The Tylosin-resistance Methyltransferase RlmAII (TlrB) Modifies the N-1 Position of 23 S rRNA Nucleotide G748

The methyltransferase RlmAII (TlrB) confers resistance to the macrolide antibiotic tylosin in the drug-producing strain Streptomyces fradiae. The resistance conferred by RlmAII is highly specific for tylosin, and no resistance is conferred to other macrolide drugs, or to lincosamide and streptogramin B (MLSB) drugs that bind to the same region on the bacterial ribosome. In this study, the methylation site of RlmAII is identified unambiguously by liquid chromatography/electrospray ionization mass spectrometry as the N-1 position of 23S rRNA nucleotide G748. This position is contacted by the mycinose sugar moiety of tylosin, which is absent from the other drugs. The selective resistance to tylosin conferred by m1G748 illustrates how differences in drug structure facilitate the drug fit at the MLSB-binding site. This observation is of relevance for the rational design of novel antimicrobials targeting the MLSB site, especially if the antimicrobials are to be used against pathogens possessing m1G748.