Fig. 3. The tetracycline-induced expression of cyclin B2 allows caffeine and okadaic acid to override the HU-induced replication checkpoints. (A) Mitotic indices of the asynchronously growing transgenic cell line without (lanes 1-6) or with (lanes 7-12) tetracycline-induced cyclin B2 expression. Cells were either treated with no drug (1 and 7), with caffeine (3 and 8) or okadaic acid (4 and 9) for 1 hour, or first with hydroxyurea for 16 hours (4 and 10), and then incubated for 1 hour with caffeine (5 and 12) or okadaic acid (6 and 12). (B) Cyclin B2-associated histone H1 kinase activities after immunopurification using the HA-epitope tag (H1 CycB2-HA), immunodetection of CycB2-HA protein using an anti HA-antibody, and CDKs using a PSTAIRE antibody, from cells treated in the same way as in A.