Improving the Thermostability and Catalytic Efficiency of Bacillus deramificans Pullulanase by Site-Directed Mutagenesis

Pullulanase (EC 3.2.1.41) is a well-known starch-debranching enzyme. Its instability and low catalytic efficiency are the major factors preventing its widespread application. To address these issues, Asp437 and Asp503 of the pullulanase from Bacillus deramificans were selected in this study as targets for site-directed mutagenesis based on a structure-guided consensus approach. Four mutants (carrying the mutations D503F, D437H, D503Y, and D437H/D503Y) were generated and characterized in detail. The results showed that the D503F, D437H, and D503Y mutants had an optimum temperature of 55°C and a pH optimum of 4.5, similar to that of the wild-type enzyme. However, the half-lives of the mutants at 60°C were twice as long as that of the wild-type enzyme. In addition, the D437H/D503Y double mutant displayed a larger shift in thermostability, with an optimal temperature of 60°C and a half-life at 60°C of more than 4.3-fold that of the wild-type enzyme. Kinetic studies showed that the K_m values for the D503F, D437H, D503Y, and D437H/D503Y mutants decreased by 7.1%, 11.4%, 41.4%, and 45.7% and the K_cat/K_m values increased by 10%, 20%, 140%, and 100%, respectively, compared to those of the wild-type enzyme. Mechanisms that could account for these enhancements were explored. Moreover, in conjunction with the enzyme glucoamylase, the D503Y and D437H/D503Y mutants exhibited an improved reaction rate and glucose yield during starch hydrolysis compared to those of the wild-type enzyme, confirming the enhanced properties of the mutants. The mutants generated in this study have potential applications in the starch industry.     

Enhancing the Thermostability of Serratia plymuthica Sucrose Isomerase Using B-Factor-Directed Mutagenesis

The sucrose isomerase of Serratia plymuthica AS9 (AS9 PalI) was expressed in Escherichia coli BL21(DE3) and characterized. The half-life of AS9 PalI was 20 min at 45°C, indicating that it was unstable. In order to improve its thermostability, six amino acid residues with higher B-factors were selected as targets for site-directed mutagenesis, and six mutants (E175N, K576D, K174D, G176D, S575D and N577K) were designed using the RosettaDesign server. The E175N and K576D mutants exhibited improved thermostability in preliminary experiments, so the double mutant E175N/K576D was constructed. These three mutants (E175N, K576D, E175N/K576D) were characterized in detail. The results indicate that the three mutants exhibit a slightly increased optimal temperature (35°C), compared with that of the wild-type enzyme (30°C). The mutants also share an identical pH optimum of 6.0, which is similar to that of the wild-type enzyme. The half-lives of the E175N, K576D and E175N/K576D mutants were 2.30, 1.78 and 7.65 times greater than that of the wild-type enzyme at 45°C, respectively. Kinetic studies showed that the K_m values for the E175N, K576D and E175N/K576D mutants decreased by 6.6%, 2.0% and 11.0%, respectively, and their k_cat/K_m values increased by 38.2%, 4.2% and 19.4%, respectively, compared with those of the wild-type enzyme. After optimizing the conditions for isomaltulose production at 45°C, we found that the E175N, K576D and E175N/K576D mutants displayed slightly improved isomaltulose yields, compared with the wild-type enzyme. Therefore, the mutants produced in this study would be more suitable for industrial biosynthesis of isomaltulose.     

