Showing papers on "Heterochromatin published in 1992"

Analysis of subtelomeric heterochromatin in the Drosophila minichromosome Dp1187 by single P element insertional mutagenesis.

Abstract: We investigated whether single P element insertional mutagenesis could be used to analyze heterochromatin within the Drosophila minichromosome Dp1187. Forty-five insertions of the P[lacZ,rosy+] element onto Dp1187 (recovered among 7,825 transpositions) were highly clustered. None was recovered in centromeric heterochromatin, but 39 occurred about 40 kb from the distal telomere within a 4.7-kb hotspot containing tandem copies of a novel 1.8-kb repetitive DNA sequence. The DNA within and distal to this region lacked essential genes and displayed several other properties characteristic of heterochromatin. The rosy+ genes within the inserted transposons were inhibited by position-effect variegation, and the subtelomeric region was underrepresented in polytene salivary gland cells. These experiments demonstrated that P elements preferentially transpose into a small subset of heterochromatic sites, providing a versatile method for studying the structure and function of these chromosome regions. This approach revealed that a Drosophila chromosome contains a large region of subtelomeric heterochromatin with specific structural and genetic properties.

Polycomb and polyhomeotic are constituents of a multimeric protein complex in chromatin of Drosophila melanogaster.

Abstract: The polycomb group (Pc-G) genes are responsible for maintaining the repressed state of homeotic genes during development. It has been suggested that the Pc-G exerts its transcriptional control by regulating higher order chromatin structure. In particular, the finding of genetic and molecular similarities to components involved in heterochromatin formation, led to the proposal that homeotic genes are permanently repressed by mechanisms similar to those responsible for heterochromatin compaction. Because of synergistic effects, Pc-G gene products are thought to act in a multimeric complex. Using immunoprecipitation we show that two members of the Pc-G, Polycomb and polyhomeotic, are constituents of a soluble multimeric protein complex. Size fractionation indicates that a large portion of the two proteins are found in a distinct complex of molecular weight 2-5 x 10(6) Da. During embryogenesis the two proteins show the same spatial distribution. In addition, by double-immunofluorescence labelling we can demonstrate that Polycomb and polyhomeotic have exactly the same binding patterns on polytene chromosomes of larval salivary glands. We propose that some Pc-G proteins act in multimeric complexes to compact the chromatin of stably repressed genes like the homeotic regulators.

The heterochromatin-associated protein HP-1 is an essential protein in Drosophila with dosage-dependent effects on position-effect variegation.

Abstract: Chromosome rearrangements which place euchromatic genes adjacent to a heterochromatic breakpoint frequently result in gene repression (position-effect variegation). This repression is thought to reflect the spreading of a heterochromatic structure into neighboring euchromatin. Two allelic dominant suppressors of position-effect variegation were found to contain mutations within the gene encoding the heterochromatin-specific chromosomal protein HP-1. The site of mutation for each allele is given: one converts Lys169 into a nonsense (ochre) codon, while the other is a frameshift after Ser10. In flies heterozygous for one of the mutant alleles (Su(var)2-504), a truncated HP-1 protein was detectable by Western blot analysis. An HP-1 minigene, consisting of HP-1 cDNA under the control of an Hsp70 heat-inducible promoter, was transduced into flies by P element-mediated germ line transformation. Heat-shock driven expression of this minigene results in elevated HP-1 protein level and enhancement of position-effect variegation. Levels of variegating gene expression thus appear to depend upon the level of expression of a heterochromatin-specific protein. The implications of these observations for mechanism of heterochromatic position effects and heterochromatin function are discussed.

Position effect variegation and chromatin proteins.

Abstract: Variegated phenotypes often result from chromosomal rearrangements that place euchromatic genes next to heterochromatin In such rearrangements, the condensed structure of heterochromatin can spread into euchromatic regions, which then assume the morphology of heterochromatin and become transcriptionally inactive In position-effect variegation (PEV) therefore, gene inactivation results from a change in chromatin structure PEV has been intensively investigated in the fruitfly Drosophila, where the phenomenon allows a genetic dissection of chromatin components Consequently, many genes have been identified which, when mutated, act as dominant modifiers (suppressors or enhancers) of PEV Data available already demonstrate that genetic, molecular and developmental analysis of these genes provides an avenue to the identification of regulatory and structural chromatin components, and hence to fundamental aspects of chromosome structure and function

