Purification and properties of a diacetyl reductase from Escherichia coli

Functional analysis of BPSS2242 reveals its detoxification role in Burkholderia pseudomallei under salt stress

A bpss2242 gene, encoding a putative short-chain dehydrogenase/oxidoreductase (SDR) in Burkholderia pseudomallei, was identified and its expression was up-regulated by ten-fold when B. pseudomallei was cultured under high salt concentration. Previous study suggested that BPSS2242 plays important roles in adaptation to salt stress and pathogenesis; however, its biological functions are still unknown. Herein, we report the biochemical properties and functional characterization of BPSS2242 from B. pseudomallei. BPSS2242 exhibited NADPH-dependent reductase activity toward diacetyl and methylglyoxal, toxic electrophilic dicarbonyls. The conserved catalytic triad was identified and found to play critical roles in catalysis and cofactor binding. Tyr162 and Lys166 are involved in NADPH binding and mutation of Lys166 causes a conformational change, altering protein structure. Overexpression of BPSS2242 in Escherichia coli increased bacterial survival upon exposure to diacetyl and methylglyoxal. Importantly, the viability of B. pseudomallei encountered dicarbonyl toxicity was enhanced when cultured under high salt concentration as a result of BPSS2242 overexpression. This is the first study demonstrating that BPSS2242 is responsible for detoxification of toxic metabolites, constituting a protective system against reactive carbonyl compounds in B. pseudomallei..

Activation of alternative metabolic pathways diverts carbon flux away from isobutanol formation in an engineered Escherichia coli strain

OBJECTIVE

Metabolic engineering efforts are guided by identifying gene targets for overexpression and/or deletion. Isobutanol, a biofuel candidate, is biosynthesized using the valine biosynthesis pathway and enzymes of the Ehrlich pathway. Most reported studies for isobutanol production in Escherichia coli employ multicopy plasmids, an approach that suffers from disadvantages such as plasmid instability, increased metabolic burden, and use of antibiotics to maintain selection pressure. Cofactor imbalance is another issue that may limit production of isobutanol, as two enzymes of the pathway utilize NADPH as a cofactor.

RESULTS

To address these issues, we constructed E. coli strains with chromosomally-integrated, codon-optimized isobutanol pathway genes (ilvGM, ilvC, kivd, adh) selected on the basis of their cofactor preferences. Genes involved in diverting pyruvate flux toward fermentation byproducts were deleted. Metabolite analyses of the constructed strains revealed extracellular accumulation of significant amounts of isobutyraldehyde, a pathway intermediate, and the overflow metabolites 2,3-butanediol and acetol.

CONCLUSIONS

These results demonstrate that the genetic modifications carried out led to activation of alternative pathways that diverted carbon flux toward formation of unwanted metabolites. The present study highlights how precursor metabolites can be metabolized through enzymatic routes that have not been considered important in previous studies due to the different strategies employed therein. The insights gained from the present study will allow rational genetic modification of host cells for production of metabolites of interest.

Draft Genome Sequences of 12 Dry-Heat-Resistant Bacillus Strains Isolated from the Cleanrooms Where the Viking Spacecraft Were Assembled

Spore-forming microorganisms are of concern for forward contamination because they can survive harsh interplanetary travel. Here, we report the draft genome sequences of 12 spore-forming strains isolated from the Manned Spacecraft Operations Building (MSOB) and the Vehicle Assembly Building (VAB) in Cape Canaveral, FL, where the Viking spacecraft were assembled.

A shortened, two-enzyme pathway for 2,3-butanediol production in Escherichia coli

The platform chemical 2,3-butanediol (2,3-BDO) is produced by a number of microorganisms via a three-enzyme pathway starting from pyruvate. Here, we report production of 2,3-BDO via a shortened, two-enzyme pathway in Escherichia coli. A synthetic operon consisting of the acetolactate synthase (ALS) and acetoin reductase (AR) genes from Enterobacter under control of the T7 promoter was cloned in an episomal plasmid. E. coli transformed with this plasmid produced 2,3-BDO and the pathway intermediate acetoin, demonstrating that the shortened pathway was functional. To assemble a synthetic operon for inducer- and plasmid-free production of 2,3-BDO, ALS and AR genes were integrated in the E. coli genome under control of the constitutive ackA promoter. Shake flask-level cultivation led to accumulation of ~1 g/L acetoin and ~0.66 g/L 2,3-BDO in the medium. The novel biosynthetic route for 2,3-BDO biosynthesis described herein provides a simple and cost-effective approach for production of this important chemical.

