Origin Recognition Complex (ORC) in Transcriptional Silencing and DNA Replication in S. cerevisiae

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Abstract
In Saccharomyces cerevisiae , the HMR-E silencer blocks site-specific interactions between proteins and their recognition sequences in the vicinity of the silencer. Silencer function is correlated with the firing of an origin of replication at HMR-E . An essential gene with a role in transcriptional silencing was identified by means of a screen for mutations affecting expression of HMR . This gene, known as ORC2 , was shown to encode a component of the origin recognition complex that binds yeast origins of replication. A temperature-sensitive mutation in ORC2 disrupted silencing in cells grown at the permissive temperature. At the restrictive temperature, the orc2-1 mutation caused cell cycle arrest at a point in the cell cycle indicative of blocks in DNA replication. The orc2-1 mutation also resulted in the enhanced mitotic loss of a plasmid, suggestive of a defect in replication. These results provide strong evidence for an in vivo role of ORC in both chromosomal replication and silencing, and provide a link between the mechanism of silencing and DNA replication.