1. Division of Microbial Genetics, National Institute of Genetics, The Graduate University for Advanced Studies, Yata 1111, Mishima, Shizuoka 411-8540, Japan
2. Department of Genetics, The Graduate University for Advanced Studies, Yata 1111, Mishima, Shizuoka 411-8540, Japan
3. PRESTO, JST, Kawaguchi, Saitama 332-0012, Japan
4. Present address: Imperial Cancer Research Fund, Clare Hall Laboratories, South Mimms EN6 3LD, UK

Correspondence to: Hiroyuki Araki^1,3,3 Correspondence and requests for materials should be addressed to H.A. (e-mail: Email: hiaraki@lab.nig.ac.jp).

Abstract

Cyclin-dependent protein kinases (Cdks) in eukaryotic cells work as a key enzyme at various points in the cell cycle^{1, 2}. At the onset of S phase, active S-phase Cdks (S-Cdks) are essential for chromosomal DNA replication³. Although several replication proteins are phosphorylated in a Cdk-dependent manner³, the biological effects of phosphorylation of these proteins on the activation of DNA replication have not been elucidated. Here we show that Sld2 (ref. 4) (also known as Drc1; ref. 5), one of the replication proteins of budding yeast (Saccharomyces cerevisiae), is phosphorylated in S phase in an S-Cdk-dependent manner, and mutant Sld2 lacking all the preferred Cdk phosphorylation sites (All-A) is defective in chromosomal DNA replication. Moreover, the complex that contains, at least, Sld2 and Dpb11 (ref. 6) (the Sld2–Dpb11 complex⁴) is formed predominantly in S phase; the All-A protein is defective in this complex formation. Because this complex is suggested to be essential for chromosomal DNA replication^{4, 5}, it seems likely that S-Cdk positively regulates formation of the Sld2–Dpb11 complex and, consequently, chromosomal DNA replication.