FIG. 4.

Activation of DN Myb accelerates transit through the M phase of the cell cycle. (A) K562-MERT cells were synchronized at G₂/M by nocodazole treatment following a double-thymidine block. Subsequently, the cells were released into the cell cycle and then treated with 4-OHT to allow MERT to inhibit endogenous Myb activity. The data provided are representative of four independent experiments. (B) Cyclin B1 protein expression was determined by Western blot analysis with the lysate of the cells used in panel A. A representative Western blot analysis of three independent experiments is shown. (C) On the left is cyclin B1 protein expression in the absence (diamonds) or presence (circles) of 4-OHT from panel B quantitated by densitometry. On the right are the percentages of K562-MERT cells in the G₂/M phase of the cell cycle in the absence (open circles) or presence (filled circles) of 4-OHT from panel A. (D) K562-MERT cells were synchronized in the G₁ phase by a double-thymidine block and subsequently released into the cell cycle in medium in the absence or presence of 4-OHT.