Regulation of postembryonic G(1) cell cycle progression in Caenorhabditis elegans by a cyclin D/CDK-like complex

In many organisms, initiation and progression through the G(1) phase of the cell cycle requires the activity of G(1)-specific cyclins (cyclin D and cyclin E) and their associated cyclin-dependent kinases (CDK2, CDK4, CDK6). We show here that the Caenorhabditis elegans genes cyd-1 and cdk-4, encoding proteins similar to cyclin D and its cognate cyclin-dependent kinases, respectively, are necessary for proper division of postembryonic blast cells. Animals deficient for cyd-1 and/or cdk-4 activity have behavioral and developmental defects that result from the inability of the postembryonic blast cells to escape G(1) cell cycle arrest. Moreover, ectopic expression of cyd-1 and cdk-4 in transgenic animals is sufficient to activate a S-phase reporter gene. We observe no embryonic defects associated with depletion of either of these two gene products, suggesting that their essential functions are restricted to postembryonic development. We propose that the cyd-1 and cdk-4 gene products are an integral part of the developmental control of larval cell proliferation through the regulation of G(1) progression.