A PROTEIN THAT ACTIVATES EXPRESSION OF A MULTIDRUG EFFLUX TRANSPORTERUPON BINDING THE TRANSPORTER SUBSTRATES

Multidrug transporters are membrane proteins which, by an unknown mech anism, recognize diverse toxic compounds and efflux them from cells. W e found that two substrates of the Bacillus subtilis multidrug transpo rter Bmr, rhodamine 6G and tetraphenylphosphonium (TPP), enhance Bmr e xpression at the level of transcription. Gene knock-out experiments de monstrated that an open reading frame located immediately downstream o f the bmr gene is required for this enhancement. The protein product o f this open reading frame, BmrR, shows distinct sequence homology to s everal known bacterial transcription activator proteins, such as MerR and TipA(L). Gel-mobility shift and DNase protection assays indicated that BmrR binds specifically, as a dimer, to the bmr gene promoter. Fu rthermore, the affinity of this binding was enhanced by rhodamine and TPP, thus suggesting that these structurally dissimilar molecules inte ract directly with BmrR. Indeed, we found that BmrR bound rhodamine 6G stoichiometrically, one rhodamine molecule/BmrR dimer, and that TPP c ompeted with rhodamine for this binding. Our results indicate that the enhancement of Bmr expression by some of its substrates is due to the ability of the regulatory protein, BmrR, to bind structurally dissimi lar compounds resulting in enhanced transcription of the transporter g ene.