Differential assembly of Cdc45p and DNA polymerases at early and late origins of DNA replication

Chromosomes are replicated in characteristic, temporal patterns during S ph ase. We have compared the timing of association of replication proteins at early- and Late-replicating origins of replication. Minichromosome maintena nce proteins assemble simultaneously at early- and late-replicating origins . In contrast, Cdc45p association with late origins is delayed relative to early origins. DNA polymerase alpha association is similarly delayed at lat e origins and requires Cdc45p function. Activation of the S phase checkpoin t inhibits association of Cdc45p with late-firing origins. These studies su ggest that Cdc45p is poised to serve as a key regulatory target for both th e temporal and checkpoint-mediated regulation of replication origins.