Molecular control of gene expression by Brucella BaaR, an IclR-family repressor
The Brucella abortus general stress response sigma factor, {sigma}E1, directly and indirectly regulates the transcription of dozens of genes that influence stress survival and host infection. Characterizing the functions of {sigma}E1 regulated genes therefore contributes to understanding of B. abortus physiology and infection biology. Transcription of the IclR family regulator, Bab2_0215, is indirectly activated by {sigma}E1 but its function remains undefined. We present a structural and functional characterization of Bab2_0215, which we have named Brucella adipic acid activated regulator (BaaR). BaaR adopts a classic IclR-family fold and directly regulates the transcription of two operons with predicted roles in carboxylic acid oxidation. BaaR binds two sites on chromosome II between baaR and a divergently transcribed hydratase/dehydrogenase (acaD2), and represses transcription. We identified three carboxylic acids (adipic acid tetradecanedioic acid, {varepsilon}-aminocaproic acid) and a lactone ({varepsilon}-caprolactone) that enhance transcription from the baaR and acaD2 promoters. However, neither the activating acids nor caprolactone enhance transcription by binding directly to BaaR. Induction of baaR transcription by adipic acid requires the gene bab2_0213, which encodes a major facilitator superfamily transporter, suggesting that Bab2_0213 transports adipic acid across the inner membrane. We conclude that a set of structurally related organic molecules activate transcription of genes repressed by BaaR. Our study provides molecular-level understanding of a gene expression program regulated downstream of {sigma}E1.