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Vitamin B12 transport system in E. coli. Protein models were based on the crystal structures with PDB: 1NQG (only BtuB), 2GSK (BtuB with Cbl and TonB), 1N4A (BtuF), 6TYI (ExbB-ExbD), and 4FI3 (BtuCD). The structure of the periplasmic domain of TonB is unknown, so it was drawn schematically. IM, inner membrane; OM, outer membrane. To see this figure in color, go online.