Fermentative Pyruvate and Acetyl-Coenzyme A Metabolism

Pyruvate and acetyl-CoA form the backbone of central metabolism. The nonoxidative cleavage of pyruvate to acetyl-CoA and formate by the glycyl radical enzyme pyruvate formate lyase is one of the signature reactions of mixed-acid fermentation in enterobacteria. Under these conditions, formic acid accounts for up to one-third of the carbon derived from glucose. The further metabolism of acetyl-CoA to acetate via acetyl-phosphate catalyzed by phosphotransacetylase and acetate kinase is an exemplar of substrate-level phosphorylation. Acetyl-CoA can also be used as an acceptor of the reducing equivalents generated during glycolysis, whereby ethanol is formed by the polymeric acetaldehyde/alcohol dehydrogenase (AdhE) enzyme. The metabolism of acetyl-CoA via either the acetate or the ethanol branches is governed by the cellular demand for ATP and the necessity to reoxidize NADH. Consequently, in the absence of an electron acceptor mutants lacking either branch of acetyl-CoA metabolism fail to cleave pyruvate, despite the presence of PFL, and instead reduce it to D-lactate by the D-lactate dehydrogenase. The conversion of PFL to the active, radical-bearing species is controlled by a radical-SAM enzyme, PFL-activase. All of these reactions are regulated in response to the prevalent cellular NADH:NAD+ ratio. In contrast to Escherichia coli and Salmonella species, some genera of enterobacteria, e.g., Klebsiella and Enterobacter, produce the more neutral product 2,3-butanediol and considerable amounts of CO2 as fermentation products. In these bacteria, two molecules of pyruvate are converted to α-acetolactate (AL) by α-acetolactate synthase (ALS). AL is then decarboxylated and subsequently reduced to the product 2,3-butandiol.

Induction of some enzymes of citrate metabolism inLeuconostoc lactisand other heterofermentative lactic acid bacteria

Citrate stimulated growth, totally induced citrate lyase, partly induced acetolactate synthase activity and partly repressed both diacetyl and acetoin reductases inLeuconostoc lactisNCW1. Similar results were obtained with 2 other leuconostocs and a heterofermentative lactobacillus. In 2 of the 3 leuconostocs tested, diacetyl reductase and acetoin reductase were NADPH specific, while in the 2 heterofermentative lactobacilli, they were NADH specific.

Acetoin metabolism in bacteria

Acetoin is an important physiological metabolite excreted by many microorganisms. The excretion of acetoin, which can be diagnosed by the Voges Proskauer test and serves as a microbial classification marker, has its vital physiological meanings to these microbes mainly including avoiding acification, participating in the regulation of NAD/NADH ratio, and storaging carbon. The well-known anabolism of acetoin involves alpha-acetolactat synthase and alpha-acetolactate decarboxylase; yet its catabolism still contains some differing views, although much attention has been focused on it and great advances have been achieved. Current findings in catabolite control protein A (CcpA) mediated carbon catabolite repression may provide a fuller understanding of the control mechanism in bacteria. In this review, we first examine the acetoin synthesis pathways and its physiological meanings and relevancies; then we discuss the relationship between the two conflicting acetoin cleavage pathways, the enzymes of the acetoin dehydrogenase enzyme system, major genes involved in acetoin degradation, and the CcpA mediated acetoin catabolite repression pathway; in the end we discuss the genetic engineering progresses concerning applications. To date, this is the first integrated review on acetoin metabolism in bacteria, especially with regard to catabolic aspects. The apperception of the generation and dissimilation of acetoin in bacteria will help provide a better understanding of microbial strategies in the struggle for resources, which will consequently better serve the utilization of these microbes.

