Mebendazole-resistant mutants of Caenorhabditis elegans were isolated following mutagenesis of the wild-type strain, N2, with ethylmethane sulphonate. The mutants define a single autosomal gene and are allelic to ben-1
. They grow, move, and reproduce normally in the presence of mebendazole and are cross-resistant to albendazole, cambendazole, fenbendazole, methylbenzimidazole carbamate, and thiabendazole. Accumulations of membrane-bound, electron-dense material associated with the nervous tissue and associated cells are seen in electron micrographs of L3, L4 and adult stages of N2 grown in the presence of mebendazole, but are not found in the mutants. The electron-dense material appears to be associated with the accessory cells of the neuropil rather than with neurons. The tubulins from wild type, N2, and the mutants were characterised by two-dimensional electrophoresis followed by silver staining and Western blotting with monoclonal antibodies to a- and B-tubulin. A single isotype of a-tubulin is apparent in both N2 and the mutants. One major and three minor B-tubulin isotypes, bt-1 to bt-3, can be discerned in N2; the minor isotype bt-2 is absent in all the mutants. These findings suggest that the isotype bt-2 is the primary site of action of benzimidazole-based drugs in C. elegans.