The ges-1 gene has been used to investigate how a particular gene can be expressed in only a subset of cells within a complex multicellular organism. The C. elegans ges-1 (Ce-ges-1) gene is only expressed in the E lineage (i.e. gut) of wildtype embryos. Expression is first detected when the gut lineage has only 4-8 cells and the embryo has -150 cells. Previous analysis of the Ce-ges-1 regulatory regions uncovered a gut activator-pharynx/rectum repressor regulatory switch: deletion of a 36 bp fragment containing two tandem WGATAR sites from the upstream Ce-ges-1 promoter switches expression off in the gut and on in the pharynx and rectum (Aamodt et. al., Science, 1991; Egan et al., Dev. Biol. 1995). Experiments performed by Fukushige et al. (Dev. Biol. 1996) have shown that this gut-to-pharynx/rectum switch changes Ce-ges-1 expression from the E lineage into portions of the digestive tract derived from the ABa (anterior pharynx), MS (posterior pharynx), and ABp (rectum) lineages. Further deletions have identified a putative pharynx activator element within 68 bp upstream from the ges-1 initiator methionine (T. Fukushige, unpublished data). A ges-1 homologue from the closely related nematode, C. briggsae, is also expressed in the gut (Kennedy et al., Dev. biol. 1992). Study of the C. briggsae ges-1 (Cb-ges-1) regulatory regions has also revealed a gut to pharynx/rectum regulatory switch, similar to that found for Ce-ges-1 regulation. A tandem pair of WGATA R sites, located ~700 bp upstream from the Cb-ges-1 translational start site, appear to regulate proper Cb-ges-1 gut expression. Additionally, a 90 bp region containing a putative Cb-ges-1 pharynx/rectum activator element is located downstream from the poly(A) signal site. Comparison of the Cb-ges-1 and Ce-ges-1 pharynx activator element does not reveal any obvious sequence or functional conservation between the Cb-ges-1 and Ce-ges-1 pharynx activator elements. However, alignment of the newly finished Cb-ges-1 sequence with Ce-ges-1 reveals a high degree of sequence conservation at the 3' end of both ges-1 genes. Our main conclusion is that the Cb-ges-1 gene, like its Ce-ges-1 counterpart, is capable of switching from expression in the gut to expression in the pharynx and rectum. In both genes, the control of this switch centres on a tandem pair of WGATAR sites. Details of how the expression switch actually work are not yet known but may well differ in the two nematodes.

Affiliation:
- Department of Biochemistry and Molecular Biology, University of Calgary, Calgary, Alberta, T2N 4N1, CANADA