end-1, which encodes a GATA-type transcription factor, is a major zygotic regulator of endoderm development that is specifically expressed in the endoderm lineage from the E to E8 stage. This gene is one of the best known candidates for a zygotic gene that is directly regulated by a number of maternal factors, including POP-1, the target of the maternal Wnt pathway, and the maternal SKN-1 transcription factor. We are investigating elements in the
end-1 promoter and attempting to identify trans-acting factors that regulate the E lineage-specific expression of
end-1. We have begun deletion analysis in the
end-1 promoter to identify cis-acting elements important for its regulation. We have dissected a regulatory domain encompassing 1 kb upstream of the
end-1 transcriptional start site into three regions. Region I, the most upstream segment, which is rich in GATA and SKN-1 consensus binding sites, together with region III, confers proper E-specific expression of
end-1. Region III shows no activity alone. Purified SKN-1 protein can bind to region I and this binding can be out-competed with oligos containing consensus SKN-1 binding sites, suggesting that the E-specific expression directed by the combination of regions I and III is due at least in part to SKN-1 activity. This represents the first biochemical evidence for a direct link between a maternal factor and zygotic target gene in C. elegans. Another potential site for maternal regulation was identified in region II, which, in conjunction with region III, is also sufficient for proper
end-1 expression. Region II lacks SKN-1 consensus binding sites, suggesting the possibility of a SKN-1-independent mechanism that activates
end-1 expression in the E lineage. (Such a SKN-1-independent mechanism was previously suggested by the impenetrant phenotype of apparent
skn-1 null mutants.) Region II binds factors from crude extracts of early embryos in vitro. This binding activity may provide access to new maternal endoderm-regulating factors. Comparisons of the C. elegans
end-1 and
end-3 genes (see abstract by Maduro et al.) and the C. briggsae
end-1 gene reveal a number of potentially conserved regulatory elements. We will describe our progress in further dissecting
end-1 regulatory regions and attempts to identify the factors that regulate its endoderm-specific expression.