Roles for ORC in M Phase and S Phase

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Abstract
The origin recognition complex (ORC), a six-subunit protein, functions as the replication initiator in the yeast Saccharomyces cerevisiae . Initiation depends on the assembly of the prereplication complex in late M phase and activation in S phase. One subunit of ORC, Orc5p, was required at G 1 /S and in early M phase. Asynchronous cells with a temperature-sensitive orc5-1 allele arrested in early M phase. In contrast, cells that were first synchronized in M phase, shifted to the restrictive 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 introducing wild-type Orc5p during G 1 . Although all orc2 and orc5 mutations were recessive in the conventional sense, this dominant phenotype was shared with other orc5 alleles and an orc2 allele. The dominant inhibition to cell-cycle progression exhibited by the orc mutants was restricted to the nucleus, suggesting that chromosomes with mutant ORC complexes were capable of sending a signal that blocked initiation on chromosomes containing functional origins.