Early in embryonic development, the 5 somatic founder cells, AB, MS, E, C, and D, each have a distinct cell cycle periodicity. The cell cycle within a particular founder cell lineage is relatively synchronous within that lineage, but asynchronous with the other founder cells. The two germline cells, Z2 and Z3, are born early in the embryo, but do not divide during the remainder of embryogenesis; they do proliferate extensively during larval and adult development to generate the germline. We have initiated a project to examine the regulation of these cell cycles during C. elegans development. Biochemical and genetic experiments from other systems have revealed that serine/threonine kinases regulate the progression from one stage of the cell cycle to the next (i.e., CDC2, CDKs). These kinases in turn are regulated by phosphorylation. In these other systems, CDC2 is phosphorylated on a single tyrosine residue by WEE1 to maintain it as an inactive kinase during much of the cell cycle. This phosphotyrosine residue and an adjacent phosphothreonine residue are dephosphorylated by a dual-specificity phosphatase, CDC25, to activate CDC2 for G2-to-M phase progression. Two candidate
cdc25 genes (one on chrom. II and one on III) and a candidate
wee1 gene have been sequenced by the Genome Consortium. To begin the characterization of these genes, we performed RNA in situ analysis in embryos. Seydoux and Fire (1994) have previously observed that the
cdc2 (
ncc-1) mRNA is ubiquitously expressed throughout embryonic development. Unlike
cdc2, the mRNA expression pattern of the
cdc25 gene from chrom. III has a more restricted pattern. It is a maternal message present in one cell embryos. This message partitions to the anterior half of the embryo before its first cell division and then persists in AB and the progeny of AB for a few cell divisions. It is not present in P1 or blastomeres derived from P1. The signal fades out by the 16 cell stage. There is no detectable zygotic expression of this
cdc25 gene throughout embryogenesis. We are currently determining the mRNA expression pattern of the
cdc25 gene on chrom. II and are attempting protein in situ analyses for these two gene products. The
wee1 mRNA expression pattern was also distinct from that of
ncc-1. It is not a maternal message. Message first appears in the 12-cell embryo in the nucleus of the E blastomere, and then is observed in the nuclei of the 8 AB progeny in the 16-cell embryo. The mRNA expression pattern coincides with prometaphase and metaphase, and is not present in cells in interphase, anaphase, or telophase. We have yet to detect
wee1 message in the cytoplasm of any cell. The
wee1 message appears to remain nuclear at a time in the cell cycle when nuclear envelope breakdown has occurred. This short burst of zygotic
wee1 expression in the 12 and 16-cell embryo is the only time we detect
wee1 message in embryos. We have made GFP fusions for all the above genes and are currently analyzing their expression patterns. A number of let, emb, and mel mutations have been identified by the worm community and map near these cell cycle regulators. We are currently attempting to rescue these mutants with the appropriate transgene. Research sponsored by the National Cancer Institute, DHHS, under contract with ABL.