Interaction of Cdc2 and Cdc18 with a fission yeast ORC2-like protein

IN fission yeast, Cdc2 kinase has both positive and negative roles in regulating DNA replication, being first necessary for the transition from G(1) to S phase and later required to prevent the re-initiation of DNA replication during G(2)(1-3). We report here that Cdc2 interacts with Orp2, a protein similar to the Orc2 replication factor subunit of Saccharomyces cerevisiae origin recognition complex (ORC). ORC binds chromosomal origins and is essential for chromosomal replication initiation(4-6). Fission yeast Orp2 is required for DNA replication and interacts with the rate-limiting replication activator Cdc18. Cells lacking Orp2 undergo aberrant mitosis, indicating that Orp2 is involved in generating a checkpoint signal, These findings suggest that ORC functions are conserved among eukaryotes and provide evidence that Cdc2 controls DNA replication initiation by acting directly at chromosomal origins.