The
let-23 gene in the nematode Caenorhabditis elegans encodes a receptor tyrosine kinase and is necessary for the induction of a vulva, survival past the L1 stage, hermaphrodite fertility and for male spicule development. We sequenced the entire
let-23 genomic region of over 12 kb. The 5' end of the
let-23 mRNA was mapped by sequencing polymerase chain reaction products, and two mRNAs were found which had alternative exons and were probably initiated at different sites. One transcript was trans-spliced to SL1. Expression of either cDNA rescued a
let-23 vulvaless mutation in germline transformation. These results suggest that the
let-23 gene encodes two proteins of 1323 or 1335 amino acid residues. We prepared various 5' deletion constructs of the
let-23 gene, and examined their rescue activities for a
let-23 lethal or vulvaless mutation. The results revealed that two regions were required for
let-23 expression, one for larval survival and the other for vulva formation. We also cloned and analyzed a
let-23 homologue from Caenorhabditis vulgaris. It can encode two proteins of 77% amino acid residue identity with the Let-23 proteins. The 12 kb fragment carrying the C. vulgaris gene rescued the
let-23 vulvaless mutation in C. elegans. Seventeen sequences highly conserved between the two species were identified in the 5' upstream region or within an intron. Three of these sequences are contained in the two regions required for
let-23 expression, suggesting that they are cis-acting elements for
let-23 expression.