Functional Elements in Drosophila Melanogaster Heterochromatin

Abstract: INTRODUCTION 239 THE STRUCTURE OF DROSOPHILA MELANOGASTER HETEROCHROMATIN . 241 THE HETEROCHROMATIC FUNCTIONAL ELEMENTS . . . . . . . . . . . . . . . . 246 The Y-Chromosome Fertility Factors . . ... . .. . . . ... . ... . . .... . . . . . . 246 The bobbed Locus, The Collochores and Rex . . . .. . ... . . . . . . ... . . . .. 252 The crystal Locus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 The Responder Locus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 The ABO Elements . .... . ... . .... . ... . . .... . 259 The Autosomal Heterochromatic Loci. . . . . . . . . . . . . . . . . . . . . . . . . . . . 260 HETEROCHROMATIN AND POSITION EFFECT VARIEGATION . . . . . . . . . . 262 MOLECULAR ORGANIZATION AND EVOLUTION OF THE HETEROCHROMATIC FUNCTIONAL ELEMENTS . . . . . . . . . . . . . 264 SUMMARY AND PERSPECTIVES 266

Analysis of the functional role of the Polycomb chromo domain in Drosophila melanogaster.

Abstract: The chromo domain was identified as a homologous protein motif between Polycomb (Pc)--a member of the Pc-group genes encoding transcriptional repressors of the homeotic genes--and HP1--a heterochromatin-associated protein encoded by the suppressor of position effect variegation gene Su(var)205. Together with previous genetic studies, this molecular similarity supports the suggestion of a common mechanism used for generating heterochromatin and for repressing homeotic genes. The evolutionary conservation of the chromo domain throughout the animal and plant kingdoms implies an important functional role for this protein motif. We have used transgenic lines as well as transient expression assays employing Drosophila tissue culture cells to study the functional role of the Pc chromo domain. Wild-type Pc protein is endogenously expressed in SL2 cells and is found in large immunologically visible complexes. Mutated Pc proteins were expressed as Pc-beta-galactosidase fusion proteins, and their nuclear distribution was examined by indirect immunofluorescence in tissue culture cells and on polytene chromosomes of transgenic larvae. We show that carboxy-terminal truncations of the Pc protein do not affect chromosomal binding of the fusion protein. However, mutations affecting only the chromo domain including in vitro generated deletions, as well as point mutations, abolish chromosomal binding. Our results demonstrate for the first time that the chromo domain is important for the function of Pc and that it is absolutely required for binding of Pc protein to chromatin. Some of the nuclear patterns generated by the mutated forms of the fusion proteins suggest, furthermore, that the chromo domain could be involved in a packaging mechanism, essential for compacting chromosomal proteins within heterochromatin or heterochromatin-like complexes.

Telomere-proximal DNA in Saccharomyces cerevisiae is refractory to methyltransferase activity in vivo.

Abstract: Genes located near telomeres in Saccharomyces cerevisiae undergo position-effect variegation; their transcription is subject to reversible but mitotically heritable repression. This position effect and the finding that telomeric DNA is late replicating suggest that yeast telomeres exist in a heterochromatin-like state. Mutations in genes that suppress the telomeric position effect suggest that a special chromatin structure exists near chromosomal termini. Thus transcriptional repression may be explained by the inability of DNA binding proteins to access the DNA near telomeres. To test this hypothesis, the Escherichia coli Dam DNA methyltransferase, which modifies the sequence GATC, was introduced into S. cerevisiae cells. DNA sequences near the telomere were highly refractive to Dam methylation but were modified when located at positions more internal on the chromosome. Telomeric sequences were accessible to methyltransferase activity in strains that contained a mutation that suppressed the telomeric position effect. These data support the model that sequence-specific DNA binding proteins are excluded from telomere-proximal sequences in vivo and show that expression of DNA methyltransferase activity may serve as a useful tool for mapping chromosomal structural domains in vivo.

There are two mechanisms of achiasmate segregation in Drosophila females, one of which requires heterochromatic homology.

