Fig. 2. Cdk1 phosphorylation and S phase checkpoint. (A) Effects of inhibition of DNA synthesis by hydroxyurea on Cdk1 phosphorylation. HU (1 mg/ml) was added to an early log phase culture of FB1 cells and samples were taken at 30 and 60 minutes after HU addition. (B) Impaired tyrosine phosphorylation of Cdk1 causes sensitivity to HU. Wild-type (FB1), and UMC5 and UMC6 cells expressing myc-tagged Cdk1 (P_tef1:cdk1-myc) and the constitutively unphosphorylated Cdk1 allele (P_tef1:cdk1^AF-myc), respectively, were spotted as serial dilutions onto YPD plates containing a series of HU concentrations and the plates were incubated at 28°C for 3 days.