ROLES FOR ORC IN M-PHASE AND S-PHASE

The origin recognition complex (ORC), a six-subunit protein, functions as the replication initiator in the yeast Saccharomyces cerevisiae. I nitiation depends on the assembly of the prereplication complex in lat e M phase and activation in S phase. One subunit of ORC, Orc5p, was re quired at G(1)/S and in early M phase. Asynchronous cells with a tempe rature-sensitive orc5-1 allele arrested in early M phase. In contrast, cells that were first synchronized in M phase, shifted to the restric tive temperature, and then released from the block arrested at the G(1 )/S boundary, The G(1)/S arrest phenotype could not be suppressed by i ntroducing wild-type Orc5p during G(1). Although all orc2 and orc5 mut ations were recessive in the conventional sense, this dominant phenoty pe was shared with other orc5 alleles and an orc2 allele, The dominant inhibition to cell-cycle progression exhibited by the ore mutants was restricted to the nucleus, suggesting that chromosomes with mutant OR C complexes were capable of sending a signal that blocked initiation o n chromosomes containing functional origins.