Abstract: There are numerous examples of the regular segregation of achiasmate chromosomes at meiosis I in Drosophila melanogaster females. Classically, the choice of achiasmate segregational partners has been thought to be independent of homology, but rather made on the basis of availability or similarities in size and shape. To the contrary, we show here that heterochromatic homology plays a primary role in ensuring the proper segregation of achiasmate homologs. We observe that the heterochromatin of chromosome 4 functions as, or contains, a meiotic pairing site. We show that free duplications carrying the 4th chromosome pericentric heterochromatin induce high frequencies of 4th chromosome nondisjunction regardless of their size. Moreover, a duplication from which some of the 4th chromosome heterochromatin has been removed is unable to induce 4th chromosome nondisjunction. Similarly, in the absence of either euchromatic homology or a size similarity, duplications bearing the X chromosome heterochromatin also disrupt the segregation of two achiasmate X chromosome centromeres. Although heterochromatic regions are sufficient to conjoin nonexchange homologues, we confirm that the segregation of heterologous chromosomes is determined by size, shape, and availability. The meiotic mutation Axs differentiates between these two processes of achiasmate centromere coorientation by disrupting only the homology-dependent mechanism. Thus there are two different mechanisms by which achiasmate segregational partners are chosen. We propose that the absence of diplotene-diakinesis during female meiosis allows heterochromatic pairings to persist until prometaphase and thus to co-orient homologous centromeres. We also propose that heterologous disjunctions result from a separate and homology-independent process that likely occurs during prometaphase. The latter process, which may not require the physical association of segregational partners, is similar to those observed in many insects, in Saccharomyces cerevisiae and in C. elegans males. We also suggest that the physical basis of this process may reflect known properties of the Drosophila meiotic spindle.

Frequent transpositions of Drosophila melanogaster HeT-A transposable elements to receding chromosome ends.

Abstract: HeT-A elements are a new family of transposable elements in Drosophila that are found exclusively in telomeric regions and in the pericentric heterochromatin. Transposition of these elements onto broken chromosome ends has been implicated in chromosome healing. To monitor the fate of HeT-A elements that had attached to broken ends of the X chromosome, we examined individual X chromosomes from a defined population over a period of 17 generations. The ends of the X chromosomes with new HeT-A additions receded at the same rate as the broken ends before the HeT-A elements attached. In addition, some chromosomes, approximately 1% per generation, had acquired new HeT-A sequences of an average of 6 kb at their ends with oligo(A) tails at the junctions. Thus, the rate of addition of new material per generation matches the observed rate of terminal loss (70-75 bp) caused by incomplete replication at the end of the DNA molecule. One such recently transposed HeT-A element which is at least 12 kb in length has been examined in detail. It contains a single open reading frame of 2.8 kb which codes for a gag-like protein.

The mouse has a Polycomb-like chromobox gene.

Abstract: The Drosophila gene Polycomb (Pc) has been implicated in the clonal inheritance of determined states and is a trans-regulator of the Antennapedia-like homeobox genes. Pc shares a region of homology (the chromobox) with the Drosophila gene Heterochromatin Protein 1 (HP1), a component of heterochromatin. The Pc chromobox has been used to isolate a mouse chromobox gene, M33, which encodes a predicted 519 amino acid protein. The M33 chromodomain is more similar to that in the Pc protein, than that in the HP1 protein. In addition to the chromodomain, the M33 and Pc proteins also share a region of homology at their C termini. The temporal and spatial expression patterns of M33 have been studied by in situ hybridization and northern analysis. During the final 10 days of embryonic development, M33 expression mirrors that of the cell-cycle-specific cyclin B gene. It is therefore suggested that the rate of cellular proliferation controls M33 expression. From comparisons of the characteristics of M33 with those of Pc it is proposed that M33 is a Pc-like chromobox gene. The roles of M33 and Pc in models of cellular memory are examined and implications of the memory models addressed.

Antibodies to defined histone epitopes reveal variations in chromatin conformation and underacetylation of centric heterochromatin in human metaphase chromosomes.

