DNase I sensitivity in facultative and constitutive heterochromatin

Old cells, new tricks: chromatin structure in senescence

Cellular senescence is a stable form of cell cycle arrest with roles in many pathophysiological processes including development, tissue repair, cancer, and aging. Senescence does not represent a single entity but rather a heterogeneous phenotype that depends on the trigger and cell type of origin. Such heterogeneous features include alterations to chromatin structure and epigenetic states. New technologies are beginning to unravel the distinct mechanisms regulating chromatin structure during senescence. Here, we describe the multiple levels of chromatin organization associated with senescence: global and focal, linear, and higher order.

Epigenetic modifications in sex heterochromatin of vole rodents

The genome of some vole rodents contains large blocks of heterochromatin coupled to the sex chromosomes. While the DNA content of these heterochromatic blocks has been extensively analyzed, little is known about the epigenetic modifications controlling their structure and dynamics. To better understand its organization and functions within the nucleus, we have compared the distribution pattern of several epigenetic marks in cells from two species, Microtus agrestis and Microtus cabrerae. We first could show that the heterochromatic blocks are identifiable within the nuclei due to their AT enrichment detectable by DAPI staining. By immunostaining analyses, we demonstrated that enrichment in H3K9me3 and HP1, depletion of DNA methylation as well as H4K8ac and H3K4me2, are major conserved epigenetic features of this heterochromatin in both sex chromosomes. Furthermore, we provide evidence of transcriptional activity for some repeated DNAs in cultivated cells. These transcripts are partially polyadenylated and their levels are not altered during mitotic arrest. In summary, we show here that enrichment in H3K9me3 and HP1, DNA demethylation, and transcriptional activity are major epigenetic features of sex heterochromatin in vole rodents.

Role of histone deacetylases in gene regulation at nuclear lamina

Theoretical models suggest that gene silencing at the nuclear periphery may involve “closing” of chromatin by transcriptional repressors, such as histone deacetylases (HDACs). Here we provide experimental evidence confirming these predictions. Histone acetylation, chromatin compactness, and gene repression in lamina-interacting multigenic chromatin domains were analyzed in Drosophila S2 cells in which B-type lamin, diverse HDACs, and lamina-associated proteins were downregulated by dsRNA. Lamin depletion resulted in decreased compactness of the repressed multigenic domain associated with its detachment from the lamina and enhanced histone acetylation. Our data reveal the major role for HDAC1 in mediating deacetylation, chromatin compaction, and gene silencing in the multigenic domain, and an auxiliary role for HDAC3 that is required for retention of the domain at the lamina. These findings demonstrate the manifold and central involvement of class I HDACs in regulation of lamina-associated genes, illuminating a mechanism by which these enzymes can orchestrate normal and pathological development.

"Chromatomics" the analysis of the chromatome

Chromatin is a highly complex mixture of proteins and DNA that is involved in the regulation and coordination of gene expression within the eukaryotic nucleus. Changes in chromatin structure can convey heritable changes of gene activity in response to external stimuli without altering the primary DNA sequence. This epigenetic inheritance of particular traits very likely plays a major role during evolutionary processes. It is however, still ill-defined how this non DNA-mediated inheritance is accomplished at a molecular level. The advent of new methods to systematically study genome-wide changes in chromatin condensation, DNA methylation levels, RNA synthesis and the association of specific proteins or protein modifications now allows a thorough investigation of changes in chromatin structure and function in response to environmental alterations. We would like to review some of these global approaches and to introduce the term "chromatomics" for the systematic analysis of the DNA, RNA and protein content of the genetic material in the eukaryotic nucleus.