Mutant analysis of glyceraldehyde 3-phosphate dehydrogenase in Escherichia coli

NAD⁺-specific glyceraldehyde 3-phosphate dehydrogenase (EC 1.2.1.12) from Escherichia coli was purified to homogeneity by a relatively simple procedure involving affinity chromatography on agarose–hexane–NAD⁺ and repeated crystallization. Rabbit antiserum directed against this protein produced one precipitin line in double-diffusion studies against the pure enzyme, and two lines against crude extracts of wild-type E. coli strains. Both precipitin lines represent the interaction of antibody with determinants specific for glyceraldehyde 3-phosphate dehydrogenase. Nine independent mutants of E. coli lacking glyceraldehyde 3-phosphate dehydrogenase activity all possessed some antigenic cross-reacting material to the wild-type enzyme. The mutants could be divided into three groups on the basis of the types and amounts of precipitin lines observed in double-diffusion experiments; one group formed little cross-reacting material. The cross-reacting material in crude cell-free extracts of several of the mutant strains were also tested for alterations in their affinity for NAD⁺ and their phosphorylative activity. The cumulative data indicate that the protein in several of the mutant strains is severely altered, and thus that glyceraldehyde 3-phosphate dehydrogenase is unlikely to have an essential, non-catalytic function such as buffering nicotinamide nucleotide or glycolytic-intermediate concentrations. Others of the mutants tested have cross-reacting material which behaved like the wild-type enzyme for the several parameters studied; the proteins from these strains, once purified, might serve as useful analogues of the wild-type enzyme.     

Nitrate Reductase Regulates Expression of Nitrite Uptake and Nitrite Reductase Activities in Chlamydomonas reinhardtii

In Chlamydomonas reinhardtii mutants defective at the structural locus for nitrate reductase (nit-1) or at loci for biosynthesis of the molybdopterin cofactor (nit-3, nit-4, or nit-5 and nit-6), both nitrite uptake and nitrite reductase activities were repressed in ammonium-grown cells and expressed at high amounts in nitrogen-free media or in media containing nitrate or nitrite. In contrast, wild-type cells required nitrate induction for expression of high levels of both activities. In mutants defective at the regulatory locus for nitrate reductase (nit-2), very low levels of nitrite uptake and nitrite reductase activities were expressed even in the presence of nitrate or nitrite. Both restoration of nitrate reductase activity in mutants defective at nit-1, nit-3, and nit-4 by isolating diploid strains among them and transformation of a structural mutant upon integration of the wild-type nit-1 gene gave rise to the wild-type expression pattern for nitrite uptake and nitrite reductase activities. Conversely, inactivation of nitrate reductase by tungstate treatment in nitrate, nitrite, or nitrogen-free media made wild-type cells respond like nitrate reductase-deficient mutants with respect to the expression of nitrite uptake and nitrite reductase activities. Our results indicate that nit-2 is a regulatory locus for both the nitrite uptake system and nitrite reductase, and that the nitrate reductase enzyme plays an important role in the regulation of the expression of both enzyme activities.     

Site-directed mutants of staphylococcal nuclease. Detection and localization by 1H NMR spectroscopy of conformational changes accompanying substitutions for glutamic acid-43

The high-resolution X-ray crystal structure of staphylococcal nuclease suggests that the gamma-carboxylate group of Glu-43 is directly involved in catalysis as a general base that facilitates the attack of water on the substrate phosphodiester. We have used primer-directed, site-specific mutagenesis to generate aspartate, glutamine, asparagine, alanine, and serine substitutions for this residue. The Vmax/Km for the aspartate mutant is reduced 1400-fold and the values for the charge-neutral mutations are reduced 5000-fold relative to the wild-type enzyme. Although these reductions in catalytic efficiency might appear useful in quantitatively estimating the importance of general basic catalysis in the reaction catalyzed by the wild-type enzyme, the thermal stabilities and 1H NMR spectral properties of the mutants suggest that such interpretations are ambiguous. All five mutants have higher melting temperatures for thermal denaturation than the wild-type enzyme, suggesting that the mutants have enhanced thermal stabilities relative to the wild-type enzyme. Chemical shift changes relative to the wild type are observed in both the aromatic and upfield-shifted methyl group regions of the 1H NMR spectra of the aspartate and serine mutants, suggesting the presence of conformational differences between the wild-type and mutant enzymes. That these conformational differences may be large enough to be mechanistically relevant is suggested by comparisons of the magnitudes of nuclear Overhauser effect (NOE) correlations between the aromatic and upfield-shifted methyl group regions observed via two-dimensional nuclear Overhauser effect correlation spectroscopy.(ABSTRACT TRUNCATED AT 250 WORDS)     

