Kup is the major K + uptake system in Escherichia coli upon hyper-osmotic stress at a low pH

Abstract

The K + uptake was observed in washed cells of Escherichia coli, wild-type, upon hyper-osmotic stress at pH 5.5 when glucose was supplemented. This uptake had apparent a K m of 0.58 mM and V max of 0.10 μmol K +/min/mg protein. Such a K + uptake was investigated using a mutant defective in Kdp and TrkA but with a functional Kup and a mutant defective in Kdp and Kup but having an active TrkA. The K + uptake to reach the steady state level as well as the initial K + influx rate in the first mutant were at least 3.5-fold greater than these values with the second mutant and similar to those of the wild-type. Such differences in the K + uptake activity were correlated with K + requirements for growth of these mutants. Moreover, the K + uptake in the wild-type was blocked by a protonophore (carbonyl cyanide m-chlorophenylhydrazone). Valinomycin, arsenate and N, N′-dicyclohexylcarbodiimide were not effective in changing the K + uptake. It is suggested that Kup is the major K + uptake system in E. coli upon hyper-osmotic stress at a low pH.