Role of Acetohydroxy Acid Isomeroreductase in Biosynthesis of Pantothenic Acid in Salmonella typhimurium

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Structural genes have been identified for all of the enzymes involved in the biosynthesis of pantothenic acid in Salmonella typhimurium and Escherichia coli K-12, with the exception of ketopantoic acid reductase, which catalyzes the conversion of α-ketopantoate to pantoate. The acetohydroxy acid isomeroreductase from S. typhimurium efficiently bound α-ketopantoate (Km = 0.25 mM) and catalyzed its reduction at 1/20 the rate at which α-acetolactate was reduced. Since two enzymes could apparently participate in the synthesis of pantoate, a S. typhimurium ilvC8 strain was mutagenized to derive strains completely blocked in the conversion of α-ketopantoate to pantoate. Several isolates were obtained that grew in isoleucine-valine medium supplemented with either pantoate or pantothenate, but not in the same medium supplemented with α-ketopantoate or β-alanine. The mutations that conferred pantoate auxotrophy (designated panE) to these isolates appeared to be clustered, but were not linked to panB or panC. All panE strains tested had greatly reduced levels of ketopantoic acid reductase (3 to 12% of the activity present in DU201). The capacity of the isomeroreductase to synthesize pantoate in vivo was assessed by determining the growth requirements of ilvC+ derivatives of panE ilvC8 strains. These strains required either α-ketopantoate, pantoate, or pantothenate when the isomeroreductase was present at low levels; when the synthesis of isomeroreductase was induced, panE ilvC+ strains grew in unsupplemented medium. These phenotypes indicate that a high level of isomeroreductase is sufficient for the synthesis of pantoate. panE ilvC+ strains also grew in medium supplemented with lysine and methionine. This phenotype resembles that of some S. typhimurium ilvG mutants (e.g., DU501) which are partially blocked in the biosynthesis of coenzyme A and are limited for succinyl coenzyme A. panE ilvC+ strains which lack the acetohydroxy acid synthases required only methionine for growth (in the presence of leucine, isoleucine, and valine). This and other evidence suggested that the synthesis of pantoic acid by isomeroreductase was blocked by the α-acetohydroxy acids and that pantoic acid synthesis was enhanced in the absence of these intermediates, even when the isomeroreductase was at low levels. panE ilvC+ strains reverted to pantothenate independence. Several of these revertants were shown to have elevated isomeroreductase levels under noninduced and induced conditions; the suppressing mutation in each revertant was shown to be closely linked to ilvC by P22 transduction. This procedure presents a means for obtaining mutants with altered regulation of isomeroreductase.

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