Mutual inhibition of cobalamin and siderophore uptake systems suggests their competition for TonB function

Vitamin B12 (CN-Cbl) and iron-siderophore complexes are transported into Escherichia coli in two energy-dependent steps. The first step is mediated by substrate-specific outer membrane transport proteins and the energy-coupling TonB protein complex, and the second step uses separate periplasmic permeases for transport across the cytoplasmic membrane. Genetic and biochemical evidence suggests that the TonB-dependent outer membrane transporters contact TonB directly, and thus they might compete for limiting amounts of functional TonB. The transport of iron-siderophore complexes, such as ferrichrome, causes a partial decrease in the rate of CN-Cbl transport. Although CN-Cbl uptake does not inhibit ferrichrome uptake in wild-type cells, in which the amount of the outer membrane ferrichrome transporter FhuA far exceeds that of the cobalamin transporter BtuB, CN-Cbl does inhibit ferrichrome uptake when BtuB is overexpressed from a multicopy plasmid. This inhibition by CN-Cbl is increased when the expression of FhuA and TonB is repressed by growth with excess iron and is eliminated when BtuB synthesis is repressed by CN-Cbl. The mutual inhibition of CN-Cbl and ferrichrome uptake is overcome by increased expression of TonB. Additional evidence for interaction of the Cbl and iron transport systems is provided by the strong stimulation of the BtuB- and TonB-dependent transport of CN-Cbl into a nonexchangeable, presumably cytoplasmic pool by preincubation of cells with the iron(II) chelator 2,2'-dipyridyl. Other metal ion chelators inhibited CN-Cbl uptake across the outer membrane. Although the effects of chelators are multiple and complex, they indicate competition or interaction among TonB-dependent transport systems.