TonB, C-terminal <p>The sequences in this set all contain a conserved C-terminal domain which is characteristic of TonB and is homologs. TonB is an energy-transducer for TonB-dependent receptors of Gram-negative bacteria [<cite idref="PUB00006671"/>]. Most members are designated as TonB or TonB-related proteins, but a few represent the paralogous TolA protein. Several bacteria have up to four TonB paralogs. In nearly every case, a proline-rich repetitive region is found N-terminal to this domain; these low-complexity regions are highly divergent and cannot readily be aligned. The region is suggested to span the periplasm. </p><p>Iron is essential for growth in both bacteria and mammals. Controlling the amount of free iron in solution is often used as a tactic by hosts to limit invasion of pathogenic microbes; binding iron tightly within protein molecules can accomplish this. Some bacteria express surface receptors to capture eukaryotic iron-binding compounds, while others have evolved siderophores to scavenge iron from iron-binding host proteins [<cite idref="PUB00006650"/>].</p><p> The absence of free iron molecules in the surrounding environment triggers transcription of gene clusters that encode both siderophore-synthesis ezymes, and receptors that recognise iron-bound siderophores [<cite idref="PUB00006626"/>]. An example of the latter is <taxon tax_id="562">Escherichia coli</taxon> fepA, which resides in the outer envelope and captures iron-bound enterobactin [<cite idref="PUB00006673"/>]. </p><p>To complete transport of bound iron across the inner membrane, a second receptor complex is needed. The major component of this is tonB, a 27kDa protein that facilitates energy transfer from the proton motive force to outer receptors. B-12 and colicin receptors also make use of the tonB system to drive active transport at the outer membrane.</p>