THE GUT ESTERASE GENE (GES-1) FROM THE NEMATODES CAENORHABDITIS-ELEGANS AND CAENORHABDITIS-BRIGGSAE

The ges-1 gene codes for a non-specific carboxylesterase that is normally expressed only in the intestine of the nematode Cnenorhabditis elegans. In the current paper, we describe the cloning and characterization of the ges-1 gene from C, elegans, as well as the homologous gene from the nematode Caenorhabditis briggsae. The ges-1 esterases from the two nematodes are 83% identical at the amino acid level and contain regions of significant similarity to insect and mammalian esterases; these conserved regions can be identified with residues believed to be necessary for esterase function. The ges-1 mRNAs from both C. elegans and C. briggsae are trans-spliced. The coding regions, the codon bias and the splicing signals of the two ges-1 genes are quite similar and most (6/7) of the intron positions are retained precisely. Yet, the flanking sequences of the two ges-1 genes appear to have diverged almost completely. For example, the C. elegans ges-1 5′-flanking region (as well as several introns) contains copies of three different SINE-like sequences, previously identified near the hsp-16 genes, near the unc-22 gene and in a repetitive element CeRep-3; none of these elements are found in the C. briggsae ges-1 gene. We show that: (1) the C. elegans ges-1 gene can be used to transform C. briggsae, whereupon expression of the exogenous ges-1 gene is confined to the C. briggsae intestine; (2) the ges-1 homologue cloned from C. briggsae can be transformed into C. elegans, whereupon it is expressed largely in the C. elegans intestine; and (3) a 5′-deletion of the C. elegans ges-1 gene that we have previously shown to be expressed in the C. elegans pharynx is also expressed in the pharynx of C. briggsae (either in the presence or absence of vector sequences). These results suggest that the ges-1 gene control circuits have been maintained between the two nematode species, despite the divergent 5′-flanking sequences of the gene. This raises the question of the evolutionary distance between C. elegans and C. briggsae and we attempt to estimate the C. elegans-C. briggsae divergence time by analysing the rate of synonomous substitutions in coding regions of ges-1 and six other C. elegans-C. briggsae gene pairs. We propose a new method of analysis, which attempts to remove rate differences found between different genes by extrapolating to zero codon bias. We estimate that the two nematode species have been separated for at least 40 million years.