Elucidating the origin of the ExbBD components of the TonB system through Bayesian inference and maximum-likelihood phylogenies

- ExbBD and MotAB have distinct evolutionary histories and are separated into different clades, but share a common ancestor.
- The ExbBD complex has been horizontally transferred throughout bacteria phyla several times.
- The TonB system probably evolved from a primitive proton channel consisting of ExbB/ExbD precursors.

Uptake of ferric siderophores, vitamin B12, and other molecules in gram-negative bacteria is mediated by a multi-protein complex known as the TonB system. The ExbB and ExbD protein components of the TonB system play key energizing roles and are homologous with the flagellar motor proteins MotA and MotB. Here, the phylogenetic relationships of ExbBD and MotAB were investigated using Bayesian inference and the maximum-likelihood method. Phylogenetic trees of these proteins suggest that they are separated into distinct monophyletic groups and have originated from a common ancestral system. Several horizontal gene transfer events for ExbB-ExbD are also inferred, and a model for the evolution of the TonB system is proposed.