Epigenetic mechanisms for primary differentiation in mammalian embryos

This review examines main developments related to the interface between primary mammalian cell differentiation and various aspects of chromosomal structure changes, such as heterochromatin dynamics, DNA methylation, mitotic recombination, and inter- and intrachromosomal differentiation. In particular, X chromosome difference, imprinting, chromosomal banding, methylation pattern, single-strand DNA breaks, sister chromatid exchanges (SCEs), and sister chromatid asymmetry are considered. A hypothesis is put forward which implies the existence of an epigenetic asymmetry versus mirror symmetry of sister chromatids for any DNA sequences. Such epigenetic asymmetry appears as a result of asymmetry of sister chromatid organization and of SCE and is a necessary (not sufficient) condition for creating cell diversity. The sister chromatid asymmetry arises as a result of consecutive rounds of active and passive demethylation which leads after chromatin assembly events to chromatid difference. Single-strand DNA breaks that emerge during demethylation trigger reparation machinery, provend as sister chromatid exchanges, which are not epigenetically neutral in this case. Taken together, chromatid asymmetry and SCE lead to cell diversity regarding their future fate. Such cells are considered pluripotent stem cells which after interplay between a set of chromosomal domains and certain substances localized within the cytoplasmic compartments (and possibly cell interactions) can cause sister cells to express different gene chains. A model is suggested that may be useful for stem cell technology and studies of carcinogenesis.

Quantification of DNaseI-sensitivity by real-time PCR: quantitative analysis of DNaseI-hypersensitivity of the mouse beta-globin LCR

We employ real-time PCR to allow us to quantify the sensitivity of chromatin to digestion by DNaseI. This approach has three clear advantages over the more conventional use of the Southern hybridization assay: the accuracy of quantification is improved; the resolution of the assay is enhanced, by designing primers to amplify small amplicons it is possible to analyze sequences both co-incident and proximal to sites of DNaseI-hypersensitivity; less material is needed, as little as 5 ng of treated genomic DNA. We applied this method in an analysis of the chromatin structure of the previously described mouse beta-globin locus control region (LCR) using fetal liver cells. The four hypersensitive sites of the canonical mouse LCR, HS1 to HS4, are shown to have kinetics of digestion consistent with these sequences being nucleosome-free in vivo. A different pattern was seen for HS6, a recently described "weak" hypersensitive site. The site was also rapidly lost but more of the sites proved resistant, we interpreted this to show that this hypersensitive was only forming in a portion of the erythroid cells. This finding implies that in vivo the LCR is structurally heterogeneous. Sequences proximal to the hypersensitive sites show a third pattern of intermediate sensitivity, consistent with the chromatin being unfolded but the sites still bound by a continual nucleosomal array. Our results demonstrate that this method has the potential to achieve accurate and detailed mapping of chromatin structure from small amounts of tissue samples.

Ultrastructural methods for nucleic acid detection by immunocytology

In the present review are summarized recent developments in immunocytochemical detection of nucleic acids in biological materials at the ultrastructural level. Not only the approaches using antibodies to natural nucleic acids are described but also the techniques involving the use of antibodies raised against various nucleotide analogs incorporated beforehand into nucleic acids. Special emphasis is placed on each method's potential and limitations. These methods, combined or not with molecular biotechnology, are powerful tools for studying the structure and function of nucleic acids. They can be used to investigate the distribution and topological organization of DNA and RNA molecules or of specialized within these molecules in the cells.

Proteins recognized by antibodies against isolated cytological heterochromatin from rat liver cells change their localization between cell species and between stages of mitosis (interphase vs metaphase)

Heterochromatin in the cell nucleus seems to concentrate various proteins, such as Drosophila heterochromatin protein 1, which maintain the repressed state of gene expression. However, it still remains obscure how protein composition related to chromatin structure is different between heterochromatin and euchromatin in interphase nuclei. We isolated cytological heterochromatin from sonicated interphase nuclei obtained from rat liver cells and prepared antisera against it. The dense heterochromatic bodies seen in the preparation of intact nuclei were duplicated in a relatively pure form during the preparation of heterochromatin. In the sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis, differences between the fractions of heterochromatin and euchromatin were noted by their protein composition. Isolated heterochromatin was then digested by DNase after partial digestion with trypsin and its dense structure changed to become highly sensitive to DNase. The prepared antibodies reacted with the heterochromatin region of rat liver cell nuclei and isolated cytological heterochromatin; however, they did not react with euchromatin. Using immunohistochemistry, the antibodies bound to each cell nucleus in all tissues observed; some cell types were distinguished by their differential stainability (e.g. staining in the cytoplasm). Staining of the mitotic cells showed that the proteins recognized by the antibodies were localized in the cytoplasm and, in part, on the chromosomes. Based on the results of molecular cloning from rat liver cDNA library using the antibodies as a probe, it seemed that the antibodies mainly recognized two proteins similar to arginase and general vesicular transport factor p115, respectively. The results obtained from these experiments reveal that some proteins located in the heterochromatin of interphase liver cell nuclei seem to play important roles in condensing a portion of the chromatin structure during interphase and suggest that proteins composing heterochromatin might be changed according to cell types or the stage of the cell cycle.

