DtxR is a global iron-dependent regulatory protein with multiple roles in the control of gene expression in <i>Corynebacterium diphtheriae</i>

<i>Corynebacterium diphtheriae</i> is the causative agent of diphtheria, a severe respiratory disease in humans. Diphtheria toxin, encoded by the <i>tox</i> gene, is a potent exotoxin secreted by <i>C. diphtheriae</i> that is responsible for much of the morbidity and mortality of diphtheria. Expression of the <i>tox</i> gene is regulated by iron and the diphtheria toxin repressor (DtxR). In addition to controlling toxin expression, DtxR functions as a global iron-dependent regulatory factor that mediates iron homeostasis in <i>C. diphtheriae</i>. While numerous genes regulated by DtxR and iron are known, a genome-wide study of both the iron and DtxR regulons is lacking in <i>C. diphtheriae</i>. Here, we report novel iron- and DtxR-regulated genes revealed by a comprehensive transcriptomic analysis in <i>C. diphtheriae</i>. The findings from the study show that iron and DtxR not only result in the repression of transcription of numerous genes but also induce gene expression. Some of the genes induced by iron and DtxR were found to be regulated by the RipA repressor, which is encoded by the <i>ripA</i> gene whose transcription is repressed by iron and DtxR. Electrophoretic mobility shift assays confirmed DtxR binding to sequences upstream of several of the newly identified genes. Expression of the gene encoding ferritin (<i>ftn</i>) was induced by iron and DtxR and required multiple DtxR binding sites upstream of the <i>ftn</i> promoter for optimal expression. Collectively, these results expand our knowledge on the function of DtxR and the diverse roles of this regulatory protein in controlling gene expression.IMPORTANCE<i>Corynebacterium diphtheriae</i> is an important human pathogen that has served as a model system for the study of iron- and diphtheria toxin repressor (DtxR)-gene regulation in gram-positive bacteria. In <i>C. diphtheriae</i>, we found that iron and DtxR are involved in the regulation of iron acquisition systems and iron-stress tolerance mechanisms, which are both considered important virulence factors in bacterial pathogens. Iron-regulated genes may encode components to be considered in the formulation of new vaccines or be used as therapeutic targets in the treatment of disease caused by <i>C. diphtheriae</i>. The findings in this report also expand our knowledge as to how DtxR functions and the diverse roles of this regulatory protein in controlling gene expression.