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

Abstract

ABSTRACT 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 K s . We provide evidence that the K s of the Δ ptsN mutant is associated with K + limitation. Accordingly, the moderate K s displayed by the Δ 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 K s . Expression of PtsN variants bearing the H73A, H73D, and H73E substitutions of the phosphorylation site histidine of PtsN complemented the K s . Absence of the predicted inner membrane protein YcgO (also called CvrA) suppressed the K s , which was correlated with elevated cellular K + content in the Δ ptsN mutant, but the Δ ptsN mutation did not alter YcgO levels. Heterologous overexpression of ycgO also led to K s 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 K s in the Δ 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.