Pyochelin-mediated iron transport in Pseudomonas aeruginosa: involvement of a high-molecular-mass outer membrane...

An iron-regulated outer membrane protein of 75,000 daltons was strongly expressed following iron limitation of strains of Pseudomonas aeruginosa which fail to produce pyoverdine. A mutant nonderepressible for this protein (K372) was deficient in pyochelin-mediated iron transport at 150 nM FeCl3, consistent with a role for the 75-kDa protein in ferripyochelin transport. Moreover, ferripyochelin specifically protected the 75-kDa protein against trypsin digestion, supporting an interaction between ferripyochelin and the 75-kDa protein. Previous reports implicated a 14,000-dalton outer membrane protein as the receptor for ferripyochelin (P.A. Sokol and D.E. Woods, Infect. Immun. 40:665-669, 1983) and demonstrated that a mutant (FBP-28) expressing a defective 14-kDa outer membrane protein did not exhibit pyochelin-mediated iron transport (P.A. Sokol, J. Bacteriol. 169:3365-3368, 1987). Nonetheless, we were able to demonstrate (i) that FBP-28 was inducible for the 75-kDa protein under iron-limiting conditions and (ii) that concomitant with the induction of this protein in FBP-28, pyochelin-mediated iron uptake at 150 nM FeCl3 was observed. Interestingly, strain K372 did transport ferripyochelin at higher (750 nM) FeCl3 concentrations, suggesting that a second pyochelin-mediated iron transport system, perhaps involving the 14-kDa outer membrane protein identified previously, operates in P. aeruginosa.