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 G1/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 G1/S boundary. The G1/S arrest phenotype could not be suppressed by introducing wild-type Orc5p during G1. 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.
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| Gene | Phenotype | Experiment Type | Mutant Information | Strain Background | Chemical | Details |
|---|---|---|---|---|---|---|
| ORC5 | cell cycle progression in M phase: arrested | classical genetics | conditional Allele: orc5-1 | W303 | Temperature: elevated temperature, 37 °C Details: asynchronous cultures arrest in early M phase with 2C DNA content and fully replicated DNA | |
| ORC5 | cell cycle progression through the G1/S phase transition: arrested | classical genetics | conditional Allele: orc5-1 | W303 | Temperature: elevated temperature, 37 °C Treatment: nocodazole block release Details: arrest after bud emergence with a 1C DNA content and unreplicated DNA |