early embryogenesis, can account for the developmental decision to selectively repress the oocyte genes, while retaining the somatic genes in the transcriptionally active state. The 5 S ribosomal genes of Xenopus are perhaps the simplest eukaryotic genes to show regulated expression during development. A large multigene family (oocyte 5 S DNA) is transcriptionally active in oocytes but is repressed in somatic cells, whereas a small multigene family (somatic 5 S DNA) is active in both cell types. A potential molecular mechanism to explain the developmental switch that turns off oocyte 5 S DNA transcription has been experimentally reconstructed in vitro and more recently tested in vivo. Central to this mechanism is the specific association of both transcription factors and histones with 5 S RNA genes. How the interplay of histones and transcription factors is thought to affect transcription, and how their respective contributions might change during development from an oocyte, to an embryo and eventually to a somatic cell is the focus of this review.

Transcription: in tune with the histones

The regulation of transcription in eukaryotes relies upon the histone proteins in several essential ways. The incorporation of the binding sites for the basal transcriptional machinery into nucleosomes serves to repress transcription. Specific regulatory molecules other than the basal transcriptional machinery exist that can associate with nucleosomal DNA and initiate a chain of events that disrupt repressive histone-DNA complexes. The main players in this story have been defined physically and genetically and include positioned nucleosomes, interactions of the histone tetramer (H3-H4)2 with DNA, the N-terminal tails of histones H3 and H4, and a large general activator complex. How they fit together biochemically is yet to be defined. The genetic data demonstrate that the disruption of histone-DNA complexes plays a major role in the induction of transcription from many genes. However, not all genes are repressed by nucleosome assembly: certain promoters make use of the staged assembly of chromatin in vivo and a rapid and tight association of transacting factors with promoter elements to remain constitutively active. Moreover, nucleosome assembly is not necessarily repressive, since the folding of DNA by the histones can facilitate the activation of genes by bringing widely separated regulatory elements into juxtaposition. Thus, histones provide the necessary infrastructure for the correct and efficient operation of the transcriptional machinery; however, their exact contributions to the transcriptional regulation of an individual gene may depend on the spatial distribution of regulatory elements, the transcription factors involved, and the three-dimensional folding of DNA that they direct.

Inheritance of chromatin states

The packaging of regulatory DNA within the eukaryotic chromosome has considerable potential not only for modulating the transcriptional activity of genes, but also for propagating states that are permissive or restrictive for transcription. Sequence-specific transcription factors, histones and their modifications, chromodomain proteins and enzymes that modify histones, DNA methylation and proteins that recognize methylated DNA could all play independent or interrelated roles in regulating gene activity. They all also have the potential of propagating their interactions with nascent DNA following replication. However, observations on the phenomenon of X chromosome inactivation suggest that the formation and stability of specific histone-DNA interactions through replication may be central to the inheritance of chromatin states, and that other molecular mechanisms have supporting roles. The future offers the exciting prospect of reconstructing the propagation of stable active or repressed chromatin states in vitro, and consequently understanding the events occurring at the replication fork in molecular detail.

Parental nucleosomes segregated to newly replicated chromatin are underacetylated relative to those assembled de novo

Antibodies specific for acetylated histone H4 were used to examine the acetylation state of parental histones that segregate to newly replicated DNA. To generate newly replicated chromatin containing only segregated parental nucleosomes, isolated nuclei were labeled with [3H]TTP in vitro; alternatively, whole cells were labeled with [3H]thymidine in the presence of cycloheximide. Soluble chromatin was prepared by micrococcal nuclease digestion, and subjected to immunoprecipitation with "penta" antibodies (Lin et al., 1989). In sharp contrast to nucleosomes containing newly synthesized, diacetylated H4 (Perry et al., 1993), chromatin replicated in vitro was only marginally susceptible to immunoprecipitation. Control experiments established that bona fide acetylated chromatin was selectively immunoprecipitated by the same techniques and that segregated nucleosomes were not disassembled during treatment with "penta" antibodies. When replication was coupled to an in vitro histone acetylation system, the enrichment for segregated nucleosomes in the immunopellet increased approximately 3-fold, demonstrating that changes in the acetylation state of segregated histones can be detected immunologically and that parental histones on new DNA are accessible to acetyltransferases during, or immediately after, DNA replication. In vivo pulse-chase experiments, performed in the presence of cycloheximide, confirmed these results. Uptake experiments further established that concurrent histone acetylation did not alter the rate of DNA synthesis in vitro. Our results provide evidence that replication-competent chromatin is not obligatorily acetylated, and indicate that the acetylation status of segregated histones may be maintained during chromatin replication. The possible significance of this, with respect to the regulation of chromatin higher order structures during DNA replication, and the propagation of transcriptionally active vs inactive chromatin structures, is discussed.

Histone deacetylase. A key enzyme for the binding of regulatory proteins to chromatin

Core histones can be modified by reversible, posttranslational acetylation of specific lysine residues within the N-terminal protein domains. The dynamic equilibrium of acetylation is maintained by two enzyme activities, histone acetyltransferase and histone deacetylase. Recent data on histone deacetylases and on anionic motifs in chromatin- or DNA-binding regulatory proteins (e.g. transcription factors, nuclear proto-oncogenes) are summarized and united into a hypothesis which attributes a key function to histone deacetylation for the binding of regulatory proteins to chromatin by a transient, specific local increase of the positive charge in the N-terminal domains of nucleosomal core histones. According to our model, the rapid deacetylation of distinct lysines in especially H2A and H2B would facilitate the association of anionic protein domains of regulatory proteins to specific nucleosomes. Therefore histone deacetylation (histone deacetylases) may represent a unique regulatory mechanism in the early steps of gene activation, in contrast to the more structural role of histone acetylation (histone acetyltransferases) for nucleosomal transitions during the actual transcription process.

A positive role for histone acetylation in transcription factor access to nucleosomal DNA

Acetylation of the N-terminal tails of the core histones directly facilitates the recognition by TFIIIA of the 5S RNA gene within model chromatin templates. This effect is independent of a reduction in the extent of histone-DNA interactions or a change in DNA helical repeat; it is also independent of whether a histone tetramer or octamer inhibits TFIIIA binding. Removal of the N-terminal tails from the core histones also facilitates the association of TFIIIA with nucleosomal templates. We suggest that the histone tails have a major role in restricting transcription factor access to DNA and that their acetylation releases this restriction by directing dissociation of the tails from DNA and/or inducing a change in DNA configuration on the histone core to allow transcription factor binding. Acetylation of core histones might be expected to exert a major influence on the accessibility of chromatin to regulatory molecules.

Influence of chromatin folding on transcription initiation and elongation by RNA polymerase III

Nucleosomes were assembled onto either closed circular plasmids containing a single Xenopus 5S RNA gene or a linear tandemly repeated array of Lytechinus 5S RNA genes. Both chromatin templates were found to vary in their extent of compaction, depending upon the type and concentration of cation in solution. Compaction of these chromatin templates led to a significant inhibition of both transcription initiation and elongation by RNA polymerase III. Thus, the transcriptional repression observed after incorporation of genes into chromatin depends not only on occlusion of the promoter elements through direct contact with histones but also on compaction of nucleosomal arrays which occurs under the conditions of the transcription reactions.

Chromatin dynamics and the modulation of genetic activity

Chromatin, the genetic material of eukaryotes, is a dynamic macromolecular assembly that continuously changes its composition and conformation to accommodate different stages of genetic activity, e.g. transcription and replication. Evidence is accumulating that the dynamic behavior of chromatin has important functional roles in the modulation of genetic activity, largely due to the intrinsic properties of arrays of nucleosome cores.