Structures of AcrR and CmeR: Insight into the mechanisms of transcriptional repression and multi-drug recognition in the TetR family of regulators

Abstract

The transcriptional regulators of the TetR family act as chemical sensors to monitor the cellular environment in many bacterial species. To perform this function, members of the TetR family harbor a diverse ligandbinding domain capable of recognizing the same series of compounds as the transporters they regulate. Many of the regulators can be induced by a wide array of structurally unrelated compounds. Binding of these structurally unrelated ligands to the regulator results in a conformational change that is transmitted to the DNA-binding region, causing the repressor to lose its DNA-binding capacity and allowing for the initiation of transcription. The multi-drug binding proteins AcrR of Escherichia coli and CmeR from Campylobacter jejuni are members of the TetR family of transcriptional repressors that regulate the expression of the multidrug resistant efflux pumps AcrAB and CmeABC, respectively. To gain insights into the mechanisms of transcriptional regulation and how multiple ligands induce the same physiological response, we determined the crystal structures of the AcrR and CmeR regulatory proteins. In this review, we will summarize the new findings with AcrR and CmeR, and discuss the novel features of these two proteins in comparison with other regulators in the TetR family.

Epoxide-Mediated CifR Repression of cif Gene Expression Utilizes Two Binding Sites in Pseudomonas aeruginosa

“…The 35 nM affinity of CifR for the cifR proximal binding site is consistent with other TetR-family proteins, such as AcrR (20.2 nM) and CmeR (88 nM) in their affinity for their binding sites (23,35). The apparent dissociation constant for the morB proximal site is higher than that of the cifR proximal site, which may be attributable to differences in the sequences in these sites ( Fig.…”

“…Binding at a similar promoter-proximal site has been shown recently for another TetR-family regulator, RamR, of Salmonella enterica (4), in contrast to other family members that have been shown to bind at a location more distal to the transcriptional start (33,35). However, these repressors have all been shown to bind at a single site to mediate repression of the operon as either a dimer or a dimer of dimers (4,33,35), whereas CifR binds at two distinct operators, separated by 28 bp, to control the expression of divergently transcribed loci. The auto-regulation exhibited by CifR, common in the TetR family, is believed to be a feedback control mechanism that ensures optimal repressor concentration (33).…”

Mutational and Transcriptomic Changes Involved in the Development of Macrolide Resistance in Campylobacter jejuni

“…The overexpression of cmeABC during stepwise selection could be partly explained by mutations in CmeR, such as the nucleotide deletion at 490A and the H174N change in the protein (Tables 2 and 3). On the basis of the sequence and crystal structure of CmeR, these mutations are located in the C-terminal domain of CmeR (36)(37)(38)(39). The deletion at 490A is expected to truncate the CmeR protein, which may explain the significant upregulation of cmeABC in the 68Ex-1 lineage (Table 2; Fig.…”

Crystal structures of CmeR‐bile acid complexes from Campylobacter jejuni

“…16 To date, CmeR is the only regulator in the TetR family that lacks the N-terminal helixturn-helix DNA-binding motif, in which the recognition helix a3 is replaced by a random coil. 16,17 Along with this unique characteristic, a large center-tocenter distance (54 Å as measured by the separation between Ca atoms of Y51 and Y51 0 from the other subunit) was observed between the two N-termini of the dimer, making it incompatible with the distance between two consecutive major grooves of B-DNA. Each monomer of CmeR consists of nine helices, and numbered with helix a3 being skipped to facilitate comparisons to other members of the TetR family.…”