The
ges-1 gene codes for a non-specific carboxylesterase that is normally expressed only in the intestine of the nematode Caenorhabditis 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 synonymous 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.(ABSTRACT TRUNCATED AT 400 WORDS)