Genome duplication is precisely regulated by cyclin-dependent kinases CDKs, which bring about the onset of S phase by activating replication origins and then prevent re-licensing of origins until mitosis is completed. The optimum sequence motif for CDK phosphorylation is S/T-P-K/R-K/R, and Drc1-Sld2 is found to have at least 11 potential phosphorylation sites. Drc1 is required for DNA synthesis and S-M replication checkpoint control. Drc1 associates with Cdc2 and is phosphorylated at the onset of S phase when Cdc2 is activated. Thus Cdc2 promotes DNA replication by phosphorylating Drc1 and regulating its association with Cut5 [1]. Sld2 and Sld3 represent the minimal set of S-CDK substrates required for DNA replication [2].

1: CDK phosphorylation of Drc1 regulates DNA replication in fission yeast. Noguchi E, Shanahan P, Noguchi C, Russell P; Curr Biol. 2002;12:599-605. PMID:11937031

2: CDK-dependent phosphorylation of Sld2 and Sld3 initiates DNA replication in budding yeast. Tanaka S, Umemori T, Hirai K, Muramatsu S, Kamimura Y, Araki H; Nature. 2007;445:328-332. PMID:17167415