2000 West Coast Worm Meeting abstract 57
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Department of MCD Biology, UC Santa Barbara, Santa Barbara, CA 93106
EMS produces daughters with different fates: E produces the endoderm while MS makes mesoderm, including the posterior pharynx. Depletion of the zygotically expressed MED-1 and -2 GATA transcription factors by RNAi results in a penetrant MS ’ C transformation and an impenetrant E ’ C transformation, similar to that seen in maternal skn-1 mutants. However, unlike skn-1 mutants, ABa still makes anterior pharynx in med-1,2(RNAi) embryos, indicating that the med genes are not required for the GLP-1-mediated signal that induces ABa pharynx. pos-1 mutants also misspecify MS, yet retain GLP-1-dependent pharynx (Tabara et al. 1999). These observations suggest that POS-1 is also required for an activity downstream of SKN-1.
The med genes appear to act at the convergence of SKN-1 and POS-1 function in the EMS lineage. While expression of a med-1::GFP::MED-1 reporter is detected in the early E and MS lineages, it is undetectable in both skn-1(RNAi) and pos-1(RNAi) embryos. Ectopic med expression is detected in embryos expressing SKN-1 ectopically. In addition, expression requires a cluster of upstream conserved SKN-1 binding sites, which bind bacterially expressed SKN-1 in vitro. We propose that POS-1, which is primarily cytoplasmic, modulates an activity required for transcriptional activation of the meds by SKN-1.
end-1 and end-3 specify E fate, and are likely downstream targets of med-1,2 in the E lineage. Indeed, GFP-tagged MED-1 binds in vivo to an extrachromosomal array containing the end-3 promoter. This interaction also occurs in MS, where end-1 and end-3 are known to be repressed by POP-1. Thus, the mechanism by which POP-1 represses E fate in MS does not preclude binding of MED-1 to end-3.
Our data indicate that SKN-1 directly activates the med genes through a POS-1-requiring mechanism. As the meds are required for both E and MS specification, their activity appears to be modulated by POP-1, the target of the Wnt/MAPK pathways in the E blastomere, perhaps at a step after their binding to target promoters.