Mycobacterium smegmatis Erm(38) is a reluctant dimethyltransferase

The waxy cell walls of mycobacteria provide intrinsic tolerance to a broad range of antibiotics, and this effect is augmented by specific resistance determinants. The inducible determinant erm(38) in the nontuberculous species Mycobacterium smegmatis confers high resistance to lincosamides and some macrolides, without increasing resistance to streptogramin B antibiotics. This is an uncharacteristic resistance pattern falling between the type I and type II macrolide, lincosamide, and streptogramin B (MLS(B)) phenotypes that are conferred, respectively, by Erm monomethyltransferases and dimethyltransferases. Erm dimethyltransferases are typically found in pathogenic bacteria and confer resistance to all MLS(B) drugs by addition of two methyl groups to nucleotide A2058 in 23S rRNA. We show here by mass spectrometry analysis of the mycobacterial rRNA that Erm(38) is indeed an A2058-specific dimethyltransferase. The activity of Erm(38) is lethargic, however, and only a meager proportion of the rRNA molecules become dimethylated in M. smegmatis, while most of the rRNAs are either monomethylated or remain unmethylated. The methylation pattern produced by Erm(38) clarifies the phenotype of M. smegmatis, as it is adequate to confer resistance to lincosamides and 14-member ring macrolides such as erythromycin, but it is insufficient to raise the level of resistance to streptogramin B drugs above the already high intrinsic tolerance displayed by this species.