Regulation of plasmid-mediated iron transport and virulence in Vibrio anguillarum

Iron is essential for bacterial growth and metabolism. In vertebrates this metal is complexed by high-affinity iron-binding proteins, such as transferrin in serum. The fish pathogen Vibrio anguillarum possesses a very efficient iron-uptake system which is encoded in the virulence plasmid pJM1. This allows the bacterium to utilize the otherwise unavailable iron in the fish host, resulting in the septicemic disease vibriosis. This system includes the siderophore anguibactin and transport components. We have cloned this iron-uptake system and have defined several genetic units by transposition mutagenesis. Nucleotide sequence analysis identified four open reading frames in the transport region, one of these corresponding to the gene for the outer membrane protein OM2 and another to a 40-kDa polypeptide. Complementation analysis indicated that products from all four reading frames are required for the transport of iron-anguibactin complexes. We have also identified positive and negative-acting regulatory elements that modulate in concert the expression of anguibactin biosynthetic genes and iron transport. The deletion or mutation of the positive-acting regulatory genes results in an iron-uptake-deficient phenotype and leads to an attenuation of virulence, underscoring the importance of this iron-uptake system as a virulence attribute of V. anguillarum.