Macrolide-lincosamide-streptogramin B resistance in Staphylococcus lentus results from the integration of part of a transposon into a small plasmid

The 8.0 kbp plasmid pSES20 from Staphylococcus lentus mediated constitutive resistance to macrolide-lincosamide-streptogramin B (MLS) antibiotics. The resistance gene, designated ermB, was locted on a 1.46 kbp ClaI-fragment and shown to belong to the class ermAM by hybridiation with a respective gene probe. This ClaI-fragment revealed extended restriction map and nucleoride sequence homology to the corresponding parts of transposons Tn917 and Tn551, wheareas the remaining pats of plasmid pSES20 did not correspond to any part of these transposons. Sequence analyses identified an open reading frame for the 245 amino acids ermB methylase which was preceded by a small open reading frame for a regulatory peptide of 27 amino acids. A duplication of 4 bp generated a stop codon which shortened this reading frame by 9 amino acids in comparison to that of Tn917. The ermB gene and its reglatory region were flanked by almost perfect repeats of 73 bp in length. The upstream repeat represented one the terminal repeats of the original erm-encoding transposon, as reported for Tn917. The downstream repeat was located in close proximity to a sequence of pSES20 which revealed considerable homology to the recombination site rs1054 of Staphylococcus aureus. This sequence as well as the terminal direct repeat may have been involved in the integration of a section of an erm-encoding transposon into the small S. lentus plasmid.