THE PSEUDOMONAS-AERUGINOSA TONB GENE ENCODES A NOVEL TONB PROTEIN

The Pseudomonas aeruginosa tonB gene was cloned by complementation of the tonB mutation of Pseudomonas putida strain TE516 (W. Bitter, J. To mmassen & P. J. Weisbeek, 1993, Mol Microbiol 7, 117-130). The gene wa s 1025 bp in length, capable of encoding a protein of 36 860 Da. As wi th previously described TonB proteins, the P. aeruginosa TonB (TonB,,. ) was rich in Pro residues (18.1%) and contained Glu-Pro/Lys-Pro repea ts. Unlike previously described TonB proteins, however, TonB(P.a.) lac ked an N-terminal membrane anchor (signal) sequence and contained, ins tead, a predicted internal signal/anchor sequence, expected to yield a n atypical N-terminal cytoplasmic domain in this protein. TonB protein s are essential components in iron-siderophore uptake in bacteria, app arently functioning as energy transducers in coupling the energized st ate of the cytoplasmic membrane to outer-membrane receptor function. A s expected, tonB derivatives of P. aeruginosa were defective in sidero phore-mediated iron acquisition. tonB gene expression was inducible by iron-limitation, consistent with the identification of a Fur consensu s binding sequence upstream of the gene. TonB(P.a.) showed substantial ly greater similarity to the Escherichia coli TonB protein than the Ps eudomonas putida protein (31% identity vs. 20% identity) and tonB(P.a. ) was able to complement deficiencies in the acquisition of ferric ent erobactin and vitamin B-12, and sensitivity to phage phi 80 of an E. c oli tonB strain. The larger size of TonB(P.a.) and its ability to func tion in both E. coli and P. putida make it a unique TonB protein whose characterization should enhance our understanding of TonB function in bacteria.