Patterns of DNase I sensitivity in the holocentric chromosomes of the aphid Megoura viciae

Using the in situ nick translation technique, we looked for the presence of DNase I sensitive sites in Megoura viciae chromosomes, to study the distribution of active or potentially active genes in aphids, a group of insects possessing holocentric chromosomes. Cytological preparations obtained by the spreading of embryo cells were treated in situ with increasing concentrations (ranging from 5 to 200 ng/mL) of DNase I. At DNase I concentrations below 50 ng/mL, only one hypersensitive site was observed, and this was located on a telomeric region of the X chromosome that contains transcriptionally active nucleolar organizing regions, as assayed by silver staining. Interestingly, at intermediate concentrations of DNase, the incorporation of biotinylated nucleotide occurred uniformly throughout all chromosomes, whereas at concentrations above 100 ng/mL, a C-like banding pattern was produced. Our data differ from results obtained with mammalian, frog, and grasshopper chromosomes, where it was found that DNase I nicking is concentrated at the distal regions of all chromosomes.Key words: aphids, holocentric chromosomes, DNase I sensitivity, nick translation.

Patterns of DNase I sensitivity in the chromosomes of the grasshopper Chorthippus parallelus (Orthoptera)

We have analysed the patterns of DNase I/nick translation in the chromosomes of the grasshopper Chorthippus parallelus erythropus. Sites of preferential DNase I-nicking were concentrated at the distal chromosome regions, thus showing the non-uniform DNase I sensitivity of different chromosome domains. Among centromeric C-bands, the heterochromatin of metacentric and acrocentric chromosomes differed with respect to their DNase I resistance.

Robert Feulgen Prize Lecture 1995. New approaches to in situ detection of nucleic acids

The present paper reviews recent results obtained by different molecular biology-based, immunocytological approaches to the localization and identification of nucleic acids in sections of biological material. Examples of sensitive, high-resolution detection methods for RNA, DNA or specialized DNA regions are presented. Special emphasis is placed on the potential values and limitations of these new methods.

Cytogenetic peculiarities in the Algerian hedgehog: silver stains not only NORs but also heterochromatic blocks

Hedgehogs belong to one of the several mammalian taxa in which karyotype differences are based on variations in heterochromatin content. Furthermore, the number and location of nucleolar organizer regions (NORs) can also vary widely. In the present study these cytogenetic features were investigated in the Algerian hedgehog, Erinaceus (Aethechinus) algirus. The heterochromatin and NOR distribution patterns in the karyotype of this species are new among hedgehogs, whereas the euchromatic regions, including their G-band pattern, are similar to those reported by others. In addition, silver staining revealed a cytogenetic feature exclusive to the heterochromatic blocks of E. algirus: their silver staining with standard cytogenetic procedures. Because no similar phenomenon has been described previously in a mammalian species, several hypotheses about the significance and specificity of silver staining to NOR sites are discussed. Finally, the existence of different types of heterochromatin in the species analysed here, lead us to propose that what hedgehogs have inherited from their common ancestor is a mechanism which permits the accumulation of heterochromatin on specific chromosomes, rather than the heterochromatin itself.

