Metal trafficking via siderophores in Gram-negative bacteria: Specificities and characteristics of the pyoverdine pathway

Under iron-limiting conditions, fluorescent pseudomonads secrete fluorescent siderophores called pyoverdines (Pvd), which form complexes with iron that are then taken up by the bacteria. Pvds consist of a fluorescent chromophore derived from 2,3-diamino-6,7-dihydroxyquinoline and containing one of the bidentate groups involved in iron chelation, linked to a peptide moiety containing the other two bidentate groups required for binding to Fe3+. More than 100 different Pvds have been identified, with different peptide sequences, forming a wide family of siderophores. In the human opportunistic pathogen Pseudomonas aeruginosa, Pvd is necessary for infection and is considered to be a virulence factor. This review focuses on the mechanisms underlying iron uptake by the Pvd pathway in pseudomonads, taking into account recent biochemical and biophysical studies and recently solved 3D-structures of the Pvd outer membrane transporter FpvA in four different loading states. These data are discussed and compared with the mechanisms of siderophore–Fe uptake reported for other Gram-negative bacteria.