Multiple Effects of L-Leucine in Escherichia coli Lead to L-Leucine-Sensitive Growth in the Absence of Unphosphorylated PtsN

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 triangle 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 alpha-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.