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 Staphyloco ccus 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 instertion femC::Omega 2005 was found to interrupt an open r eading frame coding for a putative protein of 121 amino acids which is highly similar to the glutamine synthetase repressors (GlnR) of Bacil lus spp. Downstream of femC, an open reading frame highly similar to B acillus sp. glutamine synthetases (GlnA) was found. Northern (RNA) blo ts 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 muta nt. Total glutamine synthetase activity was also decreased in the muta nt strain; the addition of glutamine to defined media restored the wil d-type methicillin resistance phenotype of the femC mutant. This resul t 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. Th ese results suggest that the loss of wild-type levels of glutamine syn thetase and the consequent decrease in glutamine availability cause a decreased level of methicillin resistance.