Transcriptional repression of euchromatic genes by Drosophila heterochromatin protein 1 and histone modifiers

In Drosophila, heterochromatin protein 1 (HP1) suppresses the expression of euchromatic genes that are artificially translocated adjacent to heterochromatin by expanding heterochromatin structure into neighboring euchromatin. The purpose of this study was to determine whether HP1 functions as a transcriptional repressor in the absence of chromosome rearrangements. Here, we show that Drosophila HP1 normally represses the expression of four euchromatic genes in a dosage-dependent manner. Three genes regulated by HP1 map to cytological region 31 of chromosome 2, which is immunostained by anti-HP1 antibodies in the salivary gland. The repressive effect of HP1 is decreased by mutation in Su(var)3-9, whose mammalian orthologue encodes a histone H3 methyltransferase and mutation in Su(var)2-1, which is correlated with histone H4 deacetylation. These data provide genetic evidence that an HP1-family protein represses the expression of euchromatic genes in a metazoan, and that histone modifiers cooperate with HP1 in euchromatic gene repression.