Getting started: regulating the initiation of DNA replication in yeast.

Initiation of DNA replication in yeast appears to operate through a two-step process. The first step occurs at the end of mitosis in the previous cell cycle, where, following the decrease in B cyclin-dependent kinase activity, an extended protein complex called the prereplicative complex (pre-RC) forms over the origin of replication. This complex is dependent on the association of the Cdc6 protein with the Origin Recognition Complex (ORC) and appears concomitantly with the nuclear entry of members of the Mcm family of proteins. The second step is dependent upon the cell passing through a G1 decision point called Start. If the environmental conditions are favorable, and the cells reach a critical size, then there is a rise in G1 cyclin-dependent kinase activity, which leads to the activation of downstream protein kinases; the protein kinases are, in turn, required for triggering initiation from the preformed initiation complexes. These protein kinases, Dbf4-Cdc7 and Clb5/6(B-cyclin)-Cdc28, are thought to phosphorylate targets within the pre-RC. The subsequent rise in B cyclin protein kinase activity following Start not only triggers origin firing, but also inhibits the formation of new pre-RCs, which ensures that there is only one S phase in each cell cycle. The destruction of B-cyclin protein kinase activity at the end of the cell cycle potentiates the formation of new pre-RCs-resetting origins for the next S phase.