The effects of 5-azacytidine and 5-azadeoxycytidine on chromosome structure and function: implications for methylation-associated cellular processes

5-Azacytidine (5-aza-C) analogs demonstrate a remarkable ability to induce heritable changes in gene and phenotypic expression. These cellular processes are associated with the demethylation of specific DNA sequences. On the other hand, 5-aza-C analogs have dramatic effects on chromosomes, leading to decondensation of chromatin structure, chromosomal instability and an advance in replication timing. Condensation inhibition of genetically inactive chromatin occurs when the DNA is still hemimethylated or fully methylated. In cell cultures prolonged for several replication cycles, chromosomal rearrangements and instability affect the 5-aza-C-sensitive regions. Moreover, the normally late-replicating inactive chromatin undergoes a transient temporal shift to an earlier DNA replication, characteristic of activatable chromatin. zThe induced alterations of chromosome structure and behavior may trigger the 5-aza-C-dependent process of cellular reprogramming. Apart from their differentiating and gene-modifying effects, 5-aza-C analogs can tumorigenically transform cells and modulate their metastatic potential. High doses of 5-aza-C analogs have cytotoxic and antineoplastic activities.

Widespread distribution of cells containing human DNA in embryos derived from mouse eggs injected with human chromosome fragments

The possibility that metaphase chromosomes can serve as a source of genetic material for making transgenic mice was suggested by our previous finding of the incorporation of human satellite DNA into mouse embryos that were injected with microdissected human centromeric fragments. In the present study, we further examined whether this chromosome transfer method can be used to generate transgenic mice containing a portion of human chromosome 4 spanning the Huntington's disease (HD) gene. For this purpose, we used an improved method of metaphase chromosome preparation that may minimize the potential for DNA damage. Using metaphase chromosomes prepared in this manner, chromosome fragments spanning the region of chromosome 4 containing the HD gene were microdissected, retrieved, and injected into fertilized mouse eggs. The injected eggs exhibited good viability and developed with a high efficiency when implanted into foster mothers. To determine whether the human DNA from the injected chromosome fragment had been incorporated into the mouse genome, embryos were harvested at 12.5 days of gestation (dg) and analyzed by in situ hybridization using a human Alu repetitive DNA probe. This analysis showed that most of the embryos contained cells with human Alu repeats. However, all of the embryos were mosaic, and the level of mosaicism was such that we were not able to determine the precise chromosomal origin of the human DNA insert. We discuss the possible basis for the mosaicism and the potential value of such mosaic animals for studying Huntington's disease.

The distribution of genes on chromosomes: a cytological approach

Studies during the last 20 years have shown that the chromosomes of many organisms, especially those of higher vertebrates, consist of a series of segments having different properties. These can be recognized as, for example, G- and R-bands. Recent studies have indicated that genes tend to lie in the R-bands rather than in the G-bands, although the number of genes that has been mapped with high precision is, as yet, only a very small proportion of the total, probably much less than 1%. We have therefore sought to study the distribution of genes on chromosomes using a cytological approach in conjunction with "universal" markers for genes. Such markers include mRNA and the gene-rich, G+C-rich H3 fraction of DNA, both of which can be localized using in situ hybridization, and DNase I hypersensitivity, and digestion by restriction enzymes known to show selectivity for the CpG islands associated with active genes, both of which can be detected using in situ nick translation. We have chosen to use the approaches involving in situ nick translation and have shown that the patterns of DNase I hypersensitivity and of CpG islands on human chromosomes show a strict correspondence to R-banding patterns: Deviations from R-banding patterns reported by previous investigators who have made similar studies appear to be attributable to excessive digestion.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid, localized amplification of a unique satellite DNA family in the rodent Microtus chrotorrhinus

