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Polycomb complexes assemble at their target sites and silence neighboring genes when these are not actively transcribed. The action of these complexes and of Trithorax complexes bound to the Polycomb Response Element establish alternative silent or derepressed states that are remembered through cell division and maintained for the rest of development. Recent results that may help explain the properties...
This review summarizes genetic, molecular and biochemical studies of the SU(VAR)3-9 protein and the evidence for its key role in heterochromatin formation and heterochromatic gene silencing. The Su(var)3-9 locus was first identified as a dominant modifier of position-effect variegation (PEV) in Drosophila melanogaster. Together with Su(var)2-5 and Su(var)3-7, Su(var)3-9 belongs to the group of haplo-suppressor...
One of the central problems of eukaryotic gene regulation is to understand the mechanism(s) by which the activity of enhancer elements is circumscribed such that they only act upon their cognate promoter sequences. Studies on the bithorax gene complex (BX-C) in Drosophila have highlighted the potential problem of enhancer promiscuity and detailed molecular and genetic analyses are now providing insight...
The repression of gene activity and the maintenance of the repressed state are fundamental requirements of cell differentiation, ordered embryonic development and tissue integrity. Furthermore, large regions of the genome such as centromeres and telomeres have a structural function and have to be kept transcriptionally inactive to be functional. In both cases the transcriptional silencing is accomplished...
It has been previously reported that the abundance and distribution of transposable elements (TEs) in Drosophila heterochromatin are conserved in unrelated stocks although they may greatly differ between families. The biases in genomic distribution of TEs are potentially informative for understanding host–transposon interactions. Here we report that in most stocks, one to four elements of the 1731...
Heterochromatin has been traditionally regarded as a genomic wasteland, but in the last three decades extensive genetic and molecular studies have shown that this ubiquitous component of eukaryotic chromosomes may perform important biological functions. In D. melanogaster, about 30 genes that are essential for viability and/or fertility have been mapped to the heterochromatin of the major autosomes...
The Heterochromatin Protein 1 (HP1) is a conserved protein which is best known for its strong association with the heterochromatin of Drosophila melanogaster. We previously demonstrated that another important property of HP1 is its localization to the telomeres of Drosophila, a feature that reflects its critical function as a telomere capping protein. Here we report our analysis of the euchromatic...
Autonomous P elements, inserted in heterochromatic telomeric associated sequences (TAS) at the X chromosome telomere (site 1A) have strong P element regulatory properties that include repression of P-induced hybrid-dysgenesis and of P-lacZ expression in the germline. P-lacZ insertions or defective P elements at 1A in TAS can also repress in trans a euchromatic P-lacZ in the germline. This property...
Whole genome shotgun assemblies have proven remarkably successful in reconstructing the bulk of euchromatic genes, with the only limit appearing to be determined by the sequencing depth. For genes imbedded in heterochromatin, however, the low cloning efficiency of repetitive sequences, combined with the computational challenges, demand that additional clues be used to annotate the sequences. One approach...
Drosophila telomeres contain multiple copies of HeT-A and TART retrotransposons. These elements specifically transpose to chromosomal ends, compensating for loss of terminal nucleotides that occurs at each cycle of DNA replication. We have investigated the role of these sequences in the formation of telomere–telomere attachments induced by mutations in the UbcD1 gene. We have constructed UbcD1 mutant...
In lecanoid Coccids, or mealybugs, the male development is accompanied by the facultative heterochromatization of the entire, paternally derived, haploid chromosome set. This epigenetic phenomenon occurs in all the cells of mid-cleavage male embryos. Consequently, the Coccid chromosome system offers a powerful tool for gaining insights into the structure of facultative heterochromatin, and into the...
The terminal DNA arrays on chromosomes of Drosophila melanogaster are composed of two families of non-LTR retrotransposons, HeT-A and TART. Available evidence suggests that chromosome length in this species and its close relatives is maintained by targeted transposition of these elements, with attachment of the elements to the chromosome end by their 3′ oligo(A) tails. However, the regulation of transposition...
In D. melanogaster males carrying Segregation Distorter (SD) second chromosomes, sperm receiving sensitive alleles of the Responder (Rsp) locus are subject to high rates of dysfunction. The Rsp region is located in 2R immediately adjacent to the centromere in heterochromatic band 39, and covers roughly 600 kb of material, of which approximately 85 kb is comprised of several hundred copies of a 240-bp...
The morphological characteristics of intercalary heterochromatin (IH) are compared with those of other types of silenced chromatin in the Drosophila melanogaster genome: pericentric heterochromatin (PH) and regions subject to position effect variegation (PEV). We conclude that IH regions in polytene chromosomes are binding sites of silencing complexes such as PcG complexes and of SuUR protein. Binding...
The modifier of mdg4 (mod(mdg4)) gene of Drosophila melanogaster has been identified in many different genetic assays. It has been independently identified through mutations isolated for their effects on position effect variegation (PEV), the properties of insulator sequences, correct pathfinding of growing nerve cells, meiotic pairing of chromosomes, or apoptosis. Molecular analysis of the mod(mdg4)...
A class of repetitive DNA sequences frequently found at centromeric regions are R/Y-satellites showing an asymmetric distribution of residues resulting in one strand being rich in purines (R-strand) while the complementary strand is pyrimidine-rich (Y-strand). The dodeca-satellite of Drosophila belongs to this class of centromeric satellites. In vitro, the dodeca-satellite forms altered DNA structures...
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