Figure 6.

 The S-phase progression checkpoint requires rad3, rad26, and cds1. (A) rad⁺ cells (h⁻ cyr1 sxa2) and rad3 mutant cells (h⁻cyr1 sxa2 rad3) were synchronized in G₁ by centrifugal elutriation (collecting early G₂ cells) followed by exposure to P factor. Cells were released from P factor-induced G₁ arrest in either the presence or absence of 0.033% MMS and S-phase progression followed by FACS. Attempts to synchronize other checkpoint mutants this way were complicated by the morphological changes associated with the cyr1 sxa2 background. (B) Wild-type, rad3, cds1, chk1, rad26, and rad26-T12 cells were synchronized in G₁ by nitrogen starvation, released into the cell cycle in the presence or absence of 0.033% MMS and followed by FACS. As before, rad3 cells are unable to slow S phase in response to DNA damage. Similarly, cds1 and rad26 are required for this checkpoint, whereas chk1 is not. Cells carrying the rad26.T12 mutation also have a defect in this checkpoint.