Abstract: Unfixed metaphase chromosome preparations from human lymphocyte cultures were immunofluorescently labelled using antibodies to defined histone epitopes. Both mouse monoclonal antibody HBC-7, raised against the N-terminal region of H2B, and rabbit serum R5/12, which recognizes H4 acetylated at Lys-12, gave non-uniform labelling patterns, whereas control antibodies against total histone fractions H4 and H1 produced homogeneous fluorescence. HBC-7 bound approximately uniformly to the bulk of the chromosomes, but the major heterochromatic domains of chromosomes 1, 9, 15, 16 and the Y showed significantly brighter fluorescence. Serum R5/12 indicated an overall reduction in acetylation of H4 in metaphase chromosomes compared with interphase nuclei, although some specific chromosomal locations had considerably elevated acetylation levels. Acetylation levels in the major heterochromatic domains appeared extremely low. To investigate further the differences noted in heterochromatin labelling, metaphases from cultures grown in the presence of various agents known to induce undercondensation of the major heterochromatic domains were similarly immunolabelled. Decondensed heterochromatin no longer exhibited higher than normal immunofluorescence levels with HBC-7. The higher resolution afforded by "stretching" the centromeric heterochromatin of chromosomes 1, 9 and 16 confirmed the low level of H4 acetylation in these domains. We consider the implications of these observations in relation to chromatin conformation and activity.

Evidence that intergenic spacer repeats of Drosophila melanogaster rRNA genes function as X-Y pairing sites in male meiosis, and a general model for achiasmatic pairing.

Abstract: In Drosophila melanogaster males, X-Y meiotic chromosome pairing is mediated by the nucleolus organizers (NOs) which are located in the X heterochromatin (Xh) and near the Y centromere. Deficiencies for Xh disrupt X-Y meiotic pairing and cause high frequencies of X-Y nondisjunction. Insertion of cloned rRNA genes on an Xh- chromosome partially restores normal X-Y pairing and disjunction. To map the sequences within an inserted, X-linked rRNA gene responsible for stimulating X-Y pairing, partial deletions were generated by P element-mediated destabilization of the insert. Complete deletions of the rRNA transcription unit did not interfere with the ability to stimulate X-Y pairing as long as most of the intergenic spacer (IGS) remained. Within groups of deletions that lacked the entire transcription unit and differed only in length of residual IGS material, pairing ability was proportional to the dose of 240-bp intergenic spacer repeats. Deletions of the complete rRNA transcription unit or the 28S sequences alone blocked nucleolus formation, as determined by binding of an antinucleolar antibody, yet did not interfere with pairing ability, suggesting that X-Y pairing may not be mechanistically related to nucleolus formation. A model for achiasmatic pairing in Drosophila males based upon the combined action of topoisomerase I and a strand transferase is proposed.

Dodeca satellite: a conserved G+C-rich satellite from the centromeric heterochromatin of Drosophila melanogaster.

Abstract: To identify sequences from the centromeric region, we have constructed a Drosophila melanogaster yeast artificial chromosome (YAC) library and screened it with purified DNA from the minichromosome Dp(1;f)1187 derived from the X chromosome. We describe the structure of one clone isolated in this way. This YAC is structurally unstable and contains tandemly repeated G+C-rich 11-mer and 12-mer units, which we call dodeca satellite. Most of this satellite is located near the centromere of an autosome. Cross-hybridizing sequences are found in the genomes of organisms as distant as Arabidopsis thaliana and Homo sapiens.

Identification of a subdomain of CENP-B that is necessary and sufficient for localization to the human centromere.

Abstract: We have combined in vivo and in vitro approaches to investigate the function of CENP-B, a major protein of human centromeric heterochromatin. Expression of epitope-tagged deletion derivatives of CENP-B in HeLa cells revealed that a single domain less than 158 residues from the amino terminus of the protein is sufficient to localize CENP-B to centromeres. Centromere localization was abolished if as few as 28 amino acids were removed from the amino terminus of CENP-B. The centromere localization signal of CENP-B can function in an autonomous fashion, relocating a fused bacterial enzyme to centromeres. The centromere localization domain of CENP-B specifically binds in vitro to a subset of alpha-satellite DNA monomers. These results suggest that the primary mechanism for localization of CENP-B to centromeres involves the recognition of a DNA sequence found at centromeres. Analysis of the distribution of this sequence in alpha-satellite DNA suggests that CENP-B binding may have profound effects on chromatin structure at centromeres.

Hoechst 33258, distamycin A, and high mobility group protein I (HMG-I) compete for binding to mouse satellite DNA.

