The
elt-2 gene encodes a single finger GATA transcription factor, cloned by virtue of its binding to GATA elements in the promoter of the gut specific
ges-1 gene. Previously, we reported (1) that ELT-2 protein is expressed exclusively in the gut and, in ectopic expression experiments, is sufficient to activate
ges-1 gene expression (as well as several other gut specific markers).
elt-2 null mutants die at the L1 stage. Although the gut is malformed, it is still clearly recognizable and most gut markers including
ges-1 are still expressed. This suggests that other factors may be able to compensate for the loss of
elt-2 in early gut development. The
end-1 gene is a good candidate for compensation for the loss of
elt-2.
end-1 encodes a GATA transcription factor and is transcribed from the 1E to 4E cell stage only in the gut (2). In collaboration with Joel Rothman's lab, ectopic END-1 expressed under the heat-shock promoter has been found to activate the expression of many gut markers, including
elt-2 and
ges-1. When ectopic END-1 is expressed ectopically in the
elt-2 mutant, several gut specific markers are also expressed ectopically. However,
ges-1 is expressed only in the presumptive gut, not ectopically. Furthermore, a deleted
ges-1 promoter transgene that is expressed only in the anterior cells of gut (See abstract by Schroeder) is not expressed in the
elt-2 mutant. Then, END-1 does not appear to compensate for loss of
elt-2 in the control of
ges-1 expression. What transcription factor does compensate for
elt-2? Our current best candidate is a new GATA factor (
elt-4?) that the genome sequence has identified immediately upstream of
elt-2. We are now exploring the role of
elt-4 in
ges-1 control and more generally in gut development. How is
elt-2 expression controlled? Our
elt-2 promoter analysis shows that a 600 bp region lying 3.5 kb upstream from the
elt-2 ATG is both necessary and sufficient to establish the expression of
elt-2 gene in early gut development. Numerous GATA sites and SKN-1 binding sites are contained in this 600 bp region. By producing worms transformed with both a 600 bp::reporter construct and either heat-shock::
end-1 or heat-shock::
elt-2 construct, we showed that ectopic END-1 can activate the 600 bp::reporter gene expression, but ectopic ELT-2 does not. Our current model is that END-1 may activate
elt-2 through direct binding to this 600 bp region of the
elt-2 promoter. After
end-1 levels decline at the 4E cell stage, ELT-2 maintains its own expression through the rest of the worm's life, by binding to other GATA sites in the
elt-2 promoter. Fukushige, T. et al. (1998) Dev Biol. in press Zhu, J. et al. (1997) Genes and Dev. 11, 2883-2896.