Analysis of a DtxR-regulated iron transport and siderophore biosynthesis gene cluster in Corynebacterium diphtheriae

This report describes a genetic locus associated with siderophore biosynthesis and transport in Corynebacterium diphtheriae. A BLAST search of the C. diphtheriae genome identified a seven-gene cluster that included four genes, designated ciuA, ciuB, ciuC, and ciuD, whose predicted products are related to ABC-type iron transporters. Downstream from ciuD is the ciuE gene, whose predicted product is similar to the aerobactin biosynthetic enzymes IucA and IucC. The CiuE protein, which has a predicted mass of 121,582 Da and is approximately twice the size of either lucC or IucA, is homologous to each of these proteins in both its N- and C-terminal regions. C diphtheriae ciuE deletion mutants exhibited a defect in siderophore production, iron uptake, and growth in low-iron medium. Mutations in the ciuA gene, whose predicted product is a lipoprotein component of an iron transport system, resulted in a severe defect in iron uptake and reduced ability to use the C. diphtheriae siderophore as an iron source. Site-directed mutations in irp6A, a gene previously reported to be associated with siderophore transport, had no effect on iron uptake or the utilization of the C diphtheriae siderophore as an iron source. Transcriptional analysis demonstrated that expression of ciuA and ciuE is DtxR and iron regulated, and DNase I protection experiments confirmed the presence of DtxR binding sites upstream from each of these genes. Thus, this iron- and DtxR-regulated gene cluster is involved in the synthesis and transport of the C diphtheriae siderophore.