The Pseudomonas aeruginosa tonB gene encodes a novel TonB protein

摘要

The Pseudomonas aeruginosa tonB gene was cloned by complementation of the tonB mutation of Pseudomonas putida strain TE516 (W. Bitter, J. Tommassen & P. J. Weisbeek, 1993, Mol Microbiol 7, 117-130). The gene was 1025 bp in length, capable of encoding a protein of 36860 Da. As with previously described TonB proteins, the P. aeruginosa TonB (TonBp.a.) was rich in Pro residues (18.1 %) and contained Glu-Pro/Lys-Pro repeats. Unlike previously described TonB proteins, however, TonBp.a. lacked an N-terminal membrane anchor (signal) sequence and contained, instead, a predicted internal signal/anchor sequence, expected to yield an atypical N-terminal cytoplasmic domain in this protein. TonB proteins are essential components in iron-siderophore uptake in bacteria, apparently functioning as energy transducers in coupling the energized state of the cytoplasmic membrane to outer-membrane receptor function. As expected, tonB derivatives of P. aeruginosa were defective in siderophore-mediated iron acquisition. tonB gene expression was inducible by iron-limitation, consistent with the identification of a Fur consensus binding sequence upstream of the gene. TonBp.a. showed substantially greater similarity to the Escherichia coli TonB protein than the Pseudomonas putida protein (31 % identity vs. 20 % identity) and tonBP.a. was able to complement deficiencies in the acquisition of ferric enterobactin and vitamin B12# and sensitivity to phage ?80 of an E. coli tonB strain. The larger size of TonBP.a. and its ability to function in both E. coli and P. putida make it a unique TonB protein whose characterization should enhance our understanding of TonB function in bacteria.