Bordetella bronchiseptica mutants BRM1, BRM6, and BRM9 fail to produce the native dihydroxamate siderophore alcaligin. A 4.5-kb BamHI-SmaI Bordetella pertussis genomic DNA fragment carried multiple genes required to restore alcaligin production to these siderophore-deficient mutants. Phenotypic complementation analysis using subclones of the 4.5-kb genomic region demonstrated that the closely linked BRM1 and BRM9 mutations were genetically separable from the BRM6 mutation, and both insertions exerted strong polar effects on expression of the downstream gene defined by the BRM6 mutation, suggesting a polycistronic transcriptional organization of these alcaligin biosynthesis genes. Subcloning and complementation experiments localized the putative Bordetella promoter to a 0.7-kb BamHI-SphI subregion of the cloned genomic DNA fragment. Nucleotide sequencing, phenotypic analysis of mutants, and protein expression by the 4.5-kb DNA fragment in Escherichia coli suggested the presence of three alcaligin system genes, namely, alcA, alcB, and alcC. The deduced protein products of alcA, alcB, and alcC have significant primary amino acid sequence similarities with known microbial siderophore biosynthesis enzymes. Primer extension analysis mapped the transcriptional start site of the putative alcaligin biosynthesis operon containing alcABC to a promoter region overlapping a proposed Fur repressor-binding site and demonstrated iron regulation at the transcriptional level.Iron is a fundamental nutritional requirement for virtually all cells, and its assimilation is considered essential for invading pathogenic bacteria to establish infection in the iron-limiting environment of the host (13, 56). Additionally, iron serves as an environmental modulator of the production of certain virulence factors in a number of bacteria (14,24,31,32,46). Despite host iron sequestration, mediated primarily by the glycoprotein family of iron-binding transferrins, pathogens multiply successfully in vivo because they express efficient ironscavenging systems in response to decreased iron availability. These iron retrieval systems utilize two general strategies: one involving high-affinity iron-chelating soluble siderophores (30,40) and the other using siderophore-independent cell surface receptor mechanisms allowing iron uptake directly from host sources such as transferrin, lactoferrin, and heme compounds (7,35,38,54).Bordetella pertussis, the causative agent of human whooping cough or pertussis, and Bordetella bronchiseptica, the agent of swine atrophic rhinitis and kennel cough in dogs, are bacterial pathogens that infect the respiratory epithelial mucosae of their hosts. Early reports described the production of putative siderophores by both B. pertussis and B. bronchiseptica in response to iron deficiency (1, 23). Armstrong and Clements isolated and characterized B. bronchiseptica transposon-induced siderophore-deficient mutants; DNA hybridization studies using sequences flanking those transposon insertions confirmed the existence of homologs o...

A Bordetella bronchiseptica iron transport mutant was isolated following an enrichment procedure based on streptonigrin resistance. The mutant displayed a growth defect on iron-restricted medium containing ferric alcaligin as the sole iron source. In addition to the apparent inability to acquire iron from the siderophore, the mutant failed to produce alcaligin as well as two known iron-regulated proteins, one of which is the AlcC alcaligin biosynthesis protein. A 1.6-kb KpnI-PstI Bordetella pertussis DNA fragment mapping downstream of the alcaligin biosynthesis genes alcABC restored both siderophore biosynthesis and expression of the ironregulated proteins to the mutant. Nucleotide sequencing of this complementing 1.6-kb region identified an open reading frame predicted to encode a protein with strong similarity to members of the AraC family of transcriptional regulators, for which we propose the gene designation alcR. Primer extension analysis localized an iron-regulated transcription initiation site upstream of the alcR open reading frame and adjacent to sequences homologous to the consensus Fur repressor binding site. The AlcR protein was produced by using an Escherichia coli expression system and visualized in electrophoretic gels. In -frame alcR deletion mutants of B. pertussis and B. bronchiseptica were constructed, and the defined mutants exhibited the alcR mutant phenotype, characterized by the inability to produce and transport alcaligin and express the two iron-repressed proteins. The cloned alcR gene provided in trans restored these siderophore system activities to the mutants. Together, these results indicate that AlcR is involved in the regulation of Bordetella alcaligin biosynthesis and transport genes and is required for their full expression.