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Reduction of Turgor Is Not the Stimulus for the Sensor Kinase KdpD of Escherichia coli

ABSTRACT Stimulus perception by the KdpD/KdpE two-component system of Escherichia coli is still controversial with respect to the nature of the stimulus that is perceived by the sensor kinase KdpD. Limiting potassium concentrations in the medium or high osmolality leads to KdpD/KdpE signal transduction, resulting in kdpFABC expression. It has been hypothesized that changes in turgor are sensed by KdpD through alterations in the physical state of the cytoplasmic membrane. However, in this study the quantitative determination of expression levels of the kdpFABC operon revealed that the system responds very effectively to K+-limiting conditions in the medium but barely and to various degrees to salt and sugar stress. Since the current view of stimulus perception calls for mainly intracellular parameters, which might be sensed by KdpD, we set out to test the cytoplasmic concentrations of ATP, K+, Na+, glutamate, proline, glycine, trehalose, putrescine, and spermidine under K+-limiting conditions. As a first result, the determination of the cytoplasmic volume, which is a prerequisite for such measurements, revealed that a transient shrinkage of the cytoplasmic volume, which is indicative of a reduction in turgor, occurred only under osmotic upshift but not under K+-limiting conditions. Furthermore, the intracellular ATP concentration significantly increased under osmotic upshift, whereas only a slight increase occurred after a potassium downshift. Finally, the cytoplasmic K+ concentration rose severalfold only after an osmotic upshock. For the first time, these data indicate that stimulus perception by KdpD correlates neither with changes in the cytoplasmic volume nor with changes in the intracellular ATP or K+ concentration or those of the other solutes tested. In conclusion, we propose that a reduction in turgor cannot be the stimulus for KdpD.