We cloned a gene responsible for multidrug resistance from chromosomal DNA of Serratia marcescens, and determined the nucleotide sequence. We designated the gene as ssmE. The deduced amino acid sequence of SsmE showed high similarity with the small multidrug resistance (SMR)-type multidrug efflux pumps. Cells of Escherichia coli KAM32, a drug hyper-susceptible mutant, transformed with a plasmid pESM437 carrying the ssmE gene showed elevated minimum inhibitory concentrations of structurally unrelated antimicrobial agents. E. coli KAM32/pESM437 showed elevated energy dependent efflux of ethidium. Reverse transcriptase polymerase chain reaction (RT-PCR) analysis revealed that ssmE was expressed in cells of S. marcescens.
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