Analysis of a DtxR-Like Metalloregulatory Protein, MntR, fromCorynebacterium diphtheriaeThat Controls Expression of an ABC Metal Transporter by an Mn2+-Dependent Mechanism

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Abstract
The DtxR protein is a global iron-dependent repressor in Corynebacterium diphtheriae that regulates transcription from multiple promoters. A search of the partially completed C. diphtheriae genome identified a gene, mntR, whose predicted product has significant homology with the DtxR repressor protein. The mntR gene is the terminal gene in a five-gene operon that also carries the mntABCD genes, whose predicted products are homologous to ABC metal transporters. Transcription of this genetic system, as measured by expression of an mntA-lacZ reporter fusion, is strongly repressed by Mn2+. The divalent metals Fe2+, Cu2+, and Zn2+ did not repress expression of the mntA-lacZ construct. A mutation in the mntR gene abolished Mn2+-dependent repression of the mntA-lacZ fusion, demonstrating that MntR is essential for the Mn2+-dependent regulation of this promoter. Footprinting experiments showed that MntR protects from DNase I digestion an approximately 73-bp AT-rich region that includes the entire mntA promoter. This large region protected from DNase I suggests that as many as three MntR dimer pairs may bind to this region. Binding studies also revealed that DtxR failed to bind to the MntR binding site and that MntR exhibited weak and diffuse binding at the DtxR binding site at the tox promoter. A C. diphtheriae mntA mutant grew as well as the wild type in a low-Mn2+ medium, which suggests that the mntABCD metal transporter is not required for growth in a low-Mn2+ medium and that additional Mn2+ transport systems may be present in C. diphtheriae. This study reports the characterization of MntR, a Mn2+-dependent repressor, and the second member of the family of DtxR-like metalloregulatory proteins to be identified in C. diphtheriae.