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In Drosophila, the Polycomb group and trithorax group proteins play a critical role in controlling the expression states of homeotic gene complexes during development. The common view is that these two classes of proteins bind to the homeotic complexes and regulate transcription at the level of chromatin. In the present work, we tested the involvement of both groups in mitotic heterochromatin formation in Drosophila. Using specific antibodies, we show that some of the tested Pc-G proteins are present in heterochromatin, while all the tested trx-G proteins localize to specific regions of heterochromatin in both mitotic chromosomes and interphase nuclei. We also observed that mutations in trx-G genes are recessive enhancers of position-effect variegation and are able to repress the transcription of heterochromatic genes. These results strongly suggest that trx-G proteins, along with some Pc-G proteins, play an active role in heterochromatin formation in Drosophila.

HP1 is a conserved chromosomal protein, first discovered in Drosophila, which is predominantly associated with the heterochromatin of many organisms. Recently, it has been shown that HP1 is required for telomere capping, telomere elongation, and transcriptional repression of telomeric sequences. Several studies have suggested a model for heterochromatin formation and epigenetic gene silencing in different species that is based on interactions among histone methyltransferases (HMTases), histone H3 methylated at lysine 9 (H3-MeK9), and the HP1 chromodomain. This model has been extended to HP1 telomeric localization by data showing that H3-MeK9 is present at all of the telomeres. Here, we tested this model, and we found that the capping function of HP1 is due to its direct binding to telomeric DNA, while the silencing of telomeric sequences and telomere elongation is due to its interaction with H3-MeK9.

Heterochromatin protein 1 (HP1) is a conserved nonhistone chromosomal protein, which is involved in heterochromatin formation and gene silencing in many organisms. In addition, it has been shown that HP1 is also involved in telomere capping in Drosophila. Here, we show a novel striking feature of this protein demonstrating its involvement in the activation of several euchromatic genes in Drosophila. By immunostaining experiments using an HP1 antibody, we found that HP1 is associated with developmental and heat shock-induced puffs on polytene chromosomes. Because the puffs are the cytological phenotype of intense gene activity, we did a detailed analysis of the heat shock-induced expression of the HSP70 encoding gene in larvae with different doses of HP1 and found that HP1 is positively involved in Hsp70 gene activity. These data significantly broaden the current views of the roles of HP1 in vivo by demonstrating that this protein has multifunctional roles.