The
let-60 gene of Caenorhabditis elegans controls the choice between vulval and hypodermal differentiation in response to an inductive signal from the gonad.
let-60 encodes a ras protein that acts downstream of the
let-23 receptor tyrosine kinase in a signal transduction pathway. Dominant-negative mutations of
let-60 [
let-60(dn)] cause a reduction of the gene activity in
let-60(dn)/+ heterozygotes and a vulva-less mutant phenotype. We have found that nine
let-60(dn) mutations cause replacements of conserved residues. Four are in two novel positions; others are in positions known previously to cause dominant-negative mutations in mammalian cells. The locations of these lesions suggest that they disrupt the ability of the ras protein to bind guanine nucleotides. Four
let-60(dn) mutant genes were introduced into wild-type animals in the form of extrachromosomal arrays and were found to generate three dominant phenotypes--lethality, vulva-less, or multivulva--depending on gene dose and alleles. The dominant lethality caused by high-dose transgenic
let-60(dn) genes suggests a toxic effect of these mutant genes in early development. The dominant-negative effects of these mutations in heterozygotes are likely to be caused by competition between
let-60(dn) and
let-60(+) protein for a positive regulator. All
let-60(dn) mutations interfere with
let-60(+) activity, but some alleles have partial constitutive activity, suggesting that the ability to interact with the activator is separable from the ability to exert a physiological effect (stimulation of vulval differentiation). These dn mutations might be useful for interfering with ras-mediated signal transduction pathways in other multicellular organisms.