Characterization of an iron-regulated promoter involved in desferrioxamine B synthesis in Streptomyces pilosus: repressor-binding site and homology to the diphtheria toxin gene promoter.

Desferrioxamine B is the main siderophore of Streptomyces pilosus. Its production is induced in response to iron limitation. Two genes involved in desferrioxamine production have been cloned and were found to be translated from a polycistronic mRNA that is produced only under conditions of iron limitation (T. Schupp, C. Toupet, and M. Divers, Gene 64:179-188, 1988). Here we report the nucleotide sequence of the desferrioxamine (des) operon promoter region. The transcriptional start site was localized by S1 nuclease mapping. Deletion analysis defined a 71-bp region downstream of the -35 region that is sufficient for iron regulation in the original host, S. pilosus, and also in Streptomyces lividans. Site-directed mutagenesis was used to create a mutation that abolishes iron repression. Two iron-independent mutants were obtained by deletion of part of a 19-bp region with dyad symmetry which overlaps the -10 promoter region and the transcriptional start site. The putative repressor-binding site identified by these constitutive mutations is not homologous to the consensus binding site of the Escherichia coli central iron repressor, Fur (ferric uptake regulation), but is similar to the DtxR-binding site in the iron-regulated promoter of the corynebacterial diphtheria toxin gene.