IDENTIFICATION OF A CHROMOSOMALLY ENCODED ABC-TRANSPORT SYSTEM WITH WHICH THE STAPHYLOCOCCAL ERYTHROMYCIN EXPORTER MSRA MAY INTERACT

The energy-dependent efflux of erythromycin (Er) in staphylococci is d ue to the presence of msrA, which encodes an ATP-binding protein. MsrA is related to the multi-component ATP-binding cassette (ABC) transpor ters which characteristically also contain membrane-spanning domains. Since MsrA functions in a heterologous host in the absence of other pl asmid-encoded products, the requirement for a transmembrane (TM) compl ex might be fulfilled by hijacking a chromosomally encoded protein. Tw o genes, stpA and smpA, were identified upstream from msrA on the orig inal Staphylococcus epidermidis plasmid, encoding an ATP-binding prote in and a hydrophobic TM protein, respectively. Sequences highly simila r to stpA and smpA (stpB and smpB) were also found adjacent to a chrom osomal copy of msrA in S. hominis. In Southern blots, internal fragmen ts of stpA or smpA hybridized to the chromosome of the Er-S S. aureus RN4220. Cloning and sequence analysis of the region identified reveale d the presence of two genes, stpC and smpC, related to stpA and smpA. The deduced amino-acid sequences of the gene products showed that StpA and StpC were 85% identical, whereas SmpA and SmpC were 65% identical . A gene similar to msrA was not present in the S. aureus chromosome. There was no further sequence similarity outside these conserved regio ns. These results indicate that the chromosomes of S. hominis and S. a ureus contain sequences encoding a potential TM protein with which Msr A might interact.