Abstract: The experiments described were designed to test the hypothesis that the (A+T)-specific DNA binding ligands Hoechst 33258 and distamycin A affect the condensation of mouse centromeric heterochromatin by competing for binding to satellite DNA with one or more chromosomal proteins. The studies focused on the nonhistone chromosomal protein HMG-I since its binding properties predict it would be a target for competition. Gel mobility shift assays show that HMG-I forms specific complexes with satellite DNA and that the formation of these complexes is competed for by both Hoechst and distamycin. In addition, methidium propyl EDTA Fe(II) [MPE Fe(II)] footprints of ligand-satellite DNA complexes showed essentially the same protection pattern for both drugs and a similar, but not identical, HMG-I footprint. If these in vitro results reflect the in vivo situation then the incomplete condensation of centromeric heterochromatin observed when mouse cells are grown in the presence of either chemical ligand could be a consequence of competition for binding of HMG-I (and possibly other proteins) to satellite DNA.

A highly differentiated ZZ/ZW sex chromosome system in a Characidae fish, Triportheus guentheri

Abstract: Seventeen specimens of Triportheus guentheri, a fish of the family Characidae, were submitted to chromosomal analysis, with a highly differentiated heteromorphic ZW pair being detected. Chromosome W is much smaller than chromosome Z and mostly heterochromatic. Chromosome Z is the largest in the karyotype, with heterochromatin occurring in the telomeric and centromeric regions only. The W chromosome also varies somewhat in size, the variations being probably due to its long arm. In addition to two other autosomal pairs, chromosome Z shows also an occasional Ag-NOR. Aspects of the ZW system differentiation and of the NOR presence in chromosome Z are discussed. The Characidae family includes a great deal of neotropical freshwater fish species and Triportheus appears to represent the only genus in this family having sex chromosome differentiation at a cytological level.

A superfamily of Drosophila satellite related (SR) DNA repeats restricted to the X chromosome euchromatin

Abstract: The 1.688 g/cm3 class of Drosophila satellite DNA is predominantly localized to the centromeric heterochromatin of the X chromosome. We report here the existence of 1.688 satellite related (SR) DNA arrays present at numerous locations throughout the euchromatic portion of the X. Unlike their heterochromatic counterparts, euchromatic SRs consist of a small number of repeating units (usually 2-4), each of which is 63-81% identical to the 359-bp monomer of the 1.688 satellite. Although it appears that SR DNA arrays are not transcribed, in at least two cases, they are located adjacent to transcriptionally active genes. SR sequences also have significant similarity to a previously described Drosophila middle repeat found almost exclusively in the X euchromatin. It seems likely that these X linked sequences are required for sex chromosome specific functions.

Cloning and expression of Drosophila HP1 homologs from a mealybug, Planococcus citri.

Abstract: The mealybug chromosome cycle is one of the most dramatic examples of genomic imprinting known. In embryos that are to become male the entire paternal chromosome set becomes heterochromatic and inactive at the blastoderm stage, while the maternal set remains active and euchromatic. HP1 is a protein from Drosophila melanogaster, which binds preferentially to heterochromatin on polytene chromosomes and is likely to be a modifier of position effect variegation. This paper describes the isolation and sequencing of two cDNA clones encoding HP1 homologs from the mealybug, Planococcus citri. The protein product of the cDNA clone that was closer to HP1 in sequence was expressed as a fusion protein in Escherichia coli, and polyclonal rat antibodies were raised against it. Immunohistochemistry to mealybug squash preparations showed that this protein was a male-specific nuclear protein, but that it was not specifically associated with the heterochromatic set of chromosomes.

Chromosome Studies in Hypoptopomatinae (Pisces, Siluriformes, Loricariidae) : I. XX/XY Sex Chromosome Heteromorphism in Pseudotocinclus tietensis

Abstract: The cytogenetic analysis of 13 specimens (9 males and 4 females) of Pseudotocinclus tietensis showed that this species has 2n=54 chromosomes (26M+20SM+6ST) plus one heteromorphic pair (SM/M) in the males and one homomorphic pair (SM/SM) in the females. Ag-NORs are located interstitially on the long arm of the largest metacentric pair and a small amount of constitutive heterochromatin is present in the karyotypes. The occurrence of a heteromorphic pair in the males is demonstrable by the presence of one extra C-banded block in the Y chromosome. The presence of one heterochromatin block distinguishing the sexual chromosomes suggests that this species presents a very primitive mechanism of sex chromosome differentiation.

