Abstract
The Cdc2-dependent inhibition of S phase is required in G2 for the correct ordering of the S and M phases in yeasts and Drosophila. This function of Cdc2 has been ascribed to its ability to phosphorylate replication factors to prevent the assembly of a preinitiation complex at the origin of replication. Whether this is the sole mechanism of S phase inhibition by Cdc2 in higher metazoans is not known because the pleiotropic functions of this essential cell cycle regulator make genetic analysis difficult.We show that Cdc2 co-expressed with Cyclin A inhibits the S phase in Drosophila salivary glands and diploid abdominal histoblasts. A kinase defective mutant of Cdc2 failed to promote mitosis, but was still able to inhibit the S phase with the same efficiency as the wild-type protein. In addition, Cdc2 and Cyclin A cooperatively inhibit transcriptional activation by the essential S phase regulator E2F. Cdc2 binds to E2F in vitro, and post-transcriptionally promotes its accumulation in vivo. Furthermore, the inhibitory effect of Cdc2 on S phase is overridden by E2F.The inhibition of S phase by Cdc2 is achieved in part by a kinase-independent mechanism, which is likely to be mediated by the inhibition of E2F.
