We describe the
elt-4 gene from the nematode Caenorhabditis elegans.
elt-4 is predicted to encode a very small (72 residues, 8.1 kD) GATA-type zinc finger transcription factor. The
elt-4 gene is located similar
to5 kb upstream of the C. elegans
elt-2 gene, which also encodes a GATA-type transcription factor; the zinc finger DNA-binding domains are highly conserved (24/25 residues) between the two proteins. The
elt-2 gene is expressed only in the intestine and is essential for normal intestinal development. This article explores whether
elt-4 also has a role in intestinal development. Reporter fusions to the
elt-4 promoter or reporter insertions into the
elt-4 coding regions show that
elt-4 is indeed expressed in the intestine, beginning at the 1.5-fold stage of embryogenesis and continuing into adulthood.
elt-4 reporter fusions are also expressed in nine cells of the posterior pharynx. Ectopic expression of
elt-4 cDNA within the embryo does not cause detectable ectopic expression of biochemical markers of gut differentiation; furthermore, ectopic
elt-4 expression neither inhibits nor enhances the ectopic marker expression caused by ectopic
elt-2 expression. A deletion allele of
elt-4 was isolated but no obvious phenotype could be detected, either in the grit or elsewhere; brood sizes, hatching efficiencies, and growth rates were indistinguishable from wild type. We found no evidence that
elt-4 provided backup functions for
elt-2. We used microarray analysis to search for genes that might be differentially expressed between L1 larvae of the
elt-4 deletion strain and wild-type worms. Paired hybridizations were repeated seven times, allowing us to conclude, with some confidence, that no candidate target transcript could be identified as significantly up- or downregulated by loss of
elt-4 function. In vitro binding experiments could not detect specific binding of ELT-4 protein to candidate binding sites (double-stranded oligonucleotides containing single or multiple WGATAR sequences); ELT-4 protein neither enhanced nor inhibited the strong sequence-specific binding of the ELT-2 protein. Whereas ELT-2 protein is a strong transcriptional activator in yeast, ELT-4 protein has no such activity tinder similar conditions, nor does it influence the transcriptional activity of coexpressed ELT-2 protein. Although an
elt-2 homolog was easily identified in the genomic sequence of the related nematode C. briggsae, no
elt-4 homolog could be identified. Analysis of the changes in silent third codon positions within the DNA-binding domains indicates that
elt-4 arose as a duplication of (
elt-2, some 25-55 MYA. Thus,
elt-4 has survived far longer than the average duplicated gene in C. elegans, even though no obvious biological function Could be detected.
elt-4 provides an interesting example of a tandemly duplicated gene that may originally have been the same size as
elt-2 but has gradually been whittled down to its present size of little more than a zinc finger. Although
elt-4 must confer (or must have conferred) some selective advantage to C. elegans, we suggest that its