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

The absence of PtsN, the terminal phosphoacceptor of the phosphotransferase system comprising PtsP-PtsO-PtsN, inEscherichia coli confers a potassium-sensitive (Ks ) 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 ΔptsN mutant is associated with K+ limitation. Accordingly, the moderate Ks displayed by the ΔptsN 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 ΔptsN mutant, but the ΔptsN mutation did not alter YcgO levels. Heterologous overexpression ofycgO 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 ΔptsN 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 inE. coli . Studies on the physiological defect that renders anE. 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.