Sexually reproducing organisms arise from germ cells, which have the potential to recreate the whole organism. An important question is how germ cells maintain this potential, or remain totipotent. In early C. elegans embryos, germline blastomeres divide asymmetrically to produce one somatic and one germline daughter. Maternally provided PIE-1 protein is present in the germline blastomeres throughout early development (Mello et al.). Germline blastomeres also contain (but do not respond to) maternally supplied transcription factors that cause differentiation of somatic blastomeres. In
pie-1 mutants, germline blastomeres respond to these transcription factors and acquire somatic fates. Thus, PIE-1 is required to prevent germline blastomeres from adopting somatic fates. But what is PIE-1 actually doing? Several lines of evidence suggest that PIE-1 inhibits accumulation of RNA polymerase II-dependent RNAs in the germline blastomeres, and the antibody H14 stains activated RNA polII in somatic cells, but not in germline blastomeres in
pie-1 (+) embryos (Seydoux et al.). This phenomenon is referred to as transcriptional inhibition, though the underlying mechanism has not been determined. When PIE-1 is ectopically overexpressed in the somatic blastomeres of the early embryo, it is capable of inhibiting the accumulation of mRNA in those blastomeres (Seydoux et al.). The finding that PIE-1 can cause transcriptional inhibition in all cells of the embryo suggests that PIE-1 targets are present in all cells. We are interested in the possibility of using ectopic PIE-1 in screens to identify additional genes involved in transcriptional repression. We have examined the effect on late embryonic and postembryonic development of a heat shock driven
pie-1 transgene (constructed and provided by Geraldine Seydoux). Late stage embryos and adult worms subjected to heat shock express abundant PIE-1 in somatic nuclei, however these same nuclei remain positive for activated RNA polII by H14 immunostaining. Nevertheless, heat shock causes a marked developmental delay that is not observed when larvae are treated with RNAi for
pie-1 . To examine the basis for this effect, we are screening for mutants that can suppress the developmental delay and asking whether these have affects on early embryogenesis that are consistent with PIE-1-mediated transcriptional repression. Mello C.C., Schubert C., Draper B., Zhang W., Lobel R., Priess J.R. (1996) Nature , 382(6593):710-2. Seydoux G., Mello C.C., Pettitt J., Wood W.B., Priess J.R., Fire A. (1996) Nature , 382(6593):713-6.