A COLORIMETRIC DETERMINATION OF BLOOD ACETOIN

Overall analyses of the reactions catalyzed by acetohydroxyacid synthase/acetolactate synthase using a precolumn derivatization-HPLC method

A precolumn derivatization-HPLC method using 2,4-dinitrophenylhydrazine and 4-nitro-o-phenylenediamine as respective labeling reagents for comprehensive analyses of the reactions catalyzed by acetohydroxyacid synthase (AHAS)/acetolactate synthase (ALS) is developed and evaluated in this research. Comparison with the classic Bauerle' UV assay which can analyze the enzymes only through measurement of acetoin production, the HPLC method shows advantages because it can analyze the enzymes not only via determination of consumption of the substrate pyruvate, but also via measurement of formation of the products including acetoin, 2,3-butanedione, and acetaldehyde in the enzymatic reactions. Thus the results deduced from the HPLC method can reflect the trait of each enzyme in a more precise manner. As far as we know, this is the first time that the reactions mediated by AHAS/ALS using pyruvate as a single substrate are globally analyzed and the features of the enzymes are properly discussed.

Evidence for the Chemical Mechanism of RibB (3,4-Dihydroxy-2-butanone 4-phosphate Synthase) of Riboflavin Biosynthesis

RibB (3,4-dihydroxy-2-butanone 4-phosphate synthase) is a magnesium-dependent enzyme that excises the C4 of d-ribulose-5-phosphate (d-Ru5P) as formate. RibB generates the four-carbon substrate for lumazine synthase that is incorporated into the xylene moiety of lumazine and ultimately the riboflavin isoalloxazine. The reaction was first identified by Bacher and co-workers in the 1990s, and their chemical mechanism hypothesis became canonical despite minimal direct evidence. X-ray crystal structures of RibB typically show two metal ions when solved in the presence of non-native metals and/or liganding non-substrate analogues, and the consensus hypothetical mechanism has incorporated this cofactor set. We have used a variety of biochemical approaches to further characterize the chemistry catalyzed by RibB from Vibrio cholera (VcRibB). We show that full activity is achieved at metal ion concentrations equal to the enzyme concentration. This was confirmed by electron paramagnetic resonance of the enzyme reconstituted with manganese and crystal structures liganded with Mn²⁺ and a variety of sugar phosphates. Two transient species prior to the formation of products were identified using acid quench of single turnover reactions in combination with NMR for singly and fully ¹³C-labeled d-Ru5P. These data indicate that dehydration of C1 forms the first transient species, which undergoes rearrangement by a 1,2 migration, fusing C5 to C3 and generating a hydrated C4 that is poised for elimination as formate. Structures determined from time-dependent Mn²⁺ soaks of VcRibB-d-Ru5P crystals show accumulation in crystallo of the same intermediates. Collectively, these data reveal for the first time crucial transient chemical states in the mechanism of RibB.

Effects of Mixing Volatile Fatty Acids as Carbon Sources on Rhodospirillum rubrum Carbon Metabolism and Redox Balance Mechanisms

Rhodospirillum rubrum has a versatile metabolism, and as such can assimilate a broad range of carbon sources, including volatile fatty acids. These carbon sources are gaining increasing interest for biotechnological processes, since they reduce the production costs for numerous value-added compounds and contribute to the development of a more circular economy. Usually, studies characterizing carbon metabolism are performed by supplying a single carbon source; however, in both environmental and engineered conditions, cells would rather grow on mixtures of volatile fatty acids (VFAs) generated via anaerobic fermentation. In this study, we show that the use of a mixture of VFAs as carbon source appears to have a synergy effect on growth phenotype. In addition, while propionate and butyrate assimilation in Rs. rubrum is known to require an excess of bicarbonate in the culture medium, mixing them reduces the requirement for bicarbonate supplementation. The fixation of CO2 is one of the main electron sinks in purple bacteria; therefore, this observation suggests an adaptation of both metabolic pathways used for the assimilation of these VFAs and redox homeostasis mechanism. Based on proteomic data, modification of the propionate assimilation pathway seems to occur with a switch from a methylmalonyl-CoA intermediate to the methylcitrate cycle. Moreover, it seems that the presence of a mixture of VFAs switches electron sinking from CO2 fixation to H2 and isoleucine production.

