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Multiple Paths for Nonphysiological Transport of K⁺ in Escherichia coli

Ed T. Buurman,^* Debbie McLaggan, Josef Naprstek, and Wolfgang Epstein

Department of Molecular Genetics and Cell Biology, University of Chicago, Chicago, Illinois 60637

Received 8 December 2003/ Accepted 19 March 2004

Mutants of Escherichia coli lacking all of the known saturable K⁺ transport systems, "triple mutants," require elevated K⁺ concentrations for growth. K⁺ transport activity in such mutants, called TrkF activity, has low substrate specificity and a low rate that increases with increasing external pH. Attempts to isolate mutants requiring even higher concentrations of K⁺ failed, implying that either TrkF is essential or is composed of multiple minor K⁺ transport activities. Instead, we sought mutations that allowed triple mutants to grow at lower K⁺ concentrations. Mutations so identified include ones altering MscL, the large mechanosensitive channel, or Opp, the oligopeptide permease. However, a possible contribution of wild-type Opp and MscL to TrkF activity was not proven. In contrast, expression of wild-type ProP, TrkG, and TrkH proteins increased uptake when encoded on multicopy plasmids. In all of these situations, the driving force for K⁺ appeared to be the transmembrane electric potential, and in most cases substrate specificity was low; these are characteristics of TrkF activity. These results support the view that TrkF is composed of multiple, "aberrant" K⁺ transport activities, i.e., paths that, regardless of their physiological function, allow K⁺ to cross the cell membrane by a uniport process.

Present address: Department of Biomedical Sciences, Institute of Medical Sciences, University of Aberdeen, Foresterhill, Aberdeen AB25 2ZD, Scotland, United Kingdom.

Deceased.

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