The C-terminal domain of Escherichia coli MutY is involved in DNA binding and glycosylase activities

Escherichia coli MutY is an adenine and a weak guanine DNA glycosylase involved in reducing mutagenic effects of 7,8-dihydro-8-oxo-guanine (8-oxoG). The C-terminal domain of MutY is required for 8-oxoG recognition and is critical for mutation avoidance of oxidative damage. To determine which residues of this domain are involved in 8-oxoG recognition, we constructed four MutY mutants based on similarities to MutT, which hydrolyzes specifically 8-oxo-dGTP to 8-oxo-dGMP. F294A-MutY has a slightly reduced binding affinity to A/G mismatch but has a severe defect in A/8-oxoG binding at 20°C. The catalytic activity of F294A-MutY is much weaker than that of the wild-type MutY. The DNA binding activity of R249A-MutY is comparable to that of the wild-type enzyme but the catalytic activity is reduced with both A/G and A/8-oxoG mismatches. The biochemical activities of F261A-MutY are nearly similar to those of the wild-type enzyme. The solubility of P262A-MutY was improved as a fusion protein containing streptococcal protein G (GB1 domain) at its N-terminus. The binding of GB1-P262A-MutY with both A/G and A/8-oxoG mismatches are slightly weaker than those of the wild-type protein. The catalytic activity of GB1-P262A-MutY is weaker than that of the wild-type enzyme at lower enzyme concentrations. Importantly, all four mutants can complement mutY mutants in vivo when expressed at high levels; however, F294A, R249A and P262A, but not F261A, are partially defective in vivo when they are expressed at low levels. These results strongly support that the C-terminal domain of MutY is involved not only in 8-oxoG recognition, but also affects the binding and catalytic activities toward A/G mismatches.     

Evidence for a Second Nitrate Reductase Activity That Is Distinct from the Respiratory Enzyme in Salmonella typhimurium

Significant nitrate reductase activity was detected in mutants of Salmonella typhimurium which mapped at or near chlC and which were incapable of growth with nitrate as electron acceptor. The same mutants were sensitive to chlorate and performed sufficient nitrate reduction to permit anaerobic growth with nitrate as the sole nitrogen source in media containing glucose. The mutant nitrate-reducing protein did not migrate with the wild-type nitrate reductase in polyacrylamide electrophoretic gels. Studies of the electrophoretic mobility in gels of different polyacrylamide concentration revealed that the wild-type and mutant nitrate reductases differed significantly in both size and charge. The second enzyme also differed from the wild-type major enzyme in its response to repression by low pH and its lack of response to repression by glucose. The same mutants were found to be derepressed for nitrite reductase and for a cytochrome with a maximal reduced absorbance at 555 nm at 25°C. This cytochrome was not detected in preparations of the wild type grown under the same conditions. Extracts of these mutants contained normal amounts of the b-type cytochromes which, in the wild type, were associated with nitrate reductase and formate dehydrogenase, respectively, although they could not mediate the oxidation of these cytochromes with nitrate. They were capable of oxidizing the derepressed 555-nm peak cytochrome with nitrate. It is suggested that these mutants synthesize a nitrate-reducing enzyme which is distinct from the chlC gene product and which is repressed in the wild type during anaerobic growth with nitrate.     

