Multiple Effects of L‐Leucine in <i>Escherichia coli</i> Lead to L‐Leucine‐Sensitive Growth in the Absence of Unphosphorylated <scp>PtsN</scp>

In E. coli K-12, the absence of unphosphorylated PtsN (unphospho-PtsN) has been proposed to cause an L-leucine-sensitive growth phenotype (Leu^S) by hyperactivated K⁺ uptake mediated impairment of the expression of the ilvBN operon, encoding subunits of the L-valine (Val)-sensitive acetohydroxyacid synthase I (AHAS I) that renders residual AHAS activity susceptible to inhibition by Leu and K⁺. This leads to AHAS insufficiency and a requirement for L-isoleucine (Ile). Herein, we provide an alternate mechanism for the Leu^S of the ∆ptsN mutant. Genetic and physiological studies with suppressors of the Leu^S indicate that impaired expression of the ilvBN operon jointly caused by the absence of unphospho-PtsN and the presence of Leu coupled to Leu-mediated repression of expression of AHAS III leads to AHAS insufficiency rendering residual AHAS activity susceptible to chronic Val stress that may be generated by exogenous Leu. Hyperactivated K⁺ uptake and an elevated α-ketobutyrate level mediate elevation of ilvBN expression and alleviate the Leu^S. The requirement of unphospho-PtsN as a positive regulator of ilvBN expression may buffer Ile biosynthesis against Leu-mediated AHAS insufficiency and protect AHAS I function from chronic endogenous Val generated by Leu and could be realized in certain environments that impair AHAS function.