Identification and characterization of three new promoter/operators from Corynebacterium diphtheriae that are regulated by the diphtheria toxin repressor (DtxR) and iron

DtxR is a dimeric, sequence-specific, DNA-binding protein that functions as an iron-dependent, negative global regulator in Corynebacterium diphtheriae. Under high-iron conditions, DtxR represses the synthesis of diphtheria toxin, corynebacterial siderophore, and other components of the high-affinity iron uptake system. Three DtxR-regulated promoter/operators designated ton, IRP1, and IRP2 were reported previously. In this study, we identified and characterized three additional DtxR-regulated promoter/operators from C. diphtheriae designated IRP3, IRP4, and IRP5. When beta-galactosidase was expressed from these three new promoter/operators in Escherichia coli containing dtxR(+) on pDSK29, enzyme levels were 5- to 30-fold lower during high-iron growth than during low-iron growth. In gel shift assays, the mobility of DNA fragments containing each promoter/operator decreased in the presence of purified DtxR and Co2+. In footprinting assays, DtxR protected 36-, 35-, and 30-bp regions of IRP3, IRP4, and IRP5, respectively, hom cleavage by DNase I. In the 19-bp core of each promoter/operator, 12 or 13 bp matched the consensus for the DtxR-binding site. The putative polypeptides encoded by the open reading frames (ORFs) downstream from IRP3 and IRP4 were homologous, respectively, to several bacterial transcriptional regulators and to the deduced polypeptide encoded by an ORF located between the E. coli genes for primosomal replication protein N and adenine phosphoribosyltransferase. The putative polypeptide encoded by the ORF downstream from IRP5 was not homologous to any sequence in the protein database at the National Center for Biotechnology Information. When the ORFs downstream from IRP3 and IRP4 were expressed under the control of the phage T7 promoter in E. coli, polypeptide products of the predicted sizes were detected in small amounts by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.