RsmG methylation of 16S rRNA affects the function of ribosomal protein uS12

The RsmG methyltransferase modifies G527 in bacterial 16S rRNA and its inactivation confers low level streptomycin resistance. In contrast, high level streptomycin resistance typically requires specific alterations in ribosomal protein uS12 or 16S rRNA. Here, we have asked if rsmG inactivation alters the phenotypes of any of a collection of randomly-generated Escherichia coli uS12 mutants. While several uS12 mutants show moderately increased resistance to streptomycin when rsmG is inactivated (MIC = 10–40 µg/ml), a uS12 R85H/rsmG-inactivated strain uniquely displays very high resistance (MIC > 1,024 µg/ml). Additional genetic selections showed that rsmG null mutations combined with specific alterations in uS12 can generate streptomycin-dependence, or pseudo-dependence, in addition to resistance. Moreover, growth of several of these mutants on high concentrations of streptomycin is conditional on rsmG inactivation. Thus, loss of m⁷G527 methylation affects the streptomycin phenotypes of distinct uS12 mutants and identifies an additional route to high-level streptomycin resistance in bacteria.