The proposed DDVA uptake pathway in SYK-6 cells. Outer membrane transport of DDVA is mediated by DdvT; the energy necessary for the transport derived from the proton motive force appears to be transmitted from the inner membrane-localized TonB complex consisting of TonB1−ExbB1−ExbD1/ExbD2. In the inner membrane, DdvK takes up DDVA into the cytoplasm. In the presence of high concentrations of DDVA (in the order of > µM), unidentified outer membrane transporters such as porins are likely be involved in DDVA uptake. DdvR represses the expression of both ddvT and ligXa in the absence of DDVA. A schematic diagram of the TonB complex was drawn based on the papers by Celia et al.3,53.