Minimal functional system required for expression of erythromycin resistance by msrA in Staphylococcus aureus RN4220

Previous studies have suggested that inducible erythromycin (Er) resistance in staphylococci mediated by the plasmid-borne ABC-transporter msrA is dependent on additional unidentified chromosomally encoded transmembrane (TM) domains. The requirement for two S. aureus candidate sequences, stpC and smpC, highly similar to sequences adjacent to msrA on the original S. epidermidis plasmid was investigated. Deletion of the sequences by allelic replacement was accomplished by electroporation of S. aureus RN4220 with a nonreplicating suicide vector. S. aureus strains carrying a Delta(stpC-smpC) mutation showed an identical Er-R phenotype to those arising from single crossover events and unmutated RN4220 containing msrA. This proves that neither stpC nor smpC is required for Er-R. To further define the minimal functional unit required for MS(R), the control region within the leader sequence of msrA was deleted. This resulted in constitutive resistance to Er and type B streptogramins (Sg), proving that Sg(R) does not require the presence of Er. Deletion constructs containing the N- or C-terminal ABC regions of MsrA did not confer Er-R in RN4220 singly or in combination.