Analysis of the Involvement of the Isoleucine Biosynthesis Pathway in Photoheterotrophic Metabolism of Rhodospirillum rubrum

Purple non-sulfur bacteria (PNSB) are recognized as a highly versatile group of bacteria that assimilate a broad range of carbon sources. Growing heterotrophically, PNSB such as Rhodospirillum rubrum (Rs. rubrum) generate reduced equivalents that are used for biomass production. However, under photoheterotrophic conditions, more reduced electron carriers than required to produce biomass are generated. The excess of reduced equivalents still needs to be oxidized for the metabolism to optimally operate. These metabolic reactions are known as electron sinks. Most PNSB rely on the CO2-fixing Calvin cycle and H2 production to oxidize these reduced equivalents. In addition to these well-described electron sinks, the involvement of some pathways, such as polyhydroxyalkanoate (PHA) biosynthesis, in redox poise is still controversial and requires further studies. Among them, isoleucine biosynthesis has been recently highlighted as one of these potential pathways. Here, we explore the role of isoleucine biosynthesis in Rs. rubrum. Our results demonstrate that the isoleucine content is higher under illuminated conditions and that submitting Rs. rubrum to light stress further increases this phenomenon. Moreover, we explore the production of (p)ppGpp in Rs. rubrum and its potential link with light stress. We further demonstrate that a fully functional isoleucine biosynthesis pathway could be an important feature for the onset of Rs. rubrum growth under photoheterotrophic conditions even in the presence of an exogenous isoleucine source. Altogether, our data suggest that isoleucine biosynthesis could play a key role in redox homeostasis.

Food grade microbial synthesis of the butter aroma compound butanedione using engineered and non-engineered Lactococcus lactis

The design-build-test-learn (DBTL) cycle has been implemented in metabolic engineering processes for optimizing the production of valuable compounds, including food ingredients. However, the use of recombinant microorganisms for producing food ingredients is associated with different challenges, e.g., in the EU, a content of more than 0.9% of such ingredients requires to be labeled. Therefore, we propose to expand the DBTL cycle and use the "learn" module to guide the development of non-engineered strains for clean label production. Here, we demonstrate how this approach can be used to generate engineered and natural cell factories able to produce the valuable food flavor compound - butanedione (diacetyl). Through comprehensive rerouting of the metabolism of Lactococcus lactis MG1363 and re-installment of the capacity to metabolize lactose and dairy protein, we managed to achieve a high titer of diacetyl (6.7 g/L) in pure dairy waste. Based on learnings from the engineering efforts, we successfully achieved the production of diacetyl without using recombinant DNA technology. We accomplish the latter by process optimization and by relying on high-throughput screening using a microfluidic system. Our results demonstrate the great potential that lies in combining metabolic engineering and natural approaches for achieving efficient production of food ingredients.

Harzianic Acid from Trichoderma afroharzianum Is a Natural Product Inhibitor of Acetohydroxyacid Synthase

Acetohydroxyacid synthase (AHAS) is the first enzyme in the branched-chain amino acid biosynthetic pathway and is a validated target for herbicide and fungicide development. Here we report harzianic acid (HA, 1) produced by the biocontrol fungus Trichoderma afroharzianum t-22 (Tht22) as a natural product inhibitor of AHAS. The biosynthetic pathway of HA was elucidated with heterologous reconstitution. Guided by a putative self-resistance enzyme in the genome, HA was biochemically demonstrated to be a selective inhibitor of fungal AHAS, including those from phytopathogenic fungi. In addition, HA can inhibit a common resistant variant of AHAS in which the active site proline is mutated. Structural analysis of AHAS complexed with HA revealed the molecular basis of competitive inhibition, which differs from all known commercial AHAS inhibitors. The alternative binding mode also rationalizes the selectivity of HA, as well as effectiveness toward resistant mutants. A proposed role of HA biosynthesis by Tht22 in the rhizosphere is discussed based on the data.

Modulating microbiota metabolism via bioaugmentation with Lactobacillus casei and Acetobacter pasteurianus to enhance acetoin accumulation during cereal vinegar fermentation

