Methylation of 23S ribosomal RNA due to carB, an antibiotic-resistance determinant from the carbomycin producer, Streptomyces thermotolerans.

A resistance gene, carB, originally isolated from the carbomycin-producing organism, Streptomyces thermotolerans, confers on Streptomyces lividans high-level resistance to the drug. However, ribosomes from S. lividans expressing carB show only moderate resistance to this macrolide in vitro, although they are highly resistant to the action of lincosamide antibiotics. The carB product monomethylates the amino group of the adenosine residue located at position 2058 in 23S ribosomal RNA. In contrast, ribosomes from S. lividans expressing ermE, in which 23S RNA is dimethylated at this same position, are much more highly resistant to macrolides and insensitive to lincosamides.