Resistance to macrolides and lincosamides in has been shown to be encoded by a 4.0-kb plasmid designated pSES21. It differed distinctly in its restriction map from all other staphylococcal macrolide resistance plasmids reported so far. Southern blot hybridisation with gene probes specific for staphylococcal genes demonstrated that the macrolide resistance gene belonged to hybridisation class C. Analysis of the C gene revealed that the deduced amino-acid sequence of the pSES21-encoded ErmC methylase exhibited 93% identity with the ErmC methylase encoded by plasmid pE194. The C gene of pSES21 was expressed constitutively and sequence analysis of the regulatory region showed multiple base-pair insertions and substitutions in the translational attenuator. As a consequence of these mutations, the reading frame of the small regulatory peptide was destroyed and a novel pair of inverted repeated sequences was generated. Previous studies identified sequence deletions and sequence duplications in the C regulatory region as the basis for constitutive C gene expression. The multiple point mutations shown in the pSES21-encoded C translational attenuator represent a novel kind of structural alteration in this regulatory region and may explain constitutive C gene expression by pairing of the newly generated inverted repeated segments in the presence of a functionally deleted reading frame for the small regulatory peptide.


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