No 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

No abstract

Bacillus subtilis exhibited an inducible K+-transporting ATPase activity with apparent Km and maximum velocity Vmax of 12.9 microM and 25.1 micromol x min(-1) x (g cell protein)(-1), respectively, when cultivated on a synthetic medium containing less than 400 microM K+. Due to this enzyme, the growth rate of the bacterium in synthetic medium was not changed down to 115 microM K+, and the bacterium was able to grow down to 20 microM K+. The limiting K+ concentration was higher in media with osmolarity increased by NaCl or sucrose. The ATPase was inhibited by micromolar concentrations of vanadate (Ki = 1.6 microM). The ATPase activity was not stimulated by any other monovalent cation. The subunit of this ATPase, with an Mr of 52000, covalently bound the gamma phosphate group of ATP. This phosphorylated intermediate was unstable in neutral and basic pH as well as in the presence of potassium and was stable in acid pH. The enzyme did not show immunological cross-reactivity with antibody against Kdp ATPase of Escherichia coli.