An Escherichia coli mutant devoid of enzyme IIANtr (EIIANtr) of the nitrogen PTS is extremely sensitive to leucine-containing peptides due to decreased expression of acetohydroxy acid synthase. This decreased expression is due to defective potassium homeostasis. We further elucidate here the mechanism for regulation of gene expression by the intracellular level of K+. The leucine hypersensitivity of a ptsN (encoding EIIANtr) mutant was suppressed by deleting rpoS, encoding the stationary phase σ factor. Despite intracellular levels of sigma factors comparable to the wild-type strain, most of the genes downregulated in a ptsN mutant are controlled by σ70, while all the upregulated genes are controlled by σS, implying that the balance of sigma activities is modified by ptsN deletion. This change of sigma factor activities in the deletion mutant was found to be due to increased levels of K+. In vitro transcription assays demonstrated that a σ70 controlled gene and a σS controlled gene were differentially affected by potassium concentration. Biochemical studies revealed that K+ is responsible for sigma factor competition by differentially influencing the binding of σ70 and σS to core RNA polymerase. Taken together, the data suggest that EIIANtr controls sigma factor selectivity by regulating the intracellular K+ level.