Osmotic control of kdp operon expression in Escherichia coli.

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Abstract
Turgor pressure, the difference in osmotic pressure across the inner membrane, regulates expression of the kdp operon in E. coli. The kdp operon codes for Kdp, a high-affinity repressible transport system for the uptake of K+. The regulation of Kdp expression was studied in a strain in which the gene for .beta.-galactosidase, lacZ, was placed under control of the kdp promoter. Neither internal nor external K+ concentrations directly controlled Kdp expression. Only when the external K+ concentration was reduced to the point of limiting growth was the kdp operon expressed. An increase in external osmolarity at constant K+ concentration, a procedure that reduces turgor pressure, caused expression of the kdp operon. As the magnitude of the osmotic shift was increased, corresponding to greater decreases in turgor pressure, the amount of Kdp expression also increased. The kdp operon thus appears to be controlled by changes in a physical force, the turgor pressure.