Outer-membrane siderophore receptors of heterotrophic oceanic bacteria

Pathogenic gram-negative bacteria use specific receptors to transport ferric siderophore complexes across their outer membrane during iron (Fe)-limited growth. Receptors such as these have not yet been characterized in oceanic heterotrophic bacteria. We examined four species of g-proteobacteria for the presence of Fe-siderophore receptors with the use of a nondenaturing polyacrylamide gel electrophoresis binding assay and the siderophore ferrioxamine B (FB) labeled with 55 Fe. Small-subunit rRNA sequence analysis assigned these bacteria to the genera Pseudoalteromonas and Alteromonas. Two oceanic species, Pseudoalteromonas haloplanktis (Neptune) and Alteromonas macleodii (Jul88), which were shown previously to transport and assimilate Fe bound to FB during growth, synthesized an outer-membrane FB receptor under Fe-limiting conditions. Only low concentrations of the receptors were detected in these bacteria when they were grown with high concentrations of Fe. The FB receptor of P. haloplanktis (Neptune) had an apparent molecular mass of 79 kDa and an externally oriented binding site. The molecular mass of the receptor of A. macleodii (Jul88) was 100 kDa. No FB receptors were detected by our methods in two coastal species, Pseudoalteromonas rubra(LMG1) and Pseudoalteromonas piscicida(PWF3). P. haloplanktis (Neptune) and A. macleodii (Jul88) also bound 55 Fe-ferrichrome, a trihydroxamate siderophore like FB. Binding assays conducted with 115 nmol L 2 15 5 Fe-FB in the presence of increasing concentrations of desferrioxamine B showed a progressive decrease in the amount of 55 Fe-FB bound by the receptor protein, suggesting strong affinity of the receptor for the Fe-free siderophore. Our results provide the first demonstration of Fe-siderophore receptors in oceanic heterotrophic bacteria. Heterotrophic bacteria have evolved Fe(III) transport systems that enable them to grow in environments containing extraordinarily low concentrations of Fe. When Fe is scarce, many of these organisms excrete low‐molecular mass Febinding compounds, called siderophores, that bind free Fe(III) in solution. The Fe-siderophore complex attaches to its cognate receptor on the outer membrane of the cell and is subsequently internalized (Braun and Killman 1999; Ratledge and Dover 2000). Some bacterial receptors recognize more than one siderophore, not necessarily of the same structural type (e.g., FhuE of E. coli) (van der Helm 1998). Many bacteria express receptors for siderophores released by other species or for Fe sources contained in their hosts (Braun et 1