Growth Inhibition by External Potassium of Escherichia coli Lacking PtsN (EIIANtr) Is Caused by Potassium Limitation Mediated by YcgO

The absence of PtsN, the terminal phosphoacceptor of the phosphotransferase system comprising PtsP-PtsO-PtsN, in Escherichia coli confers a potassium-sensitive (K-s) phenotype as the external K+ concentration ([K+](e)) is increased above 5 mM. A growth-inhibitory increase in intracellular K+ content, resulting from hyperactivated Trk-mediated K+ uptake, is thought to cause this Ks. We provide evidence that the Ks of the Delta ptsN mutant is associated with K+ limitation. Accordingly, the moderate Ks displayed by the Delta ptsN mutant was exacerbated in the absence of the Trk and Kup K+ uptake transporters and was associated with reduced cellular K+ content. Conversely, overproduction of multiple K+ uptake proteins suppressed the Ks. Expression of PtsN variants bearing the H73A, H73D, and H73E substitutions of the phosphorylation site histidine of PtsN complemented the Ks. Absence of the predicted inner membrane protein YcgO (also called CvrA) suppressed the Ks, which was correlated with elevated cellular K+ content in the Delta ptsN mutant, but the Delta ptsN mutation did not alter YcgO levels. Heterologous overexpression of ycgO also led to Ks that was associated with reduced cellular K+ content, exacerbated by the absence of Trk and Kup and alleviated by overproduction of Kup. Our findings are compatible with a model that postulates that Ks in the Delta ptsN mutant occurs due to K+ limitation resulting from activation of K+ efflux mediated by YcgO, which may be additionally stimulated by [K+](e), implicating a role for PtsN (possibly its dephosphorylated form) as an inhibitor of YcgO activity. IMPORTANCE This study examines the physiological link between the phosphotransferase system comprising PtsP-PtsO-PtsN and K+ ion metabolism in E. coli. Studies on the physiological defect that renders an E. coli mutant lacking PtsN to be growth inhibited by external K+ indicate that growth impairment results from cellular K+ limitation that is mediated by YcgO, a predicted inner membrane protein. Additional observations suggest that dephospho-PtsN may inhibit and external K+ may stimulate K+ limitation mediated by YcgO. It is speculated that YcgO-mediated K+ limitation may be an output of a response to certain stresses, which by modulating the phosphotransfer capacity of the PtsP-PtsO-PtsN phosphorelay leads to growth cessation and stress tolerance.