A role for a replicator dominance mechanism in silencing

The role of the natural HMR-E silencer in modulating replication initiation and silencing by the origin recognition complex (ORC) was examined. When n atural HMR-E was the only silencer controlling HMR, the silencer's ORC-bind ing site (ACS) was dispensable for replication initiation but essential for silencing, indicating that a non-silencer chromosomal replicator(s) existe d in close proximity to the silencer. Further analysis revealed that region s flanking both sides of HMR-E contained replicators. In contrast to replic ation initiation by the intact silencer, initiation by the non-silencer rep licator(s) was abolished in an orc2-1 mutant, indicating that these replica tors were extremely sensitive to defects in ORC. Remarkably, the activity o f one of the non-silencer replicators correlated with reduced silencing; in activation of these replicators caused by either the orc2-1 mutation or the deletion of flanking sequences enhanced silencing. These data were consist ent with a role for the ORC bound to the HMR-E silencer ACS in suppressing the function of neighboring ORC molecules capable of inhibiting silencing, and indicated that differences in ORC-binding sites within HMR itself had p rofound effects on ORC function. Moreover, replication initiation by natura l HMR-E was inefficient, suggesting that closely spaced replicators within HMR contributed to an inhibition of replication initiation.