A novel satellite DNA family (called MSAT-2570) was isolated and characterized from the rodent Microtus chrotorrhinus. With a length of 2,570 bp the repeat unit is among the largest yet reported in mammals and comprises a series of short direct and inverted repeats. These repeat motifs may prevent nucleosome formation or represent an endless source of genetic variation. Restriction enzyme digestion using the two pairs of isoschizomers HpaII/MspI and MboI/Sau3AI demonstrated tissue specific differences in satellite DNA methylation that may reflect variable chromatin conformation or differences in patterns of gene expression. The sex chromosomes of M. chrotorrhinus are usually large in size among mammals, comprising 15%-20% of the karyotype and containing large blocks of heterochromatin. In situ hybridization of the satellite DNA revealed chromosomal localization predominantly to sex chromosome heterochromatin. A survey of related rodents including three congeneric species also with giant sized sex chromosomes demonstrated that MSAT-2570 is present only in the genome of M. chrotorrhinus. However, another previously reported satellite DNA also isolated from M. chrotorrhinus has been shown to reside on sex chromosome heterochromatin in one of the other three species, indicating that these giant blocks of heterochromatin are complex in structure and comprise multiple, unrelated satellite DNA families.

Analysis of DNase 1 sensitivity and methylation of active and inactive X chromosomes of kangaroos (Macropus robustus) by in situ nick translation

The overall nuclease sensitivity and methylation of active and inactive X chromosomes of kangaroos were examined by in situ nick translation. Cultured fibroblasts of subspecies wallaroo-euro (Macropus robustus robustus; Macropus robustus erubescens) hybrids were used, enabling the paternally and maternally derived X chromosomes to be distinguished. No difference was found between the active and inactive X chromosomes with DNase I or MspI digestion. When chromosomes were digested with the methylation sensitive restriction enzymes HpaII and HhaI, the inactive X chromosome was labelled to a greater extent. These results indicate no overall difference in chromatin condensation between the active and inactive X chromosomes and greater overall methylation of the active X chromosome. This relative undermethylation of the inactive X chromosome may be important in X chromosome inactivation, but its function, if any, remains to be determined.

Restriction enzyme banding and in situ nick-translation on different types of hetero- and euchromatin

We studied the role of chromatin accessibility and methylation in the banding patterns produced by means of in situ nick-translation (NT) and restriction enzyme (RE) banding techniques. For these studies we used the X chromosomes of Microtus cabrerae because of their large segment with four different types of constitutive heterochromatin and because in these chromosomes we can also compare active and inactive euchromatin. The results demonstrate that constitutive heterochromatin in the X chromosomes of M. cabrerae is methylated at specific sequences in both active and inactive Xs. They also show that NT-based techniques are suitable for detecting weak differences in chromatin accessibility, such as differences between active and inactive euchromatin, and are able to distinguish methylation only at the accessible sites. Thus, when methylation has to be mapped in situ, additional experiments have to be performed in order to distinguish findings due to differential accessibility. RE banding seems less sensitive to slight differences in chromatin accessibility, and might thus be more suitable than in situ NT-based techniques for methylation mapping. In harmony with these results, HpaII-based RE banding is able to distinguish between active and inactive euchromatin, possibly depending on its methylation status.

Hypomethylation of human heterochromatin detected by restriction enzyme nick translation

Using the restriction enzymes MspI and HpaII in the nick translation procedure it has been shown that decondensation of the paracentric heterochromatin of chromosome 9 during human spermatogenesis is associated with hypomethylation of the DNA sequences in this domain. Somatic cells treated with 5'-azacytidine also showed decondensation of centromeric heterochromatin. In this instance, however, hypomethylation is detected both in the extended heterochromatin at the centromeres and in the euchromatin of the chromosome arms.

DNase I-sensitive sites within the nuclear architecture visualized by immunoelectron microscopy

Nick-translation using mild digestion with DNase I allows preferential labeling of actively transcribing or potentially active genes, as compared with inactive genes. We have adapted this method to the level of electron microscopy to see the DNase I-sensitive regions in situ in Ehrlich tumor cells. In interphase cells treated with very low concentrations of DNase I, labeled sequences are found at the borders and in the close vicinity of condensed chromatin blocks. Labeling of condensed areas of chromatin requires higher DNase I concentrations and longer incubation in the nick-translation medium. In the nucleolus, the first sites to be nick-translated are the fibrillar centers and the interstices surrounding them, whereas the dense fibrillar component never contains labeled sequences. When cells are pretreated with actinomycin D, only a few perinucleolar clumps of condensed chromatin are labeled under the same conditions. This method provides a new tool for studying the functional organization of chromatin within a cell. The precise location of nick-translated sites in nucleolar components observed could change classical views concerning the functional organization of the nucleolus.

