IN fission yeast, Cdc2 kinase has both positive and negative roles in
regulating DNA replication, being first necessary for the transition f
rom 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 initi
ation(4-6). Fission yeast Orp2 is required for DNA replication and int
eracts with the rate-limiting replication activator Cdc18. Cells lacki
ng Orp2 undergo aberrant mitosis, indicating that Orp2 is involved in
generating a checkpoint signal, These findings suggest that ORC functi
ons are conserved among eukaryotes and provide evidence that Cdc2 cont
rols DNA replication initiation by acting directly at chromosomal orig
ins.