FIG. 1.

HU-induced G₂ cell cycle delay correlates with the continued tyrosine phosphorylation of Cdc2. (A) Centrifugal elutriation was used to generate a synchronous population of wild-type (PR109) cells. The culture was treated with 12 mM HU for 90 min before synchronization, so that the cells would arrest in the first cell cycle after elutriation. Immediately after elutriation, the HU was washed out of the culture labeled HU−. The subsequent cell cycle progression of the two cultures was monitored by determining what percentage of the cells had gone through mitosis. (B) The tyrosine phosphorylation of Cdc2 in samples of the same cultures was analyzed by Western blotting with an antibody specific to Cdc2 phosphorylated on tyrosine-15. The blots were reprobed with an anti-PSTAIR antibody to visualize the total amount of Cdc2 present in the immunoprecipitates. Top two panels, HU− culture; bottom two panels, HU+ culture.