Biochemical characterization of a Haemophilus influenzae periplasmic iron transport operon

Bacterial iron transport is critical for growth of pathogens in the host environment, where iron is limited as a form of nonspecific immunity. For Gram-negative bacteria such as Haemophilus influenzae, iron first must be transported across the outer membrane and into the periplasmic space, then from the periplasm to the cytosol. H. influenzae express a periplasmic iron-binding protein encoded by the hitA gene. This gene is organized as the first of a three-gene operon purported to encode a classic high affinity iron acquisition system that includes hitA, a cytoplasmic permease (hitB), and a nucleotide binding protein (hitC). In this study we describe the cloning, overexpression, and purification of the H. influenzae hitA gene product. The function of this protein is unambiguously assigned by demonstrating its ability to compete for iron bound to the chemical iron chelator 2,2'-dipyridyl, both in vitro and within the periplasmic space of a siderophore-deficient strain of Escherichia coli. Finally, the importance of a functional hitABC operon for iron acquisition is demonstrated by complementation of this siderophore-deficient E. coli to growth on dipyridyl-containing medium. These studies represent a detailed genetic, biochemical, and physiologic description of an active transport system that has evolved to efficiently transport iron and consequently is widely distributed among Gram-negative pathogenic bacteria.