The mechanisms by which developmental signals interact with the general cell-cycle machinery remain poorly understood. To address this issue in C. elegans , we first characterized components of the basic C. elegans cell-cycle machinery. We focused on a family of key cell-cycle regulators, the Cyclin-dependent kinases (Cdks). By molecular biological approaches and BLAST database searches, we identified six putative C. elegans Cdks. In RNAi experiments, only one of these kinases blocked cell division completely. This kinase, NCC-1 (first identified by Mori et al ., 1994), could act to promote passage through a single cell-cycle transition, analogous to Cdks in higher eukaryotes, or could promote passage through multiple cell-cycle transitions, like CDC28/cdc2 in yeasts. To differentiate between these two possibilities, we characterized
ncc-1 loss-of-function mutations isolated in a genetic screen based on the phenotype of rare larval arrested
ncc-1(RNAi) animals (Boxem et al ., in press). We found that
ncc-1 is required for M phase in meiotic and mitotic divisions, but not for progression through G 1 and S phase. Our results indicate that C. elegans uses multiple Cdks to regulate cell-cycle transitions, similar to higher eukaryotes, and that
ncc-1 is the C. elegans ortholog of Cdk1/Cdc2 in other metazoans. None of the six identified kinases were found to be essential for DNA replication by RNAi experiments. Therefore, we addressed the possibility of redundancy between these kinases. Injection of dsRNA from K03E5.3 (an ORF ~40% identical to Cdk1/Cdc2, Cdk2 and Cdk3) into
ncc-1/+ animals resulted in more severely mutant progeny than injection into N2, indicating a possible redundancy between these two kinases. To identify genes involved in linking developmental processes to the intrinsic cell-cycle machinery, we initiated screens to identify mutants that arrest early in the cell cycle, when external signals are most likely to affect proliferation. We are analyzing four mutants isolated in a screen for conditional alleles that arrest cell division in L1. For this screen hydroxyurea was used to select against animals that entered S-phase at the nonpermissive temperature. In addition, we have initiated a screen based on absence of the S-phase marker RNR::GFP (R. Roy and V. Ambros) to identify mutants that arrest cell division in G 0 /G 1 .