A cytogenetic and genetic characterization of a group of closely linked second chromosome mutations that suppress position-effect variegation in Drosophila melanogaster.

Abstract: Characterization of a group of dominant second chromosome suppressor of position-effect variegation (PEV) (Su(var)) mutants has revealed a variety of interesting properties, including: maternal-effect suppression of PEV, homozygous lethality or semilethality and male-specific hemizygous lethality, female infecundity, acute sensitivity to the amount of heterochromatin in the cell and sensitivity to sodium butyrate. Deficiency/duplication mapping and complementation tests have revealed that eight of the mutants define at least two genes in section 31 of the left arm of chromosome 2 and they suggest that a ninth corresponds to an additional nonessential Su(var) gene within or near this region. The effects of specific deficiencies and a duplication on PEV indicate that the expression of one or more of the Su(var) genes in this region of the chromosome is dose-dependent, i.e., capable of haplo-abnormal suppression and triplo-abnormal enhancement. Interestingly, the appearance of certain visible phenotypes among a subset of the mutants suggests that they may possess antimorphic properties. Our results are consistent with the hypothesis that two of these Su(var) genes encode structural components of heterochromatin. We also report that two previously isolated mutants located in 31E and 31F-32A act as recessive suppressors of PEV.

Identification of C-banded chromosomes in meiosis and the analysis of nucleolar activity in Avena byzantina C. Koch cv ‘Kanota’

Abstract: The Giemsa C-banding technique was used to identify individual meiotic and somatic chromosomes in 21 monosomic lines of Avena byzantina C. Koch cv ‘Kanota’ (genome designation AACCDD). The hexaploid complement is composed of three sets of seven chromosome pairs. The heterochromatin in the putative diploid progenitors is located at the telomeres (genome A), at the centromeric and interstitial regions (genome C), or more evenly spread throughout the set (genome D). Comparisons based on C-banding between A. byzantina and its diploid progenitor species allowed us to allocate individual chromosomes into specific genomes. The C-banding technique may be useful for interspecific chromosome pairing analyses. Nucleolar activity and competition were studied using a silver-staining procedure. Only three chromosome pairs showed nucleolar organizer regions, thus indicating that nucleolar competition occurs naturally in hexaploid oats.

The organisation of repetitive DNA sequences on human chromosomes with respect to the kinetochore analysed using a combination of oligonucleotide primers and CREST anticentromere serum

Abstract: The spatial relationship between the families of repetitive DNAs present at the centromeres of human chromosomes and the position of the kinetochore was examined by combining immunocytochemistry with the PRINS oligonucleotide primer extension technique. Heterochromatic domains were decondensed with 5′-azacytidine to facilitate this study. Using this approach our results clearly show that the alphoid DNA sequences are closely associated with the kinetochore of human chromosomes. Simple-sequence satellite DNAs occupy separate, non-overlapping domains within the centromere. These two major families are separated by a third, relatively low-copy repetitive DNA family, SAU-3A. Pulse-field gel electrophoresis was employed to analyse the centromeric domain of human chromosome no. 9 in more detail and the results although preliminary support the conclusions drawn from the immunocytochemistry/PRINS approach.

Towards a physical map of the fertility genes on the heterochromatic Y chromosome of Drosophila hydei: families of repetitive sequences transcribed on the lampbrush loops Nooses and Threads are organized in extended clusters of several hundred kilobases.

Abstract: The understanding of structure and function of the so-called fertility genes of Drosophila is very limited due to their unusual size — several megabases — and their location on the heterochromatic Y chromosome. Since mapping of these genes has mainly been done by classical cytogenetic analyses using a small number of cytologically visible lampbrush loops as the sole markers for particular fertility genes, the resolution of the genetic map of the Y chromosome is restricted to 3–5 Mb. Here we demonstrate that a substantially finer subdivision of the megabase-sized fertility genes in the subtelomeric regions of the Y chromosome of Drosophila hydei can be achieved by a combination of digestion with restriction enzymes having 6 by recognition sequences, and pulsed field gel electrophoresis. The physical subdivision is based upon large conserved fragments of repetitive DNA in the size range from 50 up to 1600 kb and refers to the long-range organization of several families of repetitive DNA involved in Y chromosomal transcription processes in primary spermatocytes. We conclude from our results that at least five different families of repetitive DNA specifically transcribed on the lampbrush loops nooses and threads are organized as extended clusters of several hundred kb, essentially free of interspersed non-repetitive sequences.

