ASSOCIATION OF THE ORIGIN RECOGNITION COMPLEX WITH HETEROCHROMATIN AND HP1 IN HIGHER EUKARYOTES

The origin recognition complex (ORC) is required to initiate eukaryoti c DNA replication and also engages in transcriptional silencing in S. cerevisiae. We observed a striking preferential but not exclusive asso ciation of Drosophila ORC2 with heterochromatin on interphase and mito tic chromosomes. HP1, a heterochromatin-localized protein required for position effect variegation (PEV), colocalized with DmORC2 at these s ites. Consistent with this localization, intact DmORC and HP1 were fou nd in physical complex. The association was shown biochemically to req uire the chromodomain and shadow domains of HP1. The amino terminus of DmORC1 contained a strong HP1-binding site, mirroring an interaction found independently in Xenopus by a yeast two-hybrid screen. Finally, heterozygous DmORC2 recessive lethal mutations resulted in a suppressi on of PEV. These results indicate that ORC may play a widespread role in packaging chromosomal domains through interactions with heterochrom atin-organizing factors.