Introduction of a norA Promoter Region Mutation into the Chromosome of a Fluoroquinolone-Susceptible Strain of Staphylococcus aureus Using Plasmid Integration

It has been postulated that a mutation 11 bp 3′ to the −10 motif of thenorA promoter is involved in the increased expression of the gene observed in some strains ofStaphylococcus aureus exhibiting efflux-related fluoroquinolone resistance. Introduction of this mutation into the chromosome of a fluoroquinolone-susceptible strain by plasmid integration resulted in the minimum inhibitory concentrations of NorA substrates being increased, fluoroquinolone uptake being reduced, andnorA expression being enhanced. Diffuse hybridization ofnorA and integrating vector probes at a similar molecular weight range, higher than that of thenorA transcript, was observed in the integrant, suggesting the possibility of a plasmid-based promoter contributing tonorA expression. The ratio of the quantity of this transcript, which was also observed in the parent strain of the integrant, to the quantity of primarynorA transcript was 0.14, demonstrating that it was unlikely that this mRNA species contributed significantly to the results observed. It is more likely that the introduced promoter region mutation does affect the expression ofnorA.