MINIMAL FUNCTIONAL SYSTEM REQUIRED FOR EXPRESSION OF ERYTHROMYCIN RESISTANCE BY MSRA IN STAPHYLOCOCCUS-AUREUS RN4220

Previous studies have suggested that inducible erythromycin (Er) resis tance in staphylococci mediated by the plasmid-borne ABC-transporter m srA is dependent on additional unidentified chromosomally encoded tran smembrane (TM) domains. The requirement for two S. aureus candidate se quences, stpC and smpC, highly similar to sequences adjacent to msrA o n the original S. epidermidis plasmid was investigated. Deletion of th e sequences by allelic replacement was accomplished by electroporation of S. aureus RN4220 with a nonreplicating suicide vector. S. aureus s trains carrying a Delta(stpC-smpC) mutation showed an identical Er-R p henotype to those arising from single crossover events and unmutated R N4220 containing msrA. This proves that neither stpC nor smpC is requi red for Er-R. To further define the minimal functional unit required f or MS(R), the control region within the leader sequence of msrA was de leted. This resulted in constitutive resistance to Er and type B strep togramins (Sg), proving that Sg(R) does not require the presence of Er . Deletion constructs containing the N- or C-terminal ABC regions of M srA did not confer Er-R in RN4220 singly or in combination.