The femC locus of Staphylococcus aureus required for methicillin resistance includes the glutamine synthetase operon.

Tn551 insertional inactivation of femC is known to reduce methicillin resistance levels in methicillin-resistant and -susceptible Staphylococcus aureus. By use of cotransductional crosses, femC was mapped close to thrB on the SmaI-A fragment of the S. aureus NCTC 8325 chromosome. The Tn551 insertion femC::omega 2005 was found to interrupt an open reading frame coding for a putative protein of 121 amino acids which is highly similar to the glutamine synthetase repressors (GlnR) of Bacillus spp. Downstream of femC, an open reading frame highly similar to Bacillus sp. glutamine synthetases (GlnA) was found. Northern (RNA) blots probed with putative glnR or glnA fragments revealed that 1.7- and 1.9-kb transcripts characteristic of wild-type cells were replaced by less abundant 7.0- and 7.2-kb transcripts in the femC::omega 2005 mutant. Total glutamine synthetase activity was also decreased in the mutant strain; the addition of glutamine to defined media restored the wild-type methicillin resistance phenotype of the femC mutant. This result suggests that the omega 2005 insertion in glnR has a polar effect on glnA and that glnR and glnA are transcribed together as an operon. These results suggest that the loss of wild-type levels of glutamine synthetase and the consequent decrease in glutamine availability cause a decreased level of methicillin resistance.