Multiple Paths for Nonphysiological Transport of K + in Escherichia coli

Abstract

ABSTRACT 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.