CYCLIN-E CONTROLS S-PHASE PROGRESSION AND ITS DOWN-REGULATION DURING DROSOPHILA EMBRYOGENESIS IS REQUIRED FOR THE ARREST OF CELL-PROLIFERATION

Most cells of the dorsal epidermis exit from the mitotic cycle after d ivision 16 in Drosophila embryogenesis. This exit is dependent on the down-regulation of Drosaphila cyclin E (DmcycE) during the final mitot ic cycle. Ectopic expression of DmcycE after the final mitosis induces entry into S phase and reaccumulation of G2 cyclins and results in pr ogression through a complete additional cell cycle. Conversely, analys es in DmcycE mutant embryos indicate that cyclin E is required for pro gression through S phase of the mitotic cycle. Moreover, endoreplicati on, which occurs in late wild-type embryos in the same pattern as Dmcy cE expression, is not observed in the mutant embryos. Therefore, Droso phila cyclin E, which forms a complex with the Dmcdc2c kinase, control s progression through S phase and its down-regulation limits embryonic proliferation.