Mutants of Escherichia coli with altered deoxyribonucleases : I. Isolation and characterization of mutants for exonuclease III

Escherichia coli mutants deficient in exonuclease III, designated xth, were identified by means of nuclease assays performed on randomly selected, heavily mutagenized clones. The screening of large numbers of colonies was facilitated by newly developed microtechniques. Among the 10,000 clones surveyed, 23 mutants were found which had less than 25% of the parental level of exonuclease III. Seven mutants of independent origin were selected for further study. Although the mutants were isolated on the basis of a defective nuclease activity, they were also deficient in the DNA-3′-phosphatase activity associated with exonuclease III. Four of the mutants had less than 1% of the parental DNA-3′-phosphatase activity at either 25 or 42 °C; the other three strains had thermolabile enzyme activities. An altered exonuclease III, purified 710-fold from one of the latter strains (BW9093), was more thermolabile than the wild-type enzyme, and its exonuclease and DNA-3′-phosphatase activities were inactivated by heat at equal rates. These results further support the assumption that these two activities are due to a single enzyme.     

Simultaneous Enhancement of Thermostability and Catalytic Activity of Phospholipase A1 by Evolutionary Molecular Engineering

The thermal stability and catalytic activity of phospholipase A₁ from Serratia sp. strain MK1 were improved by evolutionary molecular engineering. Two thermostable mutants were isolated after sequential rounds of error-prone PCR performed to introduce random mutations and filter-based screening of the resultant mutant library; we determined that these mutants had six (mutant TA3) and seven (mutant TA13) amino acid substitutions. Different types of substitutions were found in the two mutants, and these substitutions resulted in an increase in nonploar residues (mutant TA3) or in differences between side chains for polar or charged residues (mutant TA13). The wild-type and mutant enzymes were purified, and the effect of temperature on the stability and catalytic activity of the enzymes was investigated. The melting temperatures of the TA3 and TA13 enzymes were increased by 7 and 11°C, respectively, compared with the melting temperature of the wild-type enzyme. Thus, we found that evolutionary molecular engineering was an effective and efficient approach for increasing thermostability without compromising enzyme activity.     

A kinetic and structural comparison of chorismate mutase/prephenate dehydratase from mutant strains of Escherichia coli K12 defective in the PheA gene

Three classes of mutant strains of Escherichia coli K12 defective in pheA, the gene coding for chorismate mutase/prephenate dehydratase, have been isolated: (1) those lacking prephenate dehydratase activity, (2) those lacking chorismate mutase activity, and (3) those lacking both activities. Chorismate mutase/prephenate dehydratase from the second class of mutants was less sensitive to inhibition by phenylalanine than wild-type enzyme and, along with the defective enzyme from the third class of mutants, could not be purified by affinity chromatography on Sepharosyl-phenylalanine. Pure chorismate mutase/prephenate dehydratase protein was prepared from two strains belonging to the first class. The chorismate mutase activity of these enzymes is kinetically similar to that of the wild-type enzyme except for a two- to threefold increase in both the K_a for chorismate and the K_is for inhibition by prephenate. In both cases only one change in the tryptic fingerprint was detected, resulting from a substitution of the threonine residue in the peptide Gln·Asn·Phe·Thr·Arg. This suggests that this residue is catalytically or structurally essential for the dehydratase activity.     

Attachment of the Yeast Rhodosporidium toruloides Is Mediated by Adhesives Localized at Sites of Bud Cell Development

The basidiomycetous yeast Rhodosporidium toruloides (anamorph, Rhodotorula glutinis) is a common phylloplane epiphyte with biocontrol potential. To understand how R. toruloides adheres to plant surfaces, we obtained nonadherent fungal mutants after chemical mutagenesis with methane-sulfonic acid ethyl ester. Sixteen attachment-minus (Att⁻) mutants were identified by three methods: (i) screening capsule-minus colonies for loss of adhesive ability; (ii) enrichment for mutants unable to attach to polystyrene; and (iii) selection for reduced fluorescence of fluorescein isothiocyanate-concanavalin A (Con A)-stained cells by fluorescence-activated cell sorting. None of the 16 mutants attached to polystyrene or barley leaves. The lectin Con A eliminated adhesion in all of the wild-type isolates tested. Hapten competition assays indicated that Con A bound to mannose residues on the cell surface. Adhesion of wild-type R. toruloides was transient; nonadhesive cells subsequently became adhesive, with bud development. All Att⁻ mutants and nonattaching wild-type cells lacked polar regions that stained intensely with fluorescein isothiocyanate-Con A and India ink. Lectin, enzyme, and chemical treatments showed that the polar regions consisted of alkali-soluble materials, including mannose residues. Tunicamycin treatment reduced wild-type adhesion, indicating that the mannose residues could be associated with glycoproteins. We concluded that compounds, including mannose residues, that are localized at sites of bud development mediate adhesion of R. toruloides to both polystyrene and barley leaf surfaces.     

