Role for cyclin A in the dependence of mitosis on completion of DMA replication

Abstract

THE cyclins were first identified by their cell-cycle-dependent synthesis and destruction1–3 and have a key role in the control of mitosis in Xenopusembryonic cell cycles4–6. 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 cycle2,7–10. The degradation of both cyclins is required for exit from mitosis11, but the activation and destruction of cyclin A occur earlier in the cell cycle than with the B-type cyclins9–11. This suggests that cyclin A has a distinct role in cell-cycle progression. We have used an antisense oligodeoxy-nucleotide 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.