Vitamin B12 transport in Escherichia coli: energy coupling between membranes.

Cells of Escherichia coli possess high-affinity active transport systems of vitamin B12 and iron-siderophore complexes. Specific outer-membrane proteins carry out the energy-dependent transport across the outer membrane, in conjunction with the TonB coupling protein. Mutagenesis experiments have identified a conserved region near the amino-terminus of the outer-membrane transporters that is necessary for energy-coupled transport. The ability of extragenic suppressor mutations in tonB to correct the transport defect indicates that TonB couples the proton-motive force to the outer-membrane proteins by direct contact.