INDUCIBLE ACCELERATION OF G(1) PROGRESSION THROUGH TETRACYCLINE-REGULATED EXPRESSION OF HUMAN CYCLIN-E

Cyclin E is a cell cycle-regulated protein that activates the cdc2-rel ated protein kinases cdk2 shortly before S-phase entry. In order to an alyse the biological role of cyclin E, we have generated HeLa cells th at allow the conditional expression of ectopic human cyclin E. In thes e cells, a cyclin E cDNA is under the control of a bacterial tetracycl ine repressor-VP16 activator hybrid protein. In the absence of tetracy cline, the endogenous gene becomes activated and leads to the synthesi s of elevated levels of cyclin E. Concomitant with this increase in cy clin E expression we show by a combined time-lapse video recording/5-b romo-deoxyuridine labelling procedure a significant acceleration of G( 1) transition by approximately 1.5 hours. This observation is consiste nt with the idea that cyclin E is a rate-limiting factor with respect to the control of G(1)-->S transition. The experimental system describ ed here should also prove useful to address the function of cyclin E i n further detail.