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The methyltransferase RsmG methylates the N7 position of nucleotide G535 in 16S rRNA of Bacillus subtilis (corresponding to G527 in Escherichia coli). Disruption of rsmG resulted in low-level resistance to streptomycin. A growth competition assay revealed that there are no differences in fitness between the rsmG mutant and parent strains under the various culture conditions examined. B. subtilis rsmG mutants emerged spontaneously at a relatively high frequency, 10(-6). Importantly, in the rsmG mutant background, high-level-streptomycin-resistant rpsL (encoding ribosomal protein S12) mutants emerged at a frequency 200 times greater than that seen for the wild-type strain. This elevated frequency in the emergence of high-level streptomycin resistance was facilitated by a mutation pattern in rpsL more varied than that obtained by selection of the wild-type strain.
Nishimura et al. (Sat,) studied this question.