Iron release from transferrin by pyoverdin and elastase from Pseudomonas aeruginosa

Pseudomonas aeruginosa produces the siderophores pyoverdin and pyochelin as well as receptors for siderophores in response to iron deprivation. Previously, it has been shown in vitro that at neutral pH purified pyoverdin acquires iron from transferrin only in the presence of P. aeruginosa elastase (LasB), which proteolytically degrades transferrin. We constructed a LasB-negative mutant, PAO1E, by insertional mutagenesis to investigate whether this mutant differs in growth from the parental strain PAO1 in an iron-depleted medium supplemented with transferrin or human serum. PAO1 and PAO1E did not differ in growth with 1.25 microM Fe2-transferrin as the only iron source. Urea gel electrophoresis indicated iron release from intact transferrin during the logarithmic growth phase of PAO1 and PAO1E. A total of 333 microM LasB was synthesized from PAO1 after onset of stationary-phase growth. Quantification of pyoverdin by spectroscopy revealed that up to 900 microM pyroverdin was produced during growth of the strains in medium supplemented with Fe2-transferrin or 10% human serum. Incubation of Fe2-transferrin and purified pyoverdin in concentrations similar to those found in the culture supernatant resulted in release iron from transferrin after 10 h at 37 degrees C. However, LasB significantly enhanced the rate constant for iron acquisition of pyoverdin from transferrin. We conclude that P. aeruginosa can use transferrin as an iron source without further need of LasB or pH changes. This is further supported by experiments with P. aeruginosa K437, which has a defective iron uptake system, and its LasB-negative mutant, K437E. Though K437 and K437E did not differ in growth with Fe2-transferrin as the only iron source, their growth was significantly reduced relative to that of PAO1 and PAO1E.