NORM, A PUTATIVE MULTIDRUG EFFLUX PROTEIN, OF VIBRIO-PARAHAEMOLYTICUSAND ITS HOMOLOG IN ESCHERICHIA-COLI

We found that cells of Vibrio parahaemolyticus possess an energy-depen dent efflux system for norfloxacin. We cloned a gene for a putative no rfloxacin efflux protein from the chromosomal DNA of V. parahaemolytic us by using an Escherichia coli mutant lacking the major multidrug eff lux system AcrAB as the hose and sequenced the gene (norM). Cells of E . coli transformed with a plasmid carrying the norM gene showed elevat ed energy-dependent efflux of norfloxacin. The transformants showed el evated resistance not only to norfloxacin and ciprofloxacin brat also to the structurally unrelated compounds ethidium, kanamycin, and strep tomycin, These results suggest that this is a multidrug efflux system. The hydropathy pattern of the deduced amino acid sequence of NorM sug gested the presence of 12 transmembrane domains. The deduced primary s tructure of NorM showed 57% identity and 88% similarity with that of a hypothetical E. coli membrane protein, YdhE. No reported drug efflux protein in the sequence databases showed significant sequence similari ty with NorM, Thus, NorM seems to be a novel type of multidrug efflux protein, We cloned the ydhE gene from E. coli. Cells of E. coli transf ormed with the cloned ydhE gene showed elevated resistance to norfloxa cin, ciprofloxacin, acriflavine, and tetraphenylphosphonium ion, but n ot to ethidium, when MICs were measured, Thus, it seems that NorM and YdhE differ somehow in substrate specificity.