Thelet-23 gene encodes a receptor type tyrosine kinase(1) and is necessary for vulval induction, survival past theL1 stage, hermaphrodite fertility and for male spicule development. For the vulval induction, it was proposed thatLet-23 acts in vulval precursor cells(VPCs) to receive and transduce an inductive signal from the anchor cell(1). Genetic mosaic analysis of thelet-23 gene function supports this proposal(2). We have analyzed] the 5' regulatory regions of thelet-23 gene to understand the mechanism of the expression. We cloned
alet-23 homologue from Caenorhabditis vulgaris (formerly called C. vulgarensis ) to find evolutionary conservations in the upstream sequences. The gene in C. vulgaris probably encodes a protein of 1332 amino acids, 76% of which are shared withLet-23 of 1323 amino acids in C. elegans. The exon 15 for the tyrosine kinase domain in C. elegans is split into two exons in C. vulgaris, and a total of 19 exons is found in the C. vulgaris gene. The 12kb EcoRI-SalI fragment carrying the C. vulgaris gene rescued the lethal phenotype in the germline transformation oflet-23 (
mn23)mutant. Therefore, the gene cloned from C. vulgaris is homologous to the C. eleganslet-23 gene structurally and functionally. We have found sixteen 5' upstream sequences that are conserved between C. elegans and C. vulgaris (Fig. 1, a~p). We have also examined rescue activities of variouslet-23 5' deletion constructs. A construct which has 2984bp upstream of the initiation codon ATG rescuedlet-23 (
mn23)lethal mutation. However, constructs carrying 2656bp, 2279bp or 2055bp did not rescue the mutation. The region from -2984bp to -2656bp includes two conserved sequences(Fig. 1, c and d). One of them (d) has a sequence AATCTTCA/CT which is similar to a consensus AATATNCAT for Pit-l/GHF-l binding sites(3). Constructs which has 2055bp, 1926bp,1672bp or 1487bp upstream of the ATG rescuedlet-23 (
sy97)vulvaless mutation, but one with 1074bp did not. The region from -1487bp to -1074bp includes a conserved sequence(l in Fig. l) . These results indicate that the regions between -2984 and -2656 and that between -1487 and -1074 are required for the rescue of a lethal or vulvaless mutation, respectively. Two conserved sequences in the former (c and d) may be activating elements for the transcription of thelet-23 gene in the cells important for the larval survival. The conserved sequence in the latter (l) may be a cis-acting element required for the expression in VPCs. (1) Aroian, Koga, Mendel, Ohshima & Sternberg, Nature 348, 693-699(1990) (2) Koga & Ohshima, Abstracts of C. elegans Meeting, 248(1993) (3) Ingraham, Flynn, Voss, Albert, Kapiloff, Wilson & Rosenfeld, Cell 61,1021-1033(1990)