Rapid inactivation of the Escherichia coli Kdp K+ uptake system by high potassium concentrations

The Kdp K+ uptake system of Escherichia coli is induced by limitation for K + and/or high osmolarity. In the present study, the regulation of the activ ity of the Kdp system has been investigated in E. coli mutants possessing o nly the Kdp system as the mechanism of K+ accumulation. Cells grown in the presence of low K+ (0.1-1 mM) exhibit normal growth. However, growth inhibi tion results from exposure of cells to moderate levels of external K+ (> 5 mM). Measurement of the cytoplasmic pH, of K+ pools and of transport via th e Kdp system demonstrates that the Kdp system is rapidly and irreversibly i nhibited by moderate external K+. Concentrations of K+ greater than 2 mM ar e sufficient to cause inhibition of Kdp. At pH 6, this results in rapid low ering of the capacity for pH homeostasis, but at pH 7 the intracellular pH is unaffected. Parallel analysis of the expression of the Kdp system in a K dp(+)/kdpFABC-lacZ strain shows that levels of K+ that are sufficient to in hibit Kdp activity also repress expression. As a result, growth inhibition of strains solely possessing Kdp arises jointly from inhibition of Kdp acti vity and repression of Kdp gene expression. These data identify an importan t aspect of the regulation of potassium transport via the Kdp system and al so provide support for a model of regulation of Kdp expression via at least two mechanisms: sensing of both turgor and external K+ concentration.