The gene was cloned by complementation of the mutation of strain TE516 (W. Bitter, J. Tommassen & P. J. Weisbeek, 1993, 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 TonB (TonB) was rich in Pro residues (18.1 %) and contained Glu-Pro/Lys-Pro repeats. Unlike previously described TonB proteins, however, TonB 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, derivatives of were defective in siderophore-mediated iron acquisition. gene expression was inducible by iron-limitation, consistent with the identification of a Fur consensus binding sequence upstream of the gene. TonB showed substantially greater similarity to the TonB protein than the protein (31 % identity vs. 20 % identity) and tonB was able to complement deficiencies in the acquisition of ferric enterobactin and vitamin B and sensitivity to phage ø80 of an strain. The larger size of TonB and its ability to function in both and make it a unique TonB protein whose characterization should enhance our understanding of TonB function in bacteria.


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