The Pseudomonas aeruginosa pyochelin-iron uptake pathway and its metal specificity

Pyochelin (Pch) is one of the two major siderophores produced and secreted by Pseudomonas aeruginosa PAO1 to assimilate iron. It chelates iron in the extracellular medium and transports it into the cell via a specific outer membrane transporter, FptA. We used the fluorescent properties of Pch to show that this siderophore chelates, in addition to Fe(3+) albeit with substantially lower affinities, Ag(+), Al(3+), Cd(2+), Co(2+), Cr(2+), Cu(2+), Eu(3+), Ga(3+), Hg(2+), Mn(2+), Ni(2+), Pb(2+), Sn(2+), Tb(3+), Tl(+), and Zn(2+). Surprisingly, the Pch complexes with all these metals bound to FptA with affinities in the range of 10 nM to 4.8 microM (the affinity of Pch-Fe is 10 nM) and were able to inhibit, with various efficiencies, Pch-(55)Fe uptake in vivo. We used inductively coupled plasma atomic emission spectrometry to follow metal uptake by P. aeruginosa. Energy-dependent metal uptake, in the presence of Pch, was efficient only for Fe(3+). Co(2+), Ga(3+), and Ni(2+) were also transported, but the uptake rates were 23- to 35-fold lower than that for Fe(3+). No uptake was seen for all the other metals. Thus, cell surface FptA has broad metal specificity at the binding stage but is much more selective for the metal uptake process. This uptake pathway does not appear to efficiently assimilate any metal other than Fe(3+).