Regulation of expression of the ilvB operon in Salmonella typhimurium

Biosynthesis and Regulation of the Branched-Chain Amino Acids†

This review focuses on more recent studies concerning the systems biology of branched-chain amino acid biosynthesis, that is, the pathway-specific and global metabolic and genetic regulatory networks that enable the cell to adjust branched-chain amino acid synthesis rates to changing nutritional and environmental conditions. It begins with an overview of the enzymatic steps and metabolic regulatory mechanisms of the pathways and descriptions of the genetic regulatory mechanisms of the individual operons of the isoleucine-leucine-valine (ilv) regulon. This is followed by more-detailed discussions of recent evidence that global control mechanisms that coordinate the expression of the operons of this regulon with one another and the growth conditions of the cell are mediated by changes in DNA supercoiling that occur in response to changes in cellular energy charge levels that, in turn, are modulated by nutrient and environmental signals. Since the parallel pathways for isoleucine and valine biosynthesis are catalyzed by a single set of enzymes, and because the AHAS-catalyzed reaction is the first step specific for valine biosynthesis but the second step of isoleucine biosynthesis, valine inhibition of a single enzyme for this enzymatic step might compromise the cell for isoleucine or result in the accumulation of toxic intermediates. The operon-specific regulatory mechanisms of the operons of the ilv regulon are discussed in the review followed by a consideration and brief review of global regulatory proteins such as integration host factor (IHF), Lrp, and CAP (CRP) that affect the expression of these operons.

Branched-chain amino acid biosynthesis in Salmonella typhimurium: a quantitative analysis

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.

Specific truncations of an acetolactate synthase gene from Brassica napus efficiently complement ilvB/ilvG mutants of Salmonella typhimurium

The expression of an acetolactate synthase (ALS) gene isolated from the cruciferous plant Brassica napus was investigated in Salmonella typhimurium. Using an expression plasmid containing the highly active trc (trp-lac) promoter, several plant ALS constructs were made containing successive in-frame truncations from the 5' end of the coding region. Functional complementation by these plant ALS constructs of a S. typhimurium mutant devoid of ALS enzymic activity was assayed on minimal medium. Truncations which eliminated a large portion of the transit peptide coding sequence proved to act as efficient ALS genes in the bacterial host. Truncations close to the putative processing site of the plant protein were inactive in the complementation test. A full length copy of the gene, including the entire transit peptide coding region, was also inactive. The efficiency of the complementation, estimated by comparison to the growth rate of wild-type S. typhimurium, was found to correlate with levels of ALS activity in the transformed bacteria. Specific mutations, known to produce herbicide resistance in plants, were introduced into the truncated ALS coding sequence by site-directed mutagenesis. When expressed in bacteria these constructs conferred a herbicide resistance phenotype on the host. The potential of this system for mutagenesis and enzymological studies of plant proteins is discussed.

Physiological implications of the specificity of acetohydroxy acid synthase isozymes of enteric bacteria

The rates of formation of the two alternative products of acetohydroxy acid synthase (AHAS) have been determined by a new analytical method (N. Gollop, Z. Barak, and D. M. Chipman, Anal. Biochem., 160:323-331, 1987). For each of the three distinct isozymes of AHAS in Escherichia coli and Salmonella typhimurium, a specificity ratio, R, was defined: Formula: see text, which is constant over a wide range of substrate concentrations. This is consistent with competition between pyruvate and 2-ketobutyrate for an active acetaldehyde intermediate formed irreversibly after addition of the first pyruvate moiety to the enzyme. Isozyme I showed no product preference (R = 1), whereas isozymes II and III form acetohydroxybutyrate (AHB) at approximately 180- and 60-fold faster rates, respectively, than acetolactate (AL) at equal pyruvate and 2-ketobutyrate concentrations. R values higher than 60 represent remarkably high specificity in favor of the substrate with one extra methylene group. In exponentially growing E. coli cells (under aerobic growth on glucose), which contain about 300 microM pyruvate and only 3 microM 2-ketobutyrate, AHAS I would produce almost entirely AL and only 1 to 2% AHB. However, isozymes II and III would synthesize AHB (on the pathway to Ile) and AL (on the pathway to valine-leucine) in essentially the ratio required for protein synthesis. The specificity ratio R of any AHAS isozyme was affected neither by the natural feedback inhibitors (Val, Ile) nor by the pH. On the basis of the specificities of the isozymes, the known regulation of AHAS I expression by the catabolite repression system, and the reported behavior of bacterial mutants containing single AHAS isozymes, we suggest that AHAS I enables a bacterium to cope with poor carbon sources, which lead to low endogenous pyruvate concentrations. Although AHAS II and III are well suited to producing the branched-chain amino acid precursors during growth on glucose, they would fail to provide appropriate quantities of AL when the concentration of pyruvate is relatively low.

Acetohydroxy acid synthase I is required for isoleucine and valine biosynthesis by Salmonella typhimurium LT2 during growth on acetate or long-chain fatty acids

Salmonella typhimurium LT2 normally expresses two acetohydroxy acid synthases (AHAS I and AHAS II). The function of AHAS I in this organism was unclear, since AHAS I-deficient (ilvBN) mutants of LT2 grew well on glucose or succinate minimal media, whereas AHAS II-deficient (ilvGM) mutants requried isoleucine for normal growth on glucose minimal media. We report that AHAS I-deficient mutants of S. typhimurium required isoleucine and valine for growth on acetate or oleate minimal media, whereas AHAS II-deficient mutants were able to grow on these media without isoleucine supplementation.

The ilvB locus of Escherichia coli K-12 is an operon encoding both subunits of acetohydroxyacid synthase I

The ilvB locus of Escherichia coli K-12 encloses two open reading frames defining polypeptides of 60,000 and 11,200 molecular weight. The entire locus, about 2.3 kb, is co-transcribed as an operon. The molecular weights and amino acid compositions of the presumptive operon polypeptides agree with those of the large and small subunit polypeptides of acetohydroxyacid synthase (AHAS) I, for which ilvB is the structural locus. We reserve the designation ilvB for the promoter proximal (longer) cistron and designate the promoter distal cistron ilvN. The molecular weight and amino acid sequence of the ilvB polypeptide are strikingly similar to those of the I1vI (larger subunit of AHAS III) and I1vG (larger subunit of AHAS II) polypeptides. There is less size uniformity among the I1vN, I1vH (smaller subunit of AHAS III), and I1vM (smaller subunit of AHAS II) polypeptides. Nevertheless, there is significant amino acid sequence homology among the three small subunit polypeptides. Thus, all three AHAS isozymes of E. coli K-12 probably have a common evolutionary origin.