Heterochromatic DNA sequences in the polytene chromosomes of Drosophila melanogaster salivary glands

The term ‘heterochromatin’ denotes chromosomal regions that remain condensed throughout most of the cell cycle, show late replication during normal cell cycles, under-replication in polytene chromosomes, extremely low gene density and high density of repeated sequences. Heterochromatin is characterized by modified histones H3 and H4 and high regularity of nucleosomes (reviewed by Gatti and Pimpinelli, 1992; Lohe and Hilliker, 1995; Weiler and Wakimoto, 1995; Elgin, 1996; Wallrath, 1998; Zhimulev, 1998; Dillon and Festenstein, 2002; Richards and Elgin, 2002). At the cytological level, heterochromatin in mitotic chromosomes of Drosophila melanogaster has been divided into 61 regions (designed h1-h61) according to Hand Nstaining (Gatti et al., 1994), which reflects heterogeneous contents of DNA. Sequences forming heterochromatin are divided into highly repeated (satellites), moderately repeated (mainly transposable elements) and occasional unique (genes). These sequences occur in fixed positions within the heterochromatic regions (Lohe et al., 1993; Pimpinelli et al., 1995; Carmena and Gonzalez, 1995; Makunin et al., 1999). Large blocks of simple satellite DNAs are interrupted by ‘islands’ of complex sequences (Le et al., 1995). The few heterochromatic genes are clustered within dull or moderately fluorescent regions after H-staining (Dimitri, 1991; Dimitri et al., 2003; Koryakov et al., 2002). Euchromatin and heterochromatin also differ in nucleosome and histone organization. Transcriptionally active euchromatin has long tracts without nucleosomes, which are hypersensitive for nucleases, whereas inactive heterochromatin has long stretches with regular nucleosomal pattern (Sun et al., 2001). Heterochromatic histones carry epigenetically heritable modifications including hypoacetylation of H3 and H4 histones and methylation of lysine-9 of H3 histone (reviewed by Dillon and Festenstein, 2002; Richards and Elgin, 2002). In addition to modified histones, Drosophila heterochromatin contains the non-histone proteins HP1, Su(var)3-7, Su(var)3-9 and SuUR. The HP1 protein is found mainly in pericentric heterochromatin, chromosome 4 and telomeres (James and Elgin, 1986), where it binds to methylated lysine-9 of H3 histone (Jacobs et al., 2001). This methylation is probably catalysed by Su(var)3-9 protein, as mammalian homologs of Drosophila Su(var)3-9 have been shown to have this activity (Aagaard et al., 1999; Rea et al., 2000). The Su(var)3-7 protein also interacts with HP1 (Cleard et al., 1997). The SuUR protein is present in pericentric and intercalary heterochromatin, but its function remains unclear (Makunin et al., 2002). 1035