ROLE FOR CYCLIN-A IN THE DEPENDENCE OF MITOSIS ON COMPLETION OF DNA-REPLICATION

THE cyclins were first identified by their cell-cycle-dependent synthesis and destruction 1-3 and have a key role in the control of mitosis in Xenopus embryonic cell cycles 4-6. All higher eukaryotes have at least two types of cyclins, the A- and B-type, which can be distinguished by sequence motifs and the timing of their destruction in the cell cycle 2,7-10. The degradation of both cyclins is required for exit from mitosis 11, but the activation and destruction of cyclin A occur earlier in the cell cycle than with the B-type cyclins 9-11. This suggests that cyclin A has a distinct role in cell-cycle progression. We have used an antisense oligodeoxynucleotide directed against cyclin A to investigate this role. Ablation of cyclin A messenger RNA in cytostatic factor/metaphase-arrested extracts of Xenopus eggs, followed by in vitro progression into interphase, resulted in the premature appearance of cyclin B/cdc2-associated H1 kinase activity and premature entry into mitosis. Although cyclin A-ablated extracts could initiate DNA synthesis during interphase, S phase was not completed before entry into mitosis. The effects of cyclin A ablation were reversed by the addition of cyclin A mRNA or cyclin A protein to the extracts.