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Journal of Bacteriology, August 1998, p. 4056-4067, Vol. 180, No. 16
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Branched-Chain Amino Acid Biosynthesis in Salmonella typhimurium: a Quantitative Analysis

Sabine Epelbaum,1,dagger Robert A. LaRossa,2 Tina K. VanDyk,2 T. Elkayam,1 David M. Chipman,1 and Ze'ev Barak1,*

Department of Life Sciences, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel,1 and Central Research and Development, E. I. du Pont de Nemours and Company, Wilmington, Delaware 19880-01732

Received 9 February 1998/Accepted 30 May 1998

We report here the first quantitative study of the branched-chain amino acid biosynthetic pathway in Salmonella typhimurium LT2. The intracellular levels of the enzymes of the pathway and of the 2-keto acid intermediates were determined under various physiological conditions and used for estimation of several of the fluxes in the cells. The results led to a revision of previous ideas concerning the way in which multiple acetohydroxy acid synthase (AHAS) isozymes contribute to the fitness of enterobacteria. In wild-type LT2, AHAS isozyme I provides most of the flux to valine, leucine, and pantothenate, while isozyme II provides most of the flux to isoleucine. With acetate as a carbon source, a strain expressing AHAS II only is limited in growth because of the low enzyme activity in the presence of elevated levels of the inhibitor glyoxylate. A strain with AHAS I only is limited during growth on glucose by the low tendency of this enzyme to utilize 2-ketobutyrate as a substrate; isoleucine limitation then leads to elevated threonine deaminase activity and an increased 2-ketobutyrate/2-ketoisovalerate ratio, which in turn interferes with the synthesis of coenzyme A and methionine. The regulation of threonine deaminase is also crucial in this regard. It is conceivable that, because of fundamental limitations on the specificity of enzymes, no single AHAS could possibly be adequate for the varied conditions that enterobacteria successfully encounter.

* Corresponding author. Mailing address: Department of Life Sciences, Ben-Gurion University of the Negev, P.O.B. 653, Beer-Sheva 84105, Israel. Phone: 972-7-646 1713. Fax: 972-7-647 2890. E-mail: barakz{at}bgumail.bgu.ac.il.

dagger Present address: Agricultural Products Department, E. I. du Pont de Nemours and Company, Wilmington, DE 19880.

Journal of Bacteriology, August 1998, p. 4056-4067, Vol. 180, No. 16
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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