Analysis of an avtA::Mu d1(Ap lac) mutant: metabolic role of transaminase C.

Escherichia coli can synthesize alpha-ketoisovalerate, the precursor of valine, leucine, and pantothenate, by three routes: anabolically via dihydroxyacid dehydrase and catabolically via both the branched-chain amino acid transaminase (transaminase B) and the alanine-valine transaminase (transaminase C). An E. coli K-12 mutant devoid of transaminase C (avtA) was isolated by mutagenizing an isoleucine-requiring strain devoid of transaminase B (ilvE::Tn5) with Mu d1(Ap lac) and selecting for valine-requiring derivatives which were ampicillin resistant, Lac+, able to crossfeed an ilvD mutant, and unable to grow on alpha-ketoisovalerate in place of valine. Strains defective in one, two, or all three alpha-ketoisovalerate metabolic enzymes were constructed, and their properties were analyzed. The data indicated that avtA is the structural gene for transaminase C, that transaminase C is a single enzyme species, and that the sole pathway for pantothenate biosynthesis is from alpha-ketoisovalerate. The data further showed that isoelectric inhibits the transaminase B-catalyzed deamination of valine in vivo.