Nonradioactive in situ nick translation combined with counterstaining: characterization of C-band and silver positive regions in mouse testicular cells

The DNase I sensitivity of three different chromatin regions in mouse testicular cells was analysed by in situ nick translation with biotin-dUTP combined with various counterstaining techniques. The regions were: (i) the constitutive centromeric heterochromatin, (ii) an interstitial C-band positive insertion on chromosome 1, Is(HSR1;C5)1Lub, and (iii) the chromatin containing rDNA (designated nucleolar chromatin herein). Incorporated biotin was detected either by the horseradish peroxidase reaction with diaminobenzidine (DAB) or the alkaline phosphatase reaction with fast red. The latter resulted in a water insoluble red precipitate, which was easily removable by any organic solution thus allowing the application of various counterstaining protocols. DNase I sensitivity of the three chromatin regions was screened in different cell types of the mouse testis. The interstitial Is(HSR) region was highly DNase I sensitive when it was recognizable by strong mithramycin fluorescence. The centromeric heterochromatin was DNase I resistant when it was compacted into microscopically visible chromosomal structures (mitosis, pachytene, metaphase I and II). In interphase nuclei from Sertoli cells and spermatogonia it became highly DNase I sensitive. In round spermatids it displayed medium DNase I sensitivity. Nucleolar chromatin was not labelled by in situ nick translation when silver staining demonstrated strong protein production. Sperm cells were highly DNase I sensitive from stages 11 to 15, but resistant as mature spermatozoa.

Analysis of methylation and distribution of CpG sequences on human active and inactive X chromosomes by in situ nick translation

In situ nick translation of fixed mitotic chromosomes after HpaII or MspI digestion allows us to detect different DNA methylation levels along chromosomes. We used this technique to analyse the methylation levels of CCGG sites in the active and inactive X chromosomes of female human cells. In addition, we analysed the distribution of these sites with respect to the banding pattern. Our data show that the inactive X, as a whole, is more methylated than the active one and that CCGG sequences are preferentially located on R-positive bands.

5-Azadeoxycytidine induced undercondensation in the giant X chromosomes of Microtus agrestis

Fibroblasts of female Microtus agrestis were treated with 5-azadeoxycytidine (5-aza-dCyd) at a final concentration of 10(-5) M during the last 2 h of culture. This cytidine analogue induces distinct undercondensation of the constitutive heterochromatin in the giant X chromosomes. The undercondensed heterochromatic thread exhibits longitudinal segmentation reminiscent of a chromomere pattern. In the late-replicating X chromosome, 5-aza-dCyd also inhibits condensation of the genetically inactivated euchromatin (facultative heterochromatin). The described effects of 5-aza-dCyd on the X chromosome structure appear to be incorporation independent.

Meiotic pairing constraints and the activity of sex chromosomes

The state of activity and condensation of the sex chromosomes in gametocytes is frequently different from that found in somatic cells. For example, whereas the X chromosomes of XY males are euchromatic and active in somatic cells, they are usually condensed and inactive at the onset of meiosis; in the somatic cells of female mammals, one X chromosome is heterochromatic and inactive, but both X chromosomes are euchromatic and active early in meiosis. In species in which the female is the heterogametic sex (ZZ males and ZW females), the W chromosome, which is often seen as a condensed chromatin body in somatic cells, becomes euchromatic in early oocytes. We describe an hypothesis which can explain these changes in the activity and condensation of sex chromosomes in gametocytes. It is based on the fact that normal chromosome pairing seems to be essential for the survival of sex cells; chromosomal anomalies resulting in incomplete pairing during meiosis usually result in gametogenic loss. We argue that the changes seen in the sex chromosomes reflect the need to avoid pairing failure during meiosis. Pairing normally requires structural and conformational homology of the two chromosomes, but when the regions is avoided when these regions become heterochromatinized. This hypothesis provides an explanation for the changes found in gametocytes both in species with male heterogamety and those with female heterogamety. It also suggests possible reasons for the frequent origin of large supernumerary chromosomes from sex chromosomes, and for the reported lack of dosage compensation in species with female heterogamety.