Plasmid-encoded diacetyl (acetoin) reductase in Leuconostoc pseudomesenteroides

A plasmid-borne diacetyl (acetoin) reductase (butA) from Leuconostoc pseudomesenteroides CHCC2114 was sequenced and cloned. Nucleotide sequence analysis revealed an open reading frame encoding a protein of 257 amino acids which had high identity at the amino acid level to diacetyl (acetoin) reductases reported previously. Downstream of the butA gene of L. pseudomesenteroides, but coding in the opposite orientation, a putative DNA recombinase was identified. A two-step PCR approach was used to construct FPR02, a butA mutant of the wild-type strain, CHCC2114. FPR02 had significantly reduced diacetyl (acetoin) reductase activity with NADH as coenzyme, but not with NADPH as coenzyme, suggesting the presence of another diacetyl (acetoin)-reducing activity in L. pseudomesenteroides. Plasmid-curing experiments demonstrated that the butA gene is carried on a 20-kb plasmid in L. pseudomesenteroides.

Stoichiometric growth model for riboflavin-producing Bacillus subtilis

Rate equations for measured extracellular rates and macromolecular composition data were combined with a stoichiometric model to describe riboflavin production with an industrial Bacillus subtilis strain using errors in variables regression analysis. On the basis of this combined stoichiometric growth model, we explored the topological features of the B. subtilis metabolic reaction network that was assembled from a large amount of literature. More specifically, we simulated maximum theoretical yields of biomass and riboflavin, including the associated flux regimes. Based on the developed model, the importance of experimental data on building block requirements for maximum yield and flux calculations were investigated. These analyses clearly show that verification of macromolecular composition data is important for optimum flux calculations.

Purification, characterization and some properties of diacetyl(acetoin) reductase from Enterobacter aerogenes

A new method, faster, milder and more efficient than the one previously described [Bryn, K., Hetland, O. & Stormer, F. C. (1971) Eur. J. Biochem, 18, 116-119], for purification of diacetyl(acetoin) reductase from Enterobacter aerogenes is proposed. The experiments carried out with the electrophoretically pure preparations obtained by this procedure show that the enzyme (a) produces L-glycols from the corresponding L-alpha-hydroxycarbonyls by reversible reduction of their oxo groups and also reduces the oxo group of uncharged alpha-dicarbonyls converting them into L-alpha-hydroxycarbonyls, and (b) is specific for NAD. This is a new enzyme for which we suggest the systematic name of L-glycol: NAD+ oxidoreductase and the recommended name of L-glycol dehydrogenase(NAD). The molecular mass, pI, affinity for substrates and pH profiles of this enzyme are also described.

Purification and properties of two oxidoreductases catalyzing the enantioselective reduction of diacetyl and other diketones from baker's yeast

The NADPH-linked diacetyl reductase system from the cytosolic fraction of Saccharomyces cerevisiae has been resolved into two oxidoreductases catalyzing irreversibly the enantioselective reduction of diacetyl (2,3-butanedione) to (S)- and (R)-acetoin (3-hydroxy-2-butanone) [so-called (S)- and (R)-diacetyl reductases] (EC 1.1.1.5) which have been isolated to apparent electrophoretical purity. The clean-up procedures comprising streptomycin sulfate treatment, Sephadex G-25 filtration, DEAE-Sepharose CL-6B column chromatography, affinity chromatography on Matrex Gel Red A and Superose 6 prep grade filtration led to 120-fold and 368-fold purifications, respectively. The relative molecular mass of the (R)-diacetyl reductase, estimated by means of HPLC filtration on Zorbax GF 250 and sodium dodecyl sulfate/polyacrylamide gel electrophoresis, was 36,000. The (R)-enzyme was most active at pH 6.4 and accepted in addition to diacetyl C5-, C6-2,3-diketones, 1,2-cyclohexanedione, 2-oxo aldehydes and short-chain 2- and 3-oxo esters as substrates. The enzyme was characterized by high enantioselectivity and regiospecificity. The Km values for diacetyl and 2,3-pentanedione were determined as 2.0 mM. The Mr of the (S)-diacetyl reductase was determined as 75,000 by means of HPLC filtration of Zorbax GF 250. The enzyme decomposed into subunits of Mr 48,000 and 24,000 on sodium dodecyl sulfate/polyacrylamide gel electrophoresis. The optimum pH was 6.9. The purified (S)-enzyme reduced stereospecifically a broad spectrum of substrates, comprising 2,3-, 2,4- and 2,5-diketones, 2-oxo aldehydes, 1,2-cyclohexanedione and methyl ketones as well as 3-, 4- and 5-oxo esters. The 2,3- and 2,4-diketones are transformed to the corresponding (S)-2-hydroxy ketones; 2,5-hexanedione, however, was reduced to (S,S)-2,5-hexanediol. The Km values for diacetyl and 2,3-pentanedione were estimated as 2.3 and 1.5 mM, respectively. Further characterization of the (S)-diacetyl reductase revealed that it is identical with the so-called '(S)-enzyme', involved in the enantioselective reduction of 3-, 4- and 5-oxo esters in baker's yeast.