Properties of mutant acetolactate synthases resistant to triazolopyrimidine sulfonanilide

Triazolopyrimidine sulfanilides are a class of highly active herbicides whose primary target is acetolactate synthase. Spontaneous mutants of tobacco (Nicotiana tabacum) (KS-43) and cotton (Gossypium hirsutum) (PS-3 and DO-2) resistant to triazolopyrimidine sulfonanilide were selected in tissue culture. Acetolactate synthase partially purified from the three mutants were 80- to 1000-fold less sensitive to inhibition by the compound compared with the corresponding wild-type enzyme. The mutants also varied in the cross-resistance pattern to other acetolactate synthase inhibiting herbicides in the sulfonylurea, imidazolinone, and pyrimidyl-oxy-benzoate chemical families. Thus, acetolactate synthase from KS-43, PS-3, and DO-2 cultures have different mutations. The affinities for pyruvate, thiamine pyrophosphate, as well as the activity of the mutant enzymes were found to be comparable to the corresponding wild-type enzymes. However, the enzyme from PS-3 was highly resistant to feedback inhibition by valine and leucine. In contrast, acetolactate synthase from KS-43 and DO-2 were inhibited by valine and leucine to nearly the same extent as the wild-type enzymes. Also, PS-3 cultures accumulated much higher levels of the branched chain amino acids compared to the wild-type cotton culture. The mutation in the PS-3 enzyme has therefore rendered it insensitive to feedback regulation by valine and leucine.     

R plasmid gene dosage effects in Escherichia coli K-12: copy mutants of the R plasmic R1drd-19.

Copy mutants of the R plasmid R1drd-19 were used to study gene dosage effects in Escherichia coli K-12. The specific activity of β-lactamase, chloramphenicol acetyltransferase, and streptomycin adenylylase, as well as ampicillin resistance, increased linearly with the gene dosage up to a level at least tenfold higher than that of the wild-type plasmid. This makes it possible to use ampicillin resistance to determine plasmid copy number and also to select for plasmid copy mutants with defined copy number. Chloramphenicol resistance, despite the increase in enzyme activity, reached a plateau level at a gene dosage less than twice that of the wild-type plasmid, presumably due to the high energy demand on the cells during inactivation of the antibiotic by acetylation with acetyl-coenzyme A. Similarly, resistance to streptomycin plateaued at a gene dosage about three times that of the wild-type plasmid, presumably because of a decreased efficiency of the cells' outer penetration barriers when carrying the R plasmid. The susceptibility of the cells to rifampicin was increased by the presence of plasmid copy mutants.     

Biotechnological Production of Caffeic Acid by Bacterial Cytochrome P450 CYP199A2

