PRJ5 - A NATURALLY-OCCURRING STAPHYLOCOCCUS-AUREUS PLASMID EXPRESSINGCONSTITUTIVE MACROLIDE LINCOSAMIDE STREPTOGRAMIN-B RESISTANCE CONTAINS A TANDEM DUPLICATION IN THE LEADER REGION OF THE ERMC GENE

The 2.55 kb Staphylococcus aureus plasmid, pRJ5, confers constitutive resistance to macrolide-lincosamide-streptogramin B (MLS) antibiotics. pRJ5 is nearly identical to the inducible MLS resistance plasmid pT48 , and has homology with the S. aureus plasmids pE194 and pSN2. The Hin dIII-C and/or Hind-B fragments were required for stable maintenance of the plasmid and probably carry palA. Plasmids pRJ5 and pT48 were show n to belong to the same incompatibility group, Inc12 (L). DNA sequenci ng showed that pRJ5 contains a 28 bp direct tandem duplication in the leader/attenuator region of ermC. This is likely to change the seconda ry structure of the methylase mRNA, allowing constitutive expression o f ermC. The type of mutation found on plasmid pRJ5 is different from t hose observed in similar 2.5 kb constitutive MLS-resistance plasmids i solated from other Gram-positive bacteria, including staphylococci.