DISTINCT MOLECULAR MECHANISMS REGULATE CELL-CYCLE TIMING AT SUCCESSIVE STAGES OF DROSOPHILA EMBRYOGENESIS

The conserved regulators of cell cycle progression-Cyclins, Cdc2 kinas e, and String phosphatase (Cdc25)-accommodate multiple modes of regula tion during Drosophila embryogenesis. During cell cycles 2-7, Cdc2/Cyc lin complexes are continuously present and show little fluctuation in abundance, phosphomodification, or activity. This suggests that cyclin g of the mitotic apparatus does not require cytoplasmic oscillations o f known regulatory activities. During cycles 8-13 a progressive increa se in the degradation of Cyclins at mitosis leads to increasing oscill ations of Cdc2 kinase activity. Mutants deficient in cyclin mRNAs suff er cell cycle delays during this period, suggesting that Cyclin accumu lation times these cycles. During interphase 14, programmed degradatio n of maternal String protein leads to inhibitory phosphorylation of Cd c2 and cell cycle arrest. Subsequently, mitoses 14-16 are triggered by pulses of zygotic string transcription.