Kinetics of the diacetyl and 2,3-pentanedione reduction by diacetyl reductase (alpha-diketone reductase (NAD)) from Staphylococcus aureus

The kinetic mechanism of diacetyl and 2,3-pentanedione reduction by diacetyl reductase from Staphylococcus aureus was investigated. The shape of the primary double reciprocal plots, the product inhibition pattern, and the features of the inhibition by a substrate analogue (acetone) show that diacetyl is reduced via an Ordered Bi-Bi mechanism, and 2,3-pentanedione by an Ordered Bi-Bi or Theorell-Chance mechanism. NADH is the leading substrate in both reactions. Affinity constants for the coenzyme and the substrates and inhibition constants for NAD, acetoin, and acetone were also calculated. This enzyme has a high affinity for NADH; Km (31-50 microM) and Ks (20-27 microM) for this compound are around one-tenth of the NADH intracellular concentration. Therefore, it must operate in vivo saturated with the coenzyme. This condition is not adequate to play the role, formerly proposed for diacetyl reductases, of regulating the equilibrium between oxidized and reduced forms of pyridine-nucleotides.

A novel fungal enzyme, NADPH-dependent carbonyl reductase, showing high specificity to conjugated polyketones. Purification and characterization

A novel enzyme which specifically catalyzes the reduction of conjugated polyketones was purified to homogeneity from cells of Mucor ambiguus AKU 3006. The enzyme has a strict requirement for NADPH and irreversibly reduces a number of quinones such as p-benzoquinone, alpha-naphthoquinone and acenaphthenequione. The enzyme also reduces polyketones such as isatin and ketopantoyl lactone, and their derivatives. The apparent Km values for isatin and ketopantoyl lactone are 49.9 microM and 714 microM, respectively. The reduction of ketopantoyl lactone proceeds stereospecifically to yield L-(+)-pantoyl lactone. The pro-S (A) hydrogen at C-4 of NADPH is transferred to the substrate. The enzyme is not a flavoprotein and consists of two polypeptide chains with an identical relative molecular mass of 27,500. Quercetin, dicoumarol and some SH reagents inhibit the enzyme activity. 3-Methyl-1,2-cyclopentanedione and 1,3-cyclohexanedione are uncompetitive inhibitors with Ki values of 80.9 microM and 64.5 microM, respectively, to ketopantoyl lactone.

Purification and characterization of diacetyl-reducing enzymes from Staphylococcus aureus

A method was developed to purify diacetyl-reducing enzymes from Staphylococcus aureus. Two enzymes capable of catalysing diacetyl reduction were isolated, neither of which turned out to be a specific diacetyl reductase. One of them is a lactate dehydrogenase similar to the one from Staphylococcus epidermidis, which accepts diacetyl, although poorly. The other one uses as coenzyme beta-NAD and reduces uncharged alpha-dicarbonyls with more than three carbon atoms (especially the alpha-diketones diacetyl and pentane-2,3-dione), producing the L(+) form of the corresponding alpha-hydroxycarbonyls. This enzyme has an Mr of 68,000 and is, most probably, a monomer. Its optimum pH is 6.0. Its shows a high affinity for NADH and a rather low one for diacetyl, which, at least in vitro, does not seem to be as good a substrate as pentane-2,3-dione. We propose for it the systematic name L-alpha-hydroxyketone:NAD+ oxidoreductase and the recommended name of alpha-diketone reductase (NAD). We also suggest that the diacetyl reductase entry in the I.U.B. classification be suppressed.

