ACTIVE EFFLUX OF CHLORAMPHENICOL IN SUSCEPTIBLE ESCHERICHIA-COLI STRAINS AND IN MULTIPLE-ANTIBIOTIC-RESISTANT (MAR) MUTANTS

The multiple-antibiotic resistance (mar) locus (min 34) regulates a re sistance to chloramphenicol in Escherichia coli that does not involve acetyltransferase. Transport studies showed that wild-type cells had a n apparent endogenous active efflux of chloramphenicol which depended on the proton motive force. This efflux was not altered by a 39-kb chr omosomal deletion which included the mar locus. Nevertheless, mutation s at the mar locus led to a stronger net chloramphenicol efflux. There fore, a gene encoding the putative efflux system cannot be at the mar locus but may be positively influenced by that locus.