In most eukaryotic cells, DNA replication is confined to S phase of the cell cycle . During this interval, S-phase checkpoint controls restrain mitosis until replication is complete . In budding yeast, the anaphase inhibitor Pds1p has been associated with the checkpoint arrest of mitosis when DNA is damaged or when mitotic spindles have formed aberrantly [3,4], but not when DNA replication is blocked with hydroxyurea (HU). Previous studies have implicated the protein kinase Mec1p in S-phase checkpoint control . Unlike mec1 mutants, pds1 mutants efficiently inhibit anaphase when replication is blocked. This does not, however, exclude an essential S-phase checkpoint function of Pds1 beyond the early S-phase arrest point of a HU block. Here, we show that Pds1p is an essential component of a previously unsuspected checkpoint control system that couples the completion of S phase with mitosis. Further, the S-phase checkpoint comprises at least two distinct pathways. A Mec1p-dependent pathway operates early in S phase, but a Pds1p-dependent pathway becomes essential part way through S phase.
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We thank T. Weinert for yeast strains, A. Straight and A. Murray for TUB1–GFP constructs, M. Wolff for GFP:tet system construction, and M. Smeets, who originally observed that pds1 mutants are sensitive to subarresting concentrations of HU. D.J.C. was funded by EMBO and US Army Breast Cancer Research fellowships. M.S. was funded by EMBO and HFSP fellowships.