Growth phase-dependent transcription of emrKY, a homolog of multidrug efflux emrAB genes of Escherichia coli, is induced by tetracycline.

The genes emrK and emrY were found between genes dsdA and evgA at 51 min on the Escherichia coli chromosome and form an operon. EmrK and EmrY are 50.4 and 63.3% identical in amino acid sequences to EmrA and EmrB, respectively, which together make up a multidrug resistant pump. To show that the emrKY operon can be expressed, we cloned the promoter with pMC1403 and constructed an emrK-lacZ' protein fusion plasmid, pMKD1. In E. coli MC4100 containing pMKD1, its expression was increased in the presence of a subinhibitory concentration of tetracycline, chloramphenicol or salicylate, but not by carbonylcyanide m-chlorophenylhydrazone, nalidixic acid or kanamycin. Furthermore, we have shown that emrKY transcription dependent on the growth phase is actually induced by tetracycline using a S1 nuclease protection assay.