Mutational analysis of the TonB1 energy coupler of Pseudomonas aeruginosa

Siderophore-mediated iron transport in Pseudomonas aeruginosa is dependent upon the cytoplasmic membrane-associated TonB1 energy coupling protein for activity. To assess the functional significance of the various regions of this molecule and to identify functionally important residues, the tonB1 gene was subjected to site-directed mutagenesis, and the influence on iron acquisition was determined. The novel N-terminal extension of TonB1, which is absent in all other examples of TonB, was required for TonB1 activity in both P. aeruginosa and Escherichia coli. Appending it to the N terminus of the nonfunctional (in P. aeruginosa) Escherichia coli TonB protein (TonB(Ec)) rendered TonB(Ec) weakly active in P. aeruginosa and did not compromise the activity of this protein in E. coli. Elimination of the membrane-spanning, presumed membrane anchor sequence of TonB1 abrogated TonB1 activity in P. aeruginosa and E. coli. Interestingly, however, a conserved His residue within the membrane anchor sequence, shown to be required for TonB(Ec) function in E. coli, was shown here to be essential for TonB1 activity in E. coli but not in P. aeruginosa. Several mutations within the C-terminal end of TonB1, within a region exhibiting the greatest similarity to other TonB proteins, compromised a TonB1 contribution to iron acquisition in both P. aeruginosa and E. coli, including substitutions at Tyr264, Glu274, Lys278, and Asp304. Mutations at Pro265, Gln293, and Val294 also impacted negatively on TonB1 function in E. coli but not in P. aeruginosa. The Asp304 mutation was suppressed by a second mutation at Glu274 of TonB1 but only in P. aeruginosa. Several TonB1-TonB(Ec) chimeras were constructed, and assessment of their activities revealed that substitutions at the N or C terminus of TonB1 compromised its activity in P. aeruginosa, although chimeras possessing an E. coli C terminus were active in E. coli.