A genetic and molecular characterization of two proximal heterochromatic genes on chromosome 3 of Drosophila melanogaster.

Heterochromatin comprises a transcriptionally repressive chromosome compartment in the eukaryotic nucleus; this is exemplified by the silencing effect it has on euchromatic genes that have been relocated nearby, a phenomenon known as position-effect variegation (PEV), first demonstrated in Drosophila melanogaster. However, the expression of essential heterochromatic genes within these apparently repressive regions of the genome presents a paradox, an understanding of which could provide key insights into the effects of chromatin structure on gene expression. To date, very few of these resident heterochromatic genes have been characterized to any extent, and their expression and regulation remain poorly understood. Here we report the cloning and characterization of two proximal heterochromatic genes in D. melanogaster, located deep within the centric heterochromatin of the left arm of chromosome 3. One of these genes, RpL15, is uncharacteristically small, is highly expressed, and encodes an essential ribosomal protein. Its expression appears to be compromised in a genetic background deficient for heterochromatin protein 1 (HP1), a protein associated with gene silencing in these regions. The second gene in this study, Dbp80, is very large and also appears to show a transcriptional dependence upon HP1; however, it does not correspond to any known lethal complementation group and is likely to be a nonessential gene.