Microcins are low-molecular-weight polypeptide antibiotics []. The EmrR dimer binds 2,4-dinitrophenol, carbonyl cyanide m-chlorophenylhydrazone (CCCP), and carbonyl cyanide p-(trifluoro-methoxy)phenylhydrazone, predicted ligands of the EmrAB pump, with high affinity [1, 10]. EmrR also is able to bind to salicylate and to a small drug-like molecule inhibitor of capsule biogenesis named DUO11 [10]. Binding of any of these agents interferes with the ability of EmrR to bind DNA [3]. This dimeric protein [1] recognizes and binds an imperfect inverted repeat DNA sequence [3]. EmrR belongs to the MarR family of transcriptional regulators [6], which respond to a variety of phenolic compounds and are found in gram-positive and gram-negative species as well as in mycobacteria [6]. The proteins of this family have a helix-turn-helix DNA-binding motif at the center [11], while the ligand-binding domain is possibly located at the N-terminal domain [1]. EmrR shows 28% and 47% identity and similarity, respectively, to the MarR protein [6]. Overproduction of EmrR from a multicopy plasmid causes a mutator phenotype that includes an increase in frameshift and base substitution mutations [12]. emrR is the first gene of the emrRAB operon whose transcription is induced by antimicrobial agents such as salicylic acid, CCCP, and nalidixic acid, among others; induction requires the repressor of the operon EmrR [2]. Expression of emrR is not osmoregulated [8]. Under oxidative stress conditions, translation of emrR is increased by the presence of polyamines. The increase is dependent on the suboptimal placement of the Shine-Dalgarno region upstream of the emrR open reading frame [13]. Reviews: [14, 15, 16]