Caffeic acid is a biologically active molecule that has various beneficial properties, including antioxidant, anticancer, and anti-inflammatory activities. In this study, we explored the catalytic potential of a bacterial cytochrome P450, CYP199A2, for the biotechnological production of caffeic acid. When the CYP199A2 enzyme was reacted with p-coumaric acid, it stoichiometrically produced caffeic acid. The crystal structure of CYP199A2 shows that Phe at position 185 is situated directly above, and only 6.35 Å from, the heme iron. This F185 residue was replaced with hydrophobic or hydroxylated amino acids using site-directed mutagenesis to create mutants with novel and improved catalytic properties. In whole-cell assays with the known substrate of CYP199A2, 2-naphthoic acid, only the wild-type enzyme hydroxylated 2-naphthoic acid at the C-7 and C-8 positions, whereas all of the active F185 mutants exhibited a preference for C-5 hydroxylation. Interestingly, several F185 mutants (F185V, F185L, F185I, F185G, and F185A mutants) also acquired the ability to hydroxylate cinnamic acid, which was not hydroxylated by the wild-type enzyme. These results demonstrate that F185 is an important residue that controls the regioselectivity and the substrate specificity of CYP199A2. Furthermore, Escherichia coli cells expressing the F185L mutant exhibited 5.5 times higher hydroxylation activity for p-coumaric acid than those expressing the wild-type enzyme. By using the F185L whole-cell catalyst, the production of caffeic acid reached 15 mM (2.8 g/liter), which is the highest level so far attained in biotechnological production of this compound.     

Biochemical Characterization of Soybean Mutants Lacking Constitutive NADH:Nitrate Reductase

Two nitrate reductase (NR) mutants were selected for low nitrate reductase (LNR) activity by in vivo NR microassays of M₂ seedlings derived from nitrosomethylurea-mutagenized soybean (Glycine max [L.] Merr. cv Williams) seeds. The mutants (LNR-5 and LNR-6) appeared to have normal nitrate-inducible NR activity. Both mutants, however, showed decreased NR activity in vivo and in vitro compared with the wild-type. In vitro FMNH₂-dependent nitrate reduction and Cyt c reductase activity of nitrate-grown plants, and nitrogenous gas evolution during in vivo NR assays of urea-grown plants, were also decreased in the mutants. The latter observation was due to insufficient generation of nitrite substrate, rather than some inherent difference in enzyme between mutant and wild-type plants. When grown on urea, crude extracts of LNR-5 and LNR-6 lines had similar NADPH:NR activities to that of the wild type, but both mutants had very little NADH:NR activity, relative to the wild type. Blue Sepharose columns loaded with NR extract of urea-grown mutants and sequentially eluted with NADPH and NADH yielded a NADPH:NR peak only, while the wild-type yielded both NADPH: and NADH:NR peaks. Activity profiles confirmed the lack of constitutive NADH:NR in the mutants throughout development. The results provide additional support to our claim that wild-type soybean contains three NR isozymes, namely, constitutive NADPH:NR (c₁NR), constitutive NADH:NR (c₂NR), and nitrate-inducible NR (iNR).     

Free-Energy Perturbation Simulation on Transition States and Redesign of Butyrylcholinesterase

It is recognized that an ideal anti-cocaine treatment is to accelerate cocaine metabolism by producing biologically inactive metabolites via a route similar to the primary cocaine-metabolizing pathway, i.e., butyrylcholinesterase (BChE)-catalyzed hydrolysis of cocaine. BChE mutants with a higher catalytic activity against (-)-cocaine are highly desired for use as an exogenous enzyme in humans. To develop a rational design for high-activity mutants, we carried out free-energy perturbation (FEP) simulations on various mutations of the transition-state structures in addition to the corresponding free-enzyme structures by using an extended FEP procedure. The FEP simulations on the mutations of both the free-enzyme and transition-state structures allowed us to calculate the mutation-caused shift of the free-energy change from the free enzyme (BChE) to the transition state, and thus to theoretically predict the mutation-caused shift of the catalytic efficiency (k_cat/K_M). The computational predictions are supported by the kinetic data obtained from the wet experiments, demonstrating that the FEP-based computational design approach is promising for rational design of high-activity mutants of an enzyme. One of the BChE mutants designed and discovered in this study has an ∼1800-fold improved catalytic efficiency against (-)-cocaine compared to wild-type BChE. The high-activity mutant may be therapeutically valuable.     

