TONB-DEPENDENT IRON ACQUISITION - MECHANISMS OF SIDEROPHORE-MEDIATED ACTIVE-TRANSPORT

Cells growing in aerobic environments have developed intricate strateg ies to overcome the scarcity of iron, an essential nutrient. In Gram-n egative bacteria, high-affinity iron acquisition requires outer membra ne-localized proteins that bind iron chelates at the cell surface and promote their uptake. Transport of bound chelates across the outer mem brane depends upon TonB-ExbB-ExbD, a cytoplasmic membrane-localized co mplex that transduces energy from the proton motive force to high-affi nity receptors in the outer membrane. Upon ligand binding to iron chel ate receptors, conformational changes are induced, some of which are d etected in the periplasm, These structural alterations signal the liga nd-loaded status of the receptor and, therefore, the requirement for T ons-dependent energy transduction, Thus, TonB interacts preferentially and directly with ligand-loaded receptors, Such a mechanism ensures t he productive use of cellular energy to drive active transport at the outer membrane.