Three distinct chromatin domains in telomere ends of polytene chromosomes inDrosophila melanogaster Telmutants

Abstract

IntroductionTelomeres are specialized DNA-protein complexes that cap the termini of linear chromosomes. They compensate for incomplete DNA replication and contribute to the stability of chromosomes and karyotype (Muller, 1932;Pardue and Debaryshe, 1999;Biessmann and Mason, 2003). Telomeres also participate in nuclear architecture maintenance, and are known to associate with nuclear lamina (Hochstrasser et al., 1986;Marshall et al., 1996;Hari et al., 2001) or with nuclear matrix (de Lange, 1992;Luderus et al., 1996). However, the factors that underlie these phenomena, as well as the mechanisms of telomere functioning, remain poorly understood in Drosophila melanogaster.In eukaryotes, such as budding yeast, fission yeast and humans, telomeres are considered to be essentially heterochromatic (Perrod and Gasser, 2003). Heterochromatin is characterized by a high degree of DNA compaction, late replication in S phase, and by association with specific silencing proteins (Richards and Elgin, 2002). Intercalary (IH) and pericentric heterochromatin regions in many Diptera species are characterized by DNA underreplication and nonhomologous (ectopic) pairing of chromosomal regions in polytene chromosomes (Zhimulev, 1998). The question whether D. melanogaster telomeres are heterochromatic in

Ectopic assembly of heterochromatin in Drosophila melanogaster triggered by transposable elements

“…The subtelomeric repeats are known targets for P element insertion (6). These genomic regions are enriched for Polycombgroup proteins (PcG) and the associated trimethylation of histone H3 lysine 27 (H3K27me3) mark (7,8). In 60% of the variegating lines (including those in TASs), the construct mapped to regions with a high repeat density (>30%), a property of canonical heterochromatin (9).…”

Drosophila telomeres: an exception providing new insights

“…A few other proteins, including the heterochromatin proteins SUUR, HP2 and SU(VAR)3-7 and the transcription factor WOC (Table 1), have been identified at the extreme chromosome ends by immunocytochemistry (44,45), although their functions at the tips are not known. The first three may play a role in heterochromatin formation ( Table 2).…”

“…Proteins associated with the HTT array, however, are difficult to identify, because no genetic screen can be performed for HTT-binding proteins owing to the absence of a predictable mutant phenotype. Therefore, immunohistological staining for potential candidate proteins is one of the few options, which so far has identified three proteins at the HTT array: Z4, JIL-1 and PROD (44,67). All three proteins are also associated with a multitude of other sites in the Drosophila genome, suggesting general and widespread functions ( Table 2).…”

“…Histone modifications in the pericentric heterochromatin of Drosophila and their involvement in gene silencing are well characterized (75), but analyses of the telomeric regions are just beginning. Andreyeva et al (44) have localized some characteristic histone modifications at the telomeres of polytene chromosomes by immunohistochemistry and found H3Me3K9 at HTT but not at TAS, while H3Me3K27 was absent from HTT but present at TAS and at the cap. The acetylated histone, H4AcK12, which is often associated with heterochromatin, was not found at telomeres.…”

“…Two Polycomb-group (PcG) repressors, PC and E(Z), have been immunolocalized to TAS of all telomeres, except 2R (44), where these proteins were not detected despite the presence of TAS. Consistent with the methyltransferase activity of E(Z) toward histone H3 at Lys 27, H3Me3K27 was also enriched at TAS.…”