Regulatory model of the interaction between Dpb11 and Sld2 phosphorylated by CDK. (A) The phosphorylation level of Sld2 is proportional to the level of CDK activity. However, phosphorylation of Thr84 in Sld2 requires prior phosphorylation of other CDK phosphorylation motifs. When CDK activity increases beyond threshold, Sld2 may change its conformation by multiple phosphorylations and then CDK phosphorylates Thr84. When Thr84 is phosphorylated, Sld2 forms a complex with Dpb11 to initiate DNA replication. (B) The pre-replicative complex (pre-RC) essential for the initiation of chromosomal DNA replication is formed at replication origins from late M phase to G1 phase when CDK activity is low. When CDK activity increases at G1/S phase boundary, the pre-RC components are phosphorylated and inactivated for further formation of the pre-RC before Thr84 of Sld2 is phosphorylated. Some other proteins are also phosphorylated and may bind to origins before Thr84-phosphorylation. Thus, inactivation of the pre-RC formation and preceding origin association of some replication proteins are ensured by this mechanism.