C. elegans has many characteristics that are attractive for cell cycle studies, including the fact that the location and timing of every somatic cell division are known and can be followed in the living animal. In earlier screens for animals with abnormal cell lineages, several mutants were isolated with defects in cellcycle progression (1,2). We are particularly interested in two genes,
lin-5 and
lin-6, which seem to be required both for cellcycle progression and for the control pathways that make the S and M phases interdependent. In
lin-5 animals cell division is blocked during mitosis while DNA replication continues, resulting in polyploid cells. By contrast, in postembryonic
lin-6 animals cell division continues, while DNA replication is absent in most somatic cells. We have initiated molecular and genetic characterizations of the
lin-5 and link genes. Five alleles of
lin-5 are currently available, including a temperaturesensitive allele that was kindly provided by David Merz and Joe Culotti. Using geTmline transformation experiments, we have identified an 8 kb genomic fragment that can rescue the
lin-5 phenotype. A 2.6 kb transcript from this region was found to contain mutations in three independent
lin-5 alleles and could rescue the
lin-5 phenotype when expressed from heatshock promoters. This mRNA is predicted to encode an 821 amino acid novel protein, with a large central domain that is likely to forth a coiled-coil alpha helix. Our studies are currently focused on the subcellular localization of the LIN-5 protein. The
lin-6 phenotype is defined by a single allele, which is probably a strong lossoffunction allele. Previously,
lin-6 was mapped to a region with few genetic or physical markers. We have identified a yeast artificial chromosome (YAC) that can rescue the
lin-6 defect. This YAC has been used to isolate cDNA clones from the region, which we are now testing as candidate
lin-6 cDNAs. We have established the presence of checkpoint control mechanisms in worms by studying the effects of the DNA replication inhibitor hydroxy- urea, as well as those of the microtubule-interacting drugs nocodazole and colchicine. With the ultimate goal of defining pathways for checkpoint control, we have isolated mutations in other loci that cause phenotypes similar to those of
lin-5 and
lin-6 mutants. Studies in C. elegans may teach us which aspects of checkpoint control are likely to be conserved from yeast to mammals and may reveal regulatory themes that are specific for metazoans. 1. Horvitz, H.R. and Sulston, J.E. (1980) Genetics 96; 435-454 2. Sulston, J.E. and Horvitz, H.R. (1981) Dev. Biology 82; 41-55