A New Class of Genetic Element, Staphylococcus Cassette Chromosome mec, Encodes Methicillin Resistance in Staphylococcus aureus

We have previously shown that the methicillin-resistance gene mecA of Staphylococcus aureus strain N315 is localized within a large (52-kb) DNA cassette (designated the staphylococcal cassette chromosome mec [SCCmec]) inserted in the chromosome. By sequence determination of the entire DNA, we identified two novel genes (designated cassette chromosome recombinase genes [ccrA and ccrB]) encoding polypeptides having a partial homology to recombinases of the invertase/resolvase family. The open reading frames were found to catalyze precise excision of the SCCmec from the methicillin-resistant S. aureus chromosome and site-specific as well as orientation-specific integration of the SCCmec into the S. aureus chromosome when introduced into the cells as a recombinant multicopy plasmid. We propose that SCCmec driven by a novel set of recombinases represents a new family of staphylococcal genomic elements.