Fusidic acid-resistant mutants define three regions in elongation factor G of Salmonella typhimurium.

We have sequenced fusA, the gene coding for elongation factor G (EF-G), in 18 different mutants of Salmonella typhimurium selected as fusidic acid resistant (FuR). In addition, we have sequenced two previously described FuR mutants from Escherichia coli. In all cases, the resistance is due to a mutation in one of three separate regions in fusA. The three clusters of mutant sites superimpose on regions that are well conserved, suggesting that they are of a more general functional importance. To further classify the mutants, we have measured the minimal inhibitory concentration (MIC) for Fu and for two other antibiotics which interfere with translocation on the ribosome, kanamycin (Km) and spectinomycin (Sp). The levels of resistance to Fu for each of the mutants are significantly higher than in the wild type (wt), and vary by about one order of magnitude between the highest and the lowest. Most of the mutants are also more resistant to Km than the wt, although the level of resistance is low and the variation small. In contrast, about half of the mutants are more sensitive to Sp than the wt, with only one being more resistant. Only three of the twenty mutants behave like the wt with respect to the non-selected phenotypes, KmR and SpR.