A single membrane-embedded negative charge is critical for recognizing positively charged drugs by the Escherichia coli multidrug resistance protein MdfA.
A novel gene designated cmr, which mapped to 18.8 min of the Escherichia coli K-12 genome, was shown to mediate resistance to chloramphenicol when it was expressed from a multicopy vector. The accumulation of chloramphenicol was significantly less in cells overexpressing cmr than in control cells harboring the vector without insert. After the addition of a proton motive force blocker, the level of accumulation of chloramphenicol in the resistant cells rapidly approached the levels found in sensitive cells carrying only the chromosomal cmr. Northern (RNA) blot analyses revealed that the cmr gene is expressed as a 1.3-kb transcript. This size corresponds very well with a predicted size of 1,293 nucleotides (nt) based on the mapping of the transcription initiation site to a G residue 24 nt upstream of the start codon and the presence of a putative rho-independent terminator sequence ending 36 nt downstream of the 1,233-nt open reading frame encoding the putative Cmr protein. The 411-residue-long derived amino acid sequence contains 12 putative transmembrane segments and displays significant sequence similarities 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 chloramphenicol.