![]() Transport of vitamin B12 in tonB mutants of Escherichia coli. Bassford PJ, Jr, Bradbeer C, Kadner RJ, Schnaitman CA.Links to PubMed are also available for Selected References. Get a printable copy (PDF file) of the complete article (962K), or click on a page image below to browse page by page. Full textįull text is available as a scanned copy of the original print version. I conclude that the ExbB protein is a normal component of the energy coupling system for the transport of cobalamin across the outer membrane. Cells with mutations in both exbB and tolQ had no measurable cobalamin transport and thus had a phenotype that was essentially the same as TonB. I show here that the tolQ gene product can partly replace the function of the ExbB protein. The coupling of metabolic energy to outer membrane cobalamin transport requires the TonB protein and is stimulated by the ExbB protein. Dinitrophenol eliminated cobalamin transport in all strains, but cyanide inhibited this process only in atp and btuC atp mutant cells, providing conclusive evidence that cobalamin transport across the outer membrane requires specifically the proton motive force of the inner membrane. Accordingly, I have examined the effects of cyanide and of 2,4-dinitrophenol on cobalamin uptake in btuC and atp mutants, which lack inner membrane cobalamin transport and the membrane-bound ATP synthase, respectively. The novelty of such an energy coupling mechanism and its relevance to other outer membrane transport processes have required confirmation of this conclusion by studies with cells in which cobalamin transport is limited to the outer membrane. ![]() Cells of Escherichia coli pump cobalamin (vitamin B12) across their outer membranes into the periplasmic space, and it was concluded previously that this process is potentiated by the proton motive force of the inner membrane. ![]()
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