[Characterization of the Staphylococcus aureus norA gene, which confers resistance to hydrophilic quinolones]

The norA gene cloned from chromosomal DNA of a quinolone-and methicillin-resistant Staphylococcus aureus strain conferred resistance to hydrophilic quinolones such as norfloxacin, enoxacin, ofloxacin, and ciprofloxacin, but no or less resistance to hydrophobic ones such as nalidixic acid, oxolinic acid, and sparfloxacin in S. aureus and Escherichia coli. Nucleotide sequence analysis of the cloned DNA fragment revealed that the norA gene could code for a protein consisting of 388 amino acid residues with a molecular weight of 42,265, which was consistent with the experimental value of about 49,000 obtained on DNA-directed translation. The deduced NorA polypeptide has 12 hydrophobic membrane-spanning regions and is partly homologous to tetracycline resistance protein and sugar transport proteins. The uptake of a hydrophilic quinolone, enoxacin, by S. aureus harboring a plasmid carrying the norA gene was about 50% of that by the parent strain lacking the plasmid, but it increased to almost the same level as that by the latter strain with carbonyl cyanide m-chlorophenyl hydrazone. On the other hand, the uptake of a hydrophobic quinolone, sparfloxacin, was hardly affected by the norA gene. These results suggest that the NorA polypeptide may constitute a membrane-associated active efflux pump of hydrophilic quinolones.