Kinetics and thermodynamics of diacetyl reduction with NADPH by alpha-dicarbonyl reductase from pigeon liver

alpha-dicarbonyl reductase from pigeon liver catalyzes diacetyl reduction with NADPH via an ordered Bi-Bi mechanism in which the coenzyme is the leading substrate, as deduced from the inhibition pattern by products and by acetone. The activation energy of the reaction has been calculated as 16.6 kcal/mol, delta H and delta F as 15.6 and 15.3 kcal/mol, respectively, and delta S as 1 cal/mol per k. Kinetic constants obtained for substrates (KmNADPH = 15 microM, KsNADPH = 10 microM; Kmdiacetyl = 0.5 mM, Ksdiacetyl = 0.35 mM) and products (KiNADP 50 microM; Kiacetoin = 100 mM) are about 10 times lower than those reported for this enzyme in the reduction of diacetyl with NADH. This confirms that NADPH is its physiological coenzyme.

A multifunctional fermentative alcohol dehydrogenase from the strict aerobe Alcaligenes eutrophus: purification and properties

A NAD (P)-linked alcohol dehydrogenase was isolated from the soluble extract of the strictly respiratory bacterium Alcaligenes eutrophus N9A. Derepression of the formation of this enzyme occurs only in cells incubated under conditions of restricted oxygen supply for prolonged times. The purification procedure included precipitation by cetyltrimethylammonium bromide and ammonium sulfate and subsequent chromatography on DEAE-Sephacel, Cibacron blue F3G-A Sepharose and thiol-Sepharose. The procedure resulted in a 120-fold purification of a multifunctional alcohol dehydrogenase exhibiting dehydrogenase activities for 2,3-butanediol, ethanol and acetaldehyde and reductase activities for diacetyl, acetoin and acetaldehyde. During purification the ratio between 2,3-butanediol dehydrogenase and ethanol dehydrogenase activity remained nearly constant. Recovering about 20% of the initial 2,3-butanediol dehydrogenase activity, the specific activity of the final preparation was 70.0 U X mg protein-1 (2,3-butanediol oxidation) and 2.8 U X mg protein-1 (ethanol oxidation). The alcohol dehydrogenase is a tetramer of a relative molecular mass of 156000 consisting of four equal subunits. The determination of the Km values for different substrates and coenzymes as well as the determination of the pH optima for the reactions catalyzed resulted in values which were in good agreement with the fermentative function of this enzyme. The alcohol dehydrogenase catalyzed the NAD (P)-dependent dismutation of acetaldehyde to acetate and ethanol. This reaction was studied in detail, and its possible involvement in acetate formation is discussed. Among various compounds tested for affecting enzyme activity only NAD, NADP, AMP, ADP, acetate and 2-mercaptoethanol exhibited significant effects.

Further purification and characterization of diacetyl reducing enzymes from beef liver

Two diacetyl reducing enzymes have been isolated from beef liver. One of them, a monomer of mol. wt 28-30,000 dalton and pI 6.2, corresponds to the low molecular weight diacetyl reductase formerly accounted for using preparations of this organ; it has been now identified as an L-glycol dehydrogenase. The other one, an oligomer of 78,000 dalton and pI 7.0, which matches the high molecular weight diacetyl reductase, is, in the authors' opinion, a new enzyme for which the systematic name L(+)-alpha-hydroxycarbonyl: NAD(P) oxidoreductase (EC 1.1.1...) and common name alpha-dicarbonyl reductase are proposed.

Further purification and characterization of diacetyl reductase from pigeon liver

Electrophoretically pure preparation of an enzyme from pigeon liver which is classified in the I.U.B. Enzyme List as a specific diacetyl reductase (Acetoin: NAD oxidoreductase, EC 1.1.1.5) have been obtained. This enzyme has been characterized as an alpha-dicarbonyl reductase (L(+)-alpha-hydroxicarbonyl: NAD(P) oxidoreductase, EC 1.1.1...) similar to the one recently discovered from beef liver by the authors' group. Although differences in mol wt among these two reductases had been reported, both of them are oligomers of 25,000-26,000 dalton subunits.

