Separation of origin recognition complex functions by cross-species complementation

Transcriptional silencing at the HMRa locus of Saccharomyces cerevisiae requires the function of the origin recognition complex (ORC), the replication initiator of yeast. Expression of a Drosophila melanogaster Orc2 complementary DNA in the yeast orc2-1 strain, which is defective for replication and silencing, complemented the silencing defect but not the replication defect; this result indicated that the replication and silencing functions of ORC were separable. The orc2-1 mutation mapped to the region of greatest homology between the Drosophila and yeast proteins. The silent state mediated by DmOrc2 was epigenetic; it was propagated during mitotic divisions in a relatively stable way, whereas the nonsilent state was metastable. In contrast, the silent state was erased during meiosis.