SEPARABLE FUNCTIONS OF ORC5 IN REPLICATION INITIATION AND SILENCING IN SACCHAROMYCES-CEREVISIAE

Origin recognition complex (ORC) is a six subunit complex :hat functio ns as the replication initiator and is required for silencing the HML and HMR loci in the yeast Saccharomyces cerevisiae. The roles of OXC5 in replication initiation and silencing were investigated to determine whether the two roles were mechanistically coincident or separable. S ome spontaneous revertants of ol orc5-1 were functional for replicatio n initiation, but not silencing. Other alleles of ORC5 were obtained t hat were nonfunctional for replication initiation, but fully competent for silencing. The two types of alleles, when put in the same cell, c omplemented, establishing two separable functions for ORC5. These data implied that replication initiation at HMR-E was not required for sil encing. The data were consistent with a model in which different ORC s pecies functioned at different origins within the genome and that only one Orc5p subunit functioned at any given origin.