ISOLATION OF CMR, A NOVEL ESCHERICHIA-COLI CHLORAMPHENICOL RESISTANCEGENE ENCODING A PUTATIVE EFFLUX PUMP

A novel gene designated cmr, which mapped to 18.8 min of the Escherich ia coli K-12 genome, was shown to mediate resistance to chloramphenico l when it was expressed from a multicopy vector, The accumulation of c hloramphenicol was significantly less in cells overexpressing cmr than in control cells harboring the vector without insert. After the addit ion of a proton motive force blocker, the level of accumulation of chl oramphenicol in the resistant cells rapidly approached the levels foun d in sensitive cells carrying only the chromosomal cmr. Northern (RNA) blot analyses revealed that the cmr gene is expressed as a 1.3-kb tra nscript, This size corresponds very well with a predicted size of 1,29 3 nucleotides (nt) based on the mapping of the transcription initiatio n site to a G residue 24 nt upstream of the start codon and the presen ce of a putative rho-independent terminator sequence ending 36 mt down stream of the 1,233-nt open reading frame encoding the putative Cmr pr otein, The 411-residue-long derived amino acid sequence contains 12 pu tative transmembrane segments and displays significant sequence simila rities to several known drug resistance protein sequences of the major facilitator family, We provide evidence strongly suggesting that the resistance mediated by Cmr involves active exclusion of chloramphenico l.