Cloning and nucleotide sequence of a carbomycin-resistance gene from Streptomyces thermotolerans

Two plasmids (pOJ158 and pOJ159) containing DNA fragments from the carbomycin(Cb)-producing strain Streptomyces thermotolerans were identified in Streptomyces griseofuscus based on their ability to confer resistance to Cb. The Cb-resistance determinants on pOJ158 and pOJ159 were designated carA and carB, respectively. In S. griseofuscus, pOJ159 also confers resistance to spiramycin, rosaramicin, lincomycin, and vernamycin B, but not to tylosin; in Streptomyces lividans, pOJ159 additionally confers resistance to erythromycin and oleandomycin. The carB gene was localized on pOJ159 to a 1.25-kb region whose nucleotide sequence was determined. The sequence has a G + C content of 68% and contains the coding sequence for carB and portions of the 5' and 3' untranslated regions. A comparison of the amino acid sequence of the protein encoded by carB (as deduced from the nucleotide sequence) with the deduced amino acid sequence of the RNA methylase from Streptomyces erythraeus (encoded by ermE) revealed extensive homology, suggesting that carB also encodes an RNA methylase. The region 5' to the coding sequence does not contain a small ORF or regions of complementarity that are commonly associated with translationally regulated macrolide-lincosamide-streptogramin B resistance genes. The 3' untranslated region contains an inverted repeat sequence that potentially can form a stable RNA stem-loop structure with a calculated delta G of -70 kcal.