The genetics and biochemistry of urease in Ustilago violacea

Two complementing loci in different linkage groups of the basidiomycete Ustilago violacea are involved in urease activity: a structural one (ure-1) and a second inferred to involve a permease (ure-2) locus. Two types of complementing mutations occur in the structural locus: null activity (ure-1a) and obviously reduced activity (ure-1b). The ure-2 mutants lacked urease activity in vivo on the phenol red-urea test medium, but gave extracts with wild-type activity. Extracts from wild-type strains gave one site of urease activity after polyacrylamide gel electrophoresis. A number of ure-1b mutants and active revertants from ure-1a mutants yielded electrophoretically variant urease sites. The results are discussed in terms of enzyme polymorphism in haploid eukaryotes by one (missense) or two (null, then missense) mutations.     

Hydrophobic Substitution of Surface Residues Affects Lipase Stability in Organic Solvents

A novel lipase has been recently isolated from a local Pseudomonas sp. (GQ243724). In the present study, we have tried to increase the organic solvent stability of this lipase using site-directed mutagenesis. Eight variants N219L, N219I, N219P, N219A, N219R, N219D, S251L, and S251K were designed to change the surface hydrophobicity of this enzyme with respect to the wild-type. Among these variants, the stability of N219L and N219I significantly increased in the presence of all tested organic solvents, whereas two mutants (N219R and N219D) significantly exhibited decreased stabilities in all the organic solvent studied, suggesting that improvement of hydrophobic patches on the enzyme surface enhances the stability in organic media. Furthermore, replacing Ser²⁵¹ with hydrophobic residues on the enzyme surface dramatically diminished its stability in the tested condition. In spite of the distance of the mutated sites from the active site, the values of k _cat and K _m were affected. Finally, structural analysis of the wild-type and mutated variants was carried out in the presence and absence of some organic solvents using circular dichroism and fluorescence spectroscopy.     

Directed Evolution and Structural Analysis of NADPH-Dependent Acetoacetyl Coenzyme A (Acetoacetyl-CoA) Reductase from Ralstonia eutropha Reveals Two Mutations Responsible for Enhanced Kinetics

NADPH-dependent acetoacetyl-coenzyme A (acetoacetyl-CoA) reductase (PhaB) is a key enzyme in the synthesis of poly(3-hydroxybutyrate) [P(3HB)], along with β-ketothiolase (PhaA) and polyhydroxyalkanoate synthase (PhaC). In this study, PhaB from Ralstonia eutropha was engineered by means of directed evolution consisting of an error-prone PCR-mediated mutagenesis and a P(3HB) accumulation-based in vivo screening system using Escherichia coli. From approximately 20,000 mutants, we obtained two mutant candidates bearing Gln47Leu (Q47L) and Thr173Ser (T173S) substitutions. The mutants exhibited k_cat values that were 2.4-fold and 3.5-fold higher than that of the wild-type enzyme, respectively. In fact, the PhaB mutants did exhibit enhanced activity and P(3HB) accumulation when expressed in recombinant Corynebacterium glutamicum. Comparative three-dimensional structural analysis of wild-type PhaB and highly active PhaB mutants revealed that the beneficial mutations affected the flexibility around the active site, which in turn played an important role in substrate recognition. Furthermore, both the kinetic analysis and crystal structure data supported the conclusion that PhaB forms a ternary complex with NADPH and acetoacetyl-CoA. These results suggest that the mutations affected the interaction with substrates, resulting in the acquirement of enhanced activity.     

Investigation of activity of recombinant mengovirus RNA-dependent RNA polymerase and its mutants

The activities of wild-type mengovirus RNA polymerase (RdRP) and of its three mutants with C-terminal tryp-tophan residue replaced by residues of alanine (W460A), phenylalanine (W460F), or tyrosine (W460Y) were studied. The proteins were expressed in E. coli and purified by affinity chromatography with the IMPACT system. The isolated recombinant proteins were studied using a cell-free replication system on elongation of oligo(U) primer on RNA template corresponding to the 3′-terminal 366-meric fragment of the mengovirus RNA. The activities of the mutant polymerases were comparable to that of the wild-type enzyme.