The Caenorhabditis elegans ges-1 gene (gut esterase No. 1) is expressed only in the intestinal lineage, beginning when the developing gut has only four to eight cells. We analyze the sequence requirements for this tissue-specific gene regulation by injecting deleted/mutated constructs of the ges-1 gene into a viable ges-1 (null) strain of worms and assaying heritably transformed embryos by esterase histochemistry. Many deletion constructs accurately reconstitute the wildtype gut-specific ges-1 expression. However, deletions in the neighborhood of 1100 bp upstream of the ges-1 ATG abolish ges-1 expression in the developing gut, while at the same time activating ges-1 expression in cells of the pharynx/tail that appear to belong to the sister lineage of the gut. Deletions of a 36-bp DNA region containing two tandem WGATAR sequences are sufficient to cause this gut-to-pharynx/tail switch in expression pattern. Deletion of either one of the WGATAR sites or deletion of an adjoining downstream region directs ges-1 expression only in a restricted set of cells of the anterior gut. The ges-1 GATA region acts like a gut-specific enhancer in that: (i) it restores ges-1 gut expression when reinserted elsewhere into the GATA-deleted ges-1 gene; and (ii) multiple copies direct gut expression of an hsp16-lacZ reporter gene. The ges-1 GATA-region also acts as the site of the pharynx/tail repression in that reinsertion elsewhere into the GATA-deleted ges-1 construct causes repression of ges-1 in the pharynx/tail. However, multiple copies of the GATA region are not able to repress the heat-induced expression of an hsp16-lacZ reporter gene, suggesting that the pharynx/tail repression mechanism is specific to the ges-1 environment. Finally, mutation rather than deletion of the individual GATA sequences suggests that gut activation and pharynx/tail repression may be due to separate factors. We present a molecular model that summarizes these results. The ges-1 control circuitry appears surprisingly complex for what might have been expected to be the simplest possible example of a nonessential gene expressed early in a clonal embryonic lineage.

The Caenorhabditis elegans digestive tract is composed of four distinct modules derived from separate cell lineages: anterior pharynx from the ABa lineage, posterior pharynx from the MS lineage, gut from the E lineage, and rectum from the ABp lineage. The C. elegans gut esterase gene (ges-1) is normally expressed in the embryonic gut or E lineage. However, expression ges-1 can be switched into cells of the embryonic pharynx and tail by virtue of deleting a tandem pair of WGATAR sites in the ges-1 promoter. Here, we use both laser ablation experiments and genetic analysis to show that cells expressing the WGATAR-deleted ges-1 transgene belong to all three nongut lineages of the digestive tract: ABa, MS, and ABp. We also show that the molecular size and spatial distribution of ges-1 mRNA transcripts produced by either the WGATAR-deleted ges-1 transgene or the undeleted ges-1 control transgene appear correctly regulated, suggesting that the spatial switch in ges-1 expression occurs at the level of transcription initiation. We further show that both the WGATAR-deleted and the undeleted ges-1 transgenes respond appropriately to mutations in a series of maternal effect genes (skn-1, mex-1, pie-1, and pop-1) that alter early blastomere fate. Moreover, the pharynx/tail expression of the WGATAR-deleted ges-1 transgene is abolished by mutations in the zygotic gene pha-4. Finally, we use imprecise transposon excision to produce two independent C. elegans strains with 1- to 2-kb deletions that remove the tandem WGATAR sites from the promoter of the endogenous chromosomal ges-1 gene: in both of these strains, ges-1 is not expressed in the embryonic gut but is expressed in cells of the embryonic pharynx; pharynx expression is weak but incontrovertible. Overall, our results validate previous transgenic analysis of ges-1 control and show further that ges-1 appears to be regulated in a system-specific, rather than a lineage-specific, manner. The multiple facets of ges-1 expression provide an opportunity to investigate how a multicomponent organ system such as the digestive tract is established from diverse cell lineages.