Academic journal article Genetics

The Large Isoform of Drosophila Melanogaster Heterochromatin Protein 2 Plays a Critical Role in Gene Silencing and Chromosome Structure

Academic journal article Genetics

The Large Isoform of Drosophila Melanogaster Heterochromatin Protein 2 Plays a Critical Role in Gene Silencing and Chromosome Structure

Article excerpt


Drosophila melanogaster heterochromatin protein 2 (HP2) interacts with heterochromatin protein 1 (HP1). In polytene chromosomes, HP2 and HP1 colocalize at the chromocenter, telomeres, and the small fourth chromosome. We show here that HP2 is present in the arms as well as the centromeric regions of mitotic chromosomes. We also demonstrate that Su(var)2-HP2 exhibits a dosage-dependent modification of variegation of a yellow reporter transgene, indicating a structural role in heterochromatin formation. We have isolated and characterized 14 new mutations in the Su(var)2-HP2 gene. Using w^sup m4h^, many (but not all) mutant alleles show dominant Su(var) activity. Su(var)2-HP2 mutant larvae show a wide variety of mitotic abnormalities, but not the telomere fusion seen in larvae deficient for HP1. The Su(var)2-HP2 gene codes for two isoforms: HP2-L (~365 kDa) and HP2-S (~175 kDa), lacking exons 5 and 6. In general, mutations that affect only the larger isoform result in more pronounced defects than do mutations common to both isoforms. This suggests that an imbalance between large and small isoforms is particularly deleterious. These results indicate a role for HP2 in the structural organization of chromosomes and in heterochromatin-induced gene silencing and show that the larger isoform plays a critical role in these processes.

THE DNA found inside a eukaryotic nucleus does not exist as such, but is packaged with proteins to form chromatin. By weight, chromatin is approximately one-third DNA, one-third histones, and one-third nonhistone chromosomal proteins, plus a small RNA component. The histones play a major role in packaging and organizing the very long DNA molecules of each chromosome. Analysis of post-transcriptional modifi- cations of histones shows that these basic proteins also play a significant role in determining specific modes of packaging and concomitant gene regulation, interacting with both enzymes and structural proteins that define alternative chromatin states (for reviews, see RICHARDS and ELGIN 2002; KHORASANIZADEH 2004). One level of packaging, evident by cytological examination of interphase nuclei, is the partitioning of chromatin into euchromatin and heterochromatin. While euchromatin decondenses during interphase, heterochromatin remains relatively more condensed, showing intense staining.

In Drosophila melanogaster, the entire Y chromosome, most of the fourth chromosome, the proximal 40% of the X chromosome, and the pericentric 20% of the major autosomes are heterochromatic. EuchromaticDNA has a high proportion of genes and unique sequences and tends to replicate throughout S phase. Heterochromatin has relatively few genes, is rich in repetitive sequences, and tends to replicate late in S phase; it is often found to be underreplicated (compared to euchromatin) in the polytene chromosomes of insects such as D. melanogaster. Heterochromatin is also characterized by a very low rate of meiotic recombination (WEILER and WAKIMOTO 1995; ZHIMULEV et al. 2004).

Euchromatin and heterochromatin appear to differ functionally as well. In a phenomenon termed positioneffect variegation (PEV), chromosomal rearrangements that juxtapose euchromatin with heterochromatin at breakpoints often result in the misregulation of the genes found within the regions flanking the breakpoints (SPOFFORD 1976). Misregulation can be seen not only in rearrangements, but also in transgenes, when a gene normally found in euchromatin is placed within a heterochromatic environment. These genes are appropriately expressed (time and place) in some cells but not in others, leading to a mottled or variegated pattern. For example, in the inversion In(1)w^sup m4h^, the white gene fails to express in some eye cells, leading to white patches in the eye. Such loss of normal expression, apparently the consequence of heterochromatic packaging, is described as silencing.

Many mutations that dominantly affect PEV have been recovered (REUTER and WOLFF 1981; GRIGLIATTI 1991). …

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