![]() ![]() The uptake of iron is particularly important for bacterial growth ( 71) and synthesis of outer membrane iron transporters (called TonB-dependent transporters, TBDTs) is therefore regulated in a variety of ways. Transport into Gram-negative organisms is initiated by passage of the transported species across the outer membrane and into the periplasmic space prior to inner membrane translocation. In this review, we summarize recent progress in understanding regulation, structure and function in TBDTs and questions remaining to be answered. However, the transport mechanism is still unclear. In recent years many new structures of TBDTs have been solved in various states, resulting in a more complete picture of siderophore selectivity and binding, signal transduction across the outer membrane, and interaction with TonB-ExbB-ExbD. Because iron uptake is vital for almost all bacteria, expression of TBDTs is regulated in a number of ways that include metal-dependent regulators, σ/anti-σ factor systems, small RNAs, and even a riboswitch. ![]() ![]() The siderophore substrates range in complexity from simple small molecules such as citrate to large proteins like serum transferrin and haemoglobin. The transport process requires energy in the form of protonmotive force and a complex of three inner membrane proteins, TonB-ExbB-ExbD, to transduce this energy to the outer membrane. TonB-dependent transporters (TBDTs) are bacterial outer membrane proteins that bind and transport ferric chelates called siderophores, as well as vitamin B 12, nickel complexes, and carbohydrates. ![]()
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