Expression of the Kdp ATPase is consistent with regulation by turgor pressure

The kdpFABC operon of Escherichia coli encodes the four protein subunits of the Kdp K+ transport system. Kdp is expressed when growth is limited by the availability of K+. Expression of Kdp is dependent on the products of the adjacent kdpDE operon, which encodes a pair of two-component regulators. Studies with kdp-lac fusions led to the suggestion that change in turgor pressure acts as the signal to express Kdp (L. A. Laimins, D. B. Rhoads, and W. Epstein, Proc. Natl. Acad. Sci. USA 78:464-468, 1981). More recently, effects of compatible solutes, among others, have been interpreted as inconsistent with the turgor model (H. Asha and J. Gowrishankar, J. Bacteriol. 175:4528-4537, 1993). We re-examined the effects of compatible solutes and of medium pH on expression of Kdp in studies in which growth rate was also measured. In all cases, Kdp expression correlated with the K+ concentration when growth began to slow. Making the reasonable but currently untestable assumptions that the reduction in growth rate by K+ limitation is due to a reduction in turgor and that addition of betaine does not increase turgor, we concluded that all of the data on Kdp expression are consistent with control by turgor pressure.