GLUTAMINE-SYNTHETASE AND HETERORESISTANCE IN METHICILLIN-RESISTANT STAPHYLOCOCCUS-AUREUS

Inactivation of femC in methicillin-resistant Staphylococcus aureus (M RSA) results in lowered methicillin resistance and a reduction in the amidation of the iso-D-glutamate of the peptidoglycan stem peptide, Th e femC phenotype is due to insertional inactivation of the glutamine s ynthetase repressor gene glnR by Tn551, which has a polar effect on gl utamine synthetase (glnA) transcription. The complete glutamine synthe tase operon (glnRA) of S, aureus was cloned and sequenced, and its tra nscriptional start was determined, The deduced amino acid sequence of the staphylococcal glutamine synthetase showed 76% identity and 87% si milarity to the Bacillus subtilis glutamine synthetase, The staphyloco ccal glnRA operon was shown to complement an Escherichia coli glutamin e synthetase-negative mutant and to restore methicillin resistance in femC mutants, femC mutants revert to resistance in the presence of hig h concentrations of methicillin, These revertants, which still carried the femC lesion, were shown to retain the lowered amidation of the is o-D-glutamate peptidoglycan stem peptide. A new chromosomal locus hmrC was postulated to have mutated to allow expression of high methicilli n resistance in these femC revertants, Although the highly resistant h mrC revertant resembled phenotypically the highly methicillin-resistan t subclones occurring in heterogeneously resistant MRSA, we could show by transduction that the locus hmrC was distinct from chr, a chromos omal site postulated to confer high methicillin resistance in heteroge neous MRSA, This suggests that S, aureus can adopt multiple ways to ac hieve high methicillin resistance.