Abstract
DtxR is an iron-dependent sequence-specific DNA-binding protein that binds to the tox operator, an inverted-repeat nucleotide sequence located upstream from the diphtheria toxin gene. In this study, two additional iron-regulated promoter/operator sequences (IRP1 and IRP2) that are controlled by DtxR were cloned from the chromosome of Corynebacterium diphtheriae and characterized. Operon fusions to lacZ were used to analyze expression from IRP1 and IRP2 in Escherichia coli. Transcription from both promoters was strongly repressed in high-iron medium in the presence of the cloned dtxR gene; however, transcription in the absence of dtxR was 50- to 100-fold greater, regardless of the iron concentration. Purified DtxR altered the electrophoretic mobility of DNA fragments carrying IRP1 or IRP2, and the nucleotide sequences of the two promoter/operator regions indicated that they are both homologous with the tox operator. DtxR protected an approximately 30-bp region on both IRP1 and IRP2 from DNase I digestion. A 19-bp consensus DtxR-binding site was derived from a comparison of the various DtxR-regulated operator/promoter sequences. Footprinting experiments using hydroxyl radicals and dimethyl sulfate demonstrated that DtxR interacted with these operators in a symmetrical manner, probably as a dimer or multimer. The deduced amino acid sequence of an open reading frame (ORF1) located downstream from IRP1 was homologous with a family of periplasmic proteins involved in iron transport in gram-negative bacteria and with the ferrichrome receptor, FhuD, from Bacillus subtilis. These findings suggest that ORF1 encodes a membrane-associated lipoprotein that may serve as the receptor for a ferric-siderophore complex in C. diphtheriae.