Stepwise assembly of initiation complexes at budding yeast replication origins during the cell cycle.

DNA replication is a pivotal event in the cell cycle and, as a consequence, is tightly controlled in eukaryotic cells. The initiation of DNA replication is dependent upon the completion of mitosis and upon the commitment to complete the cell cycle made during G(1). Characterisation of the protein factors required for initiating DNA replication is essential to understand how the cell cycle is regulated. Recent results indicate that initiation complexes assemble in multiple stages during the cell cycle. First, origins are bound by the multisubunit origin recognition complex (ORC) which is essential for DNA replication in vivo. ORC, present at little more than one complete complex per replication origin, binds to origins immediately after initiation in the previous cell cycle. ORC binding occurs by the recognition of a bipartite sequence that includes the essential ARS consensus sequence (ACS) and the functionally important B1 element adjacent to the ACS. A novel pre-replicative complex (pre-RC) assembles at origins at the end of mitosis in actively cycling cells and remains at origins until DNA replication initiates. Finally, Dbf4, which is periodically synthesised at the end of G(1), interacts with replication origins. Dbf4-origin interaction requires an intact ACS strongly suggesting that interaction occurs through ORC. Dbf4 interacts with and is required for the activation of the Cdc7 protein kinase and together, Dbf4 and Cdc7 are required for the G(1)-S transition. Separate regions of Dbf4 are required for Cdc7- and origin-interaction suggesting that Dbf4 may act to recruit Cdc7 to replication origins where phosphorylation of some key component may cause origin firing.