Kinetics of alpha-dicarbonyls reduction by L-glycol dehydrogenase from hen muscle

Michaelis constants of L-glycol dehydrogenase from hen muscle (isozyme of pI 7.2) for the alpha-dicarbonyls tested (glyoxal, 2,3-pentanedione, methylglyoxal, and diacetyl) range from 35 microM for pentanedione to 0.41 mM for glyoxal. The enzyme shows a high affinity for NADPH, Km (2.2-3.1 microM), and Ks (1.2-1.9 microM) being so much lower than its tissue concentration that L-glycol dehydrogenase has to operate in vivo saturated with the coenzyme; this condition is very unfavorable to play a role in regulating the equilibrium oxidized/reduced forms of the pyridine nucleotides, as it has been proposed for some similar enzymes. Convergence of the double reciprocal plots and the pattern of inhibition by products and by acetone, a substrate analog, demonstrate that glyoxal reduction--and most likely that of diacetyl--proceeds via an ordered Bi-Bi mechanism in which NADPH is fixed before the addition of the carbonyl; the reduction of methylglyoxal and 2,3-pentanedione could follow the same model, but our experimental results are also consistent with that of Theorell-Chance.

Specificity of the Westerfeld adaptation of the Voges-Proskauer test

Aliphatic chain compounds at least four carbons long with vicinal carbonyl groups in the 2,3 positions were detected by the Westerfeld test. Acetoin, which has one carbonyl group and an adjacent hydroxyl group, gave positive results, but methyl action (3-hydroxy-3-methyl-2-butanone) was negative, and subsequent tests supported the conclusion that acetoin is oxidized to diacetyl by alpha-naphthol during the Westerfeld test in the absence or presence of air. 2,3-Pentanedione and 2,3-heptanedione gave positive results, but equimolar concentrations of these compounds gave maximal absorbancy readings that were only 35% (2,3-pentanedione) and 31% (2,3-heptanedione) of those obtained with diacetyl or acetoin. Negative results were obtained with pyruvic acid, 2,3-butylene glycol, and carbon ring compounds (1,2-cyclohexanedione, alloxan, and 3,4-dihydroxy-3-cyclobutene-1,2-dione). alpha-Naphtho could not be replaced in the test by beta-naphthol, 1,2,3,4,-tetrahydroxy-1-naphthol, or 5,6,7,8-tetrahydroxy-1-naphthol. Creatine could not be replaced by arginine, guanidine . HCl, or guanidinoacetic acid.

Purification and some properties of L-glycol dehydrogenase from hen's muscle

1. An enzyme which catalyzes the NAD(P)H-linked reversible reduction of uncharged vicinal dicarbonyls and alpha-hydroxycarbonyls to L-(+)-glycols has been purified from hen's muscle. This enzyme has not been previously described. 2. According to the rules of the I.U.P.A.C.-I.U.B. Enzymes Commission, the systematic name of L-(+)-glycol:NAD(P) oxidoreductase and the trivial name of L-glycol dehydrogenase are proposed for the enzyme. 3. Three forms of this enzyme differing in pI have been isolated; two forms, which together represent about 90% of total recovered activity, and electrophoretically pure. 4. Molecular weight, pH profiles and affinity for substrates are also described.

Physiological and biochemical role of the butanediol pathway in Aerobacter (Enterobacter) aerogenes

Aerobacter (Enterobacter) aerogenes wild type and three mutants deficient in the formation of acetoin and 2,3-butanediol were grown in a glucose minimal medium. Culture densities, pH, and diacetyl, acetoin, and 2,3-butanediol levels were recorded. The pH in wild-type cultures dropped from 7.0 to 5.8, remained constant while acetoin and 2,3-butanediol were formed, and increased to pH 6.5 after exhaustion of the carbon source. More 2,3-butanediol than acetoin was formed initially, but after glucose exhaustion reoxidation to acetoin occurred. The three mutants differed from the wild type in yielding acid cultures (pH below 4.5). The wild type and one of the mutants were grown exponentially under aerobic and anaerobic conditions with the pH fixed at 7.0, 5.8, and 5.0, respectively. Growth rates decreased with decreasing pH values. Aerobically, this effect was weak, and the two strains were affected to the same degree. Under anaerobic conditions, the growth rates were markedly inhibited at a low pH, and the mutant was slightly more affected than the wild type. Levels of alcohol dehydrogenase were low under all conditions, indicating that the enzyme plays no role during exponential growth. The levels of diacetyl (acetoin) reductase, lactate dehydrogenase, and phosphotransacetylase were independent of the pH during aerobic growth of the two strains. Under anaerobic conditions, the formation of diacetyl (acetoin) reductase was pH dependent, with much higher levels of the enzyme at pH 5.0 than at pH 7.0. Lactate dehydrogenase and phosphotransacetylase revealed the same pattern of pH-dependent formation in the mutant, but not in the wild type.