DNase I sensitivity of Microtus agrestis active, inactive and reactivated X chromosomes in mouse-Microtus cell hybrids

We isolated Microtus agrestis-mouse somatic cell hybrid clones which had retained either the active or the inactive M. agrestis X chromosome. In both hybrid clones the X chromosomes retained their original chromatin conformation as studied by the in situ nick translation technique--the active X chromosome retained its high sensitivity to DNase I while the inactive one remained insensitive. A clone in which the hypoxanthine guanine phosphoribosyltransferase (HPRT) gene had been spontaneously reactivated was isolated from the hybrid containing the inactive X chromosome. The in situ nick translation technique was used to study possible DNA conformation changes in the euchromatin of the inactive X chromosome with special reference to the reactivated HPRT locus. We found that the euchromatin in this X chromosome exhibited the same low sensitivity to DNase I as is characteristic of the inactive X chromosome.

In situ nick translation distinguishes between C-band positive regions on mouse chromosomes

In situ nick translation of mouse metaphase chromosomes by non-radioactive detection means and DNase I digestion followed by Giemsa staining were used to analyse the DNase I resistance of two different C-band positive regions. These were the centromeric heterochromatin of acro- and metacentric chromosomes and an interstitial C-band on chromosome 1 of wild mice, IS(HSR;1C5D)1Lub. Whereas the centromeric heterochromatin was clearly resistant to DNase I, the interstitial C-band showed very high DNase I sensitivity. Among centromeric C-bands, the heterochromatin in Robertsonian fusion biarmed chromosomes was more resistant to DNase I action than was the centromeric heterochromatin of the acrocentric chromosomes.

Transcriptional activity of constitutive heterochromatin in the mammal Microtus agrestis (Rodentia, Cricetidae)

Two cytological approaches were applied to the analysis of genetic activity in euchromatin and heterochromatin in the European field vole Microtus agrestis. The first is based on the transcriptional activity of prematurely condensed chromosomes, the second on in situ hybridization of labeled cellular RNA to metaphase chromosomes. The results show that in cultivated fibroblasts, the transcriptional activity of constitutive heterochromatin is in the same order of magnitude as that of euchromatin; its chromosomal pattern is nonrandom. It is concluded that--as has already been shown for insects and amphibians--also in mammals, constitutive heterochromatin is transcribed in its decondensed state but is genetically inactive when forming chromocenters at interphase.

5-aza-C-induced changes in the time of replication of the X chromosomes of Microtus agrestis are followed by non-random reversion to a late pattern of replication

Treatment with 5-azacytidine (5-aza-C) causes an advance in the time of replication and enhances the DNase-I sensitivity of the inactive X chromosome in Gerbillus gerbilllus fibroblasts. We found that these changes were not stably inherited and upon removal of the drug the cells reverted to the original state of one active and one inactive X chromosome. In order to determine whether this reversion was random, we used a cell line of female Microtus agrestis fibroblasts in which the two X chromosomes are morphologically distinguishable. In this work we show that the reversion to a late pattern of replication is not random, and the originally late replicating X chromosome is preferentially "reinactivated", suggesting an imprinting-like marking of one or both X chromosomes. The changes in the replication pattern of the X chromosome were associated with changes in total DNA methylation. Double treatment of cells with 5-aza-C did not alter this pattern of euchromatin activation and reinactivation. A dramatic advance in the time of replication of the entire X linked constitutive heterochromatin (XCH) region was however, observed in the doubly treated cells. This change in the replication timing of the XCH occurred in both X chromosomes and was independent of the changes observed in the euchromatic region. These observations suggest the existence of at least two independent regulatory sites which control the timing of replication of two large chromosomal regions.