Transcriptional activation of Bordetella alcaligin siderophore genes requires the AlcR regulator with alcaligin as inducer

Genetic and biochemical studies have established that Fur and iron mediate repression of Bordetella alcaligin siderophore system (alc) genes under iro n-replete nutritional growth conditions. In this study, transcriptional ana lyses using Bordetella chromosomal alc-lacZ operon fusions determined that maximal ale gene transcriptional activity under iron starvation stress cond itions is dependent on the presence of alcaligin siderophore. Mutational an alysis and genetic complementation confirmed that alcaligin-responsive tran scriptional activation of Bordetella alcaligin system genes is dependent on AlcR, a Fur-regulated AraC-like positive transcriptional regulator encoded within the alcaligin gene cluster. AlcR-mediated transcriptional activatio n is remarkably sensitive to inducer, occurring at extremely low alcaligin concentrations. This positive autogenous control circuit involving alcaligi n siderophore as the inducer for AlcR-mediated transcriptional activation o f alcaligin siderophore biosynthesis and transport genes coordinates enviro nmental and intracellular signals for maximal expression of these genes und er conditions in which the presence of alcaligin in the environment is perc eived.