Copies of a Stellate Gene Variant Are Located in the X Heterochromatin of Drosophila Melanogaster and Are Probably Expressed

Abstract: Two variants of X chromosome Stellate genes responsible for crystal formation in XO male primary spermatocytes occupy different genome positions. The majority if not all of the 1250-bp Stellate genes are located at the 12E site where the Ste locus has been mapped and almost all of the 1150-bp Stellate repeats are concentrated in the distal X heterochromatin. Sequencing of Stellate genes derived from X heterochromatin reveals the preservation of their open reading frames and precise matching with some Stellate cDNAs reported earlier. At least some heterochromatic Stellate genes are suggested to be expressed and, therefore, involved in the interaction with the Y chromosome locus Su(Ste), as are the Stellate genes from 12E.

Heterochromatin protein 1, a known suppressor of position-effect variegation, is highly conserved in Drosophila

Abstract: The Su(var)205 gene of Drosophila melanogaster encodes heterochromatin protein 1 (HP1), a protein located preferentially within beta-heterochromatin. Mutation of this gene has been associated with dominant suppression of position-effect variegation. We have cloned and sequenced the gene encoding HP1 from Drosophila virilis, a distantly related species. Comparison of the predicted amino acid sequence with Drosophila melanogaster HP1 shows two regions of strong homology, one near the N-terminus (57/61 amino acids identical) and the other near the C-terminus (62/68 amino acids identical) of the protein. Little homology is seen in the 5' and 3' untranslated portions of the gene, as well as in the intronic sequences, although intron/exon boundaries are generally conserved. A comparison of the deduced amino acid sequences of HP1-like proteins from other species shows that the cores of the N-terminal and C-terminal domains have been conserved from insects to mammals. The high degree of conservation suggests that these N- and C-terminal domains could interact with other macromolecules in the formation of the condensed structure of heterochromatin.

In situ localization of two repetitive DNA sequences to surface-spread pachytene chromosomes of rye

Abstract: Surface-spread pollen mother cells at meiotic prophase from Secale cereale (rye) were used for fluorescent DNA:DNA in situ localization of two tandemly repeated DNA sequences: pTa71, a wheat rDNA clone, and pSc119.2, a cloned 120-bp repeat from rye heterochromatin. The fluorescent hybridization signal, consisting of many yellow-green dots, was closely associated with the bivalent axes, corresponding to the synaptonemal complex, and located in the surrounding chromatin. The rDNA signal was associated with one bivalent, the smallest of the seven, at a distance about 13% of the bivalent length from the telomere. This corresponded to the position of the nucleolar organizing region of silver-stained synaptonemal complexes analyzed under the electron microscope and published data for somatic metaphase chromosomes. The relative length of the axis covered with the rDNA signal is less than expected from somatic metaphases, but it corresponds more closely to the proportion of the sequences in the genome. The hybrid...

Cold-sensitive chromosome regions and their relation to constitutive heterochromatin in Cestrum parqui (Solanaceae)

Abstract: The structural cause for cold-induced undercondensation of certain chromosome segments is still unknown, and no consistent cytochemical correlations have emerged as yet. Cestrum parqui belongs to one of the few plant genera having chromosomes with such cold-sensitive regions. In the present study, four heterochromatin classes were distinguished in the karyotype of this species by sequential fluorochrome staining with chromomycin A3, DAPI, and D 287/170, and C-banding, N-banding, and silver impregnation. (i) Cold-sensitive regions (CSRs): Banding behaviour indicates AT-rich constitutive heterochromatin. Silver impregnation occurs after cold-treatment from interphase to metaphase. CSRs are subject to polymorphisms. Quantitatively they are only a minor component of the band endowment. (ii) Nucleolar organizers: Two pairs were identified. Banding behaviour indicates GC-rich constitutive heterochromatin. The nucleolar constrictions proper are morphologically indistinct at mitotic metaphase and lose silver impr...