Genetic Analysis of Resistance to Benzimidazoles in Physarum: Differential Expression of Beta-tubulin Genes

Physarum displays two vegetative cell types, uninucleate myxamoebae and multinucleate plasmodia. Mutant myxamoebae of Physarum resistant to the antitubulin drug methylbenzimidazole-2-yl-carbamate (MBC) were isolated. All mutants tested were cross-resistant to other benzimidazoles but not to cycloheximide or emetine. Genetic analysis showed that mutation to MBC resistance can occur at any one of four unlinked loci, benA, benB, benC or benD. MBC resistance of benB and benD mutants was expressed in plasmodia, but benA and benC mutant plasmodia were MBC sensitive, suggesting that benA and benC encode myxamoeba-specific products. Myxamoebae carrying the recessive benD210 mutation express a beta-tubulin with novel electrophoretic mobility, in addition to a beta-tubulin with wild-type mobility. This and other evidence indicates that benD is a structural gene for beta-tubulin, and that at least two beta-tubulin genes are expressed in myxamoebae. Comparisons of the beta-tubulins of wild-type and benD210 strains by gel electrophoresis revealed that, of the three (or more) beta-tubulin genes expressed in Physarum, one, benD, is expressed in both myxamoebae and plasmodia, one is expressed specifically in myxamoebae and one is expressed specifically in plasmodia. However, mutation in only one gene, benD, is sufficient to confer MBC resistance on both myxamoebae and plasmodia.