The membrane-bound sensor kinase KdpD and the cytoplasmic response regulator KdpE regulate the expression of the kdpFABC operon coding for the high affinity potassium uptake system KdpFABC in Escherichia coli. The signal transduction cascade of this two component system is activated under K(+)-limiting conditions in the medium, but is less sensitive to high osmolality. In order to test whether K(+) limitation affects membrane phospholipid composition and whether this change affects kdpFABC expression, we analysed the phospholipid composition of E. coli under these conditions. Our measurements revealed that there is an increase in the cardiolipin (CL) content during the exponential growth phase at the expense of the zwitterionic phospholipid phosphatidylethanolamine. The higher anionic phospholipid content occurs along with an increase of transcriptional activity of the cls gene coding for CL synthase. Furthermore, in vivo studies with E. coli derivatives carrying mutations in genes coding for enzymes involved in phospholipid biosynthesis revealed that the increase in the anionic lipid composition enhances the expression rate of the kdpFABC operon. Finally, we show that kinase activity of KdpD is stimulated in its native membrane environment by fusion with liposomes of anionic, but reduced with liposomes of zwitterionic phospholipids.