Acetoin, giving a creamy yogurt aroma and buttery taste, exists in cereal vinegar as an important flavor substance and is mainly produced by the metabolism of Lactobacillus and Acetobacter during multispecies solid-state acetic acid fermentation. However, the impacts of Lactobacillus-Acetobacter interactions on acetoin accumulation and the microbial metabolism during acetic acid fermentation are not completely clear. Here, six strains isolated from vinegar fermentation culture and associated with acetoin metabolism, namely, Lactobacillus reuteri L-0, L. buchneri F2-6, L. brevis 4-20, L. fermentum M10-7, L. casei M1-6 and Acetobacter pasteurianus G3-2, were selected for microbial growth and metabolism analysis in monoculture and coculture fermentations. Lactobacillus sp. and A. pasteurianus G3-2 respectively utilized glucose and ethanol preferentially. In monocultures, L. casei M1-6 (183.7 mg/L) and A. pasteurianus G3-2 (121.0 mg/L) showed better acetoin-producing capacity than the others. In the bicultures with Lactobacillus sp. and A. pasteurianus G3-2, biomass analysis in the stationary phase demonstrated that significant growth depressions of Lactobacillus sp. occurred compared with monocultures, possibly due to intolerance to acetic acid produced by A. pasteurianus G3-2. Synergistic effect between Lactobacillus sp. and A. pasteurianus G3-2 on enhanced acetoin accumulation was identified, however, cocultures of two Lactobacillus strains could not apparently facilitate acetoin accumulation. Coculture of L. casei M1-6 and A. pasteurianus G3-2 showed the best performance in acetoin production amongst all mono-, bi- and triculture combinations, and the yield of acetoin increased from 1827.7 to 7529.8 mg/L following optimization of culture conditions. Moreover, the interactions of L. casei M1-6 and A. pasteurianus G3-2 regulated the global metabolism of vinegar microbiota during fermentation through performing in situ bioaugmentation, which could accelerate the production of acetic acid, lactic acid, acetoin, ethyl acetate, ethyl lactate, ligustrazine and other important flavoring substances. This work provides a promising strategy for the production of acetoin-rich vinegar through Lactobacillus sp.-A. pasteurianus joint bioaugmentation.

Protein-based biorefining driven by nitrogen-responsive transcriptional machinery

BACKGROUND

Protein-based bioconversion has been demonstrated as a sustainable approach to produce higher alcohols and ammonia fertilizers. However, owing to the switchover from transcription mediated by the bacterial RNA polymerase σ⁷⁰ to that mediated by alternative σ factors, the biofuel production driven by σ⁷⁰-dependent promoters declines rapidly once cells enter the stationary phase or encounter stresses. To enhance biofuel production, in this study the growth phase-independent and nitrogen-responsive transcriptional machinery mediated by the σ⁵⁴ is exploited to drive robust protein-to-fuel conversion.

RESULTS

We demonstrated that disrupting the Escherichia coli ammonia assimilation pathways driven by glutamate dehydrogenase and glutamine synthetase could sustain the activity of σ⁵⁴-mediated transcription under ammonia-accumulating conditions. In addition, two σ⁵⁴-dependent promoters, argTp and glnAp2, were identified as suitable candidates for driving pathway expression. Using these promoters, biofuel production from proteins was shown to persist to the stationary phase, with the net production in the stationary phase being 1.7-fold higher than that derived from the optimal reported σ⁷⁰-dependent promoter P _LlacO₁. Biofuel production reaching levels 1.3- to 3.4-fold higher than those of the σ⁷⁰-dependent promoters was also achieved by argTp and glnAp2 under stressed conditions. Moreover, the σ⁵⁴-dependent promoters realized more rapid and stable production than that of σ⁷⁰-dependent promoters during fed-batch fermentation, producing up to 4.78 g L ^- 1 of total biofuels.

CONCLUSIONS

These results suggested that the nitrogen-responsive transcriptional machinery offers the potential to decouple production from growth, highlighting this system as a novel candidate to realize growth phase-independent and stress-resistant biofuel production.

Efficient production of α-acetolactate by whole cell catalytic transformation of fermentation-derived pyruvate

Background

Diacetyl provides the buttery aroma in products such as butter and margarine. It can be made via a harsh set of chemical reactions from sugarcane bagasse, however, in dairy products it is normally formed spontaneously from α-acetolactate, a compound generated by selected lactic acid bacteria in the starter culture used. Due to its bacteriostatic properties, it is difficult to achieve high levels of diacetyl by fermentation. Here we present a novel strategy for producing diacetyl based on whole-cell catalysis, which bypasses the toxic effects of diacetyl.

Results

By expressing a robust α-acetolactate synthase (ALS) in a metabolically optimized Lactococcus lactis strain we obtained a whole-cell biocatalyst that efficiently converted pyruvate into α-acetolactate. After process optimization, we achieved a titer for α-acetolactate of 172 ± 2 mM. Subsequently we used a two-stage production setup, where pyruvate was produced by an engineered L. lactis strain and subsequently used as the substrate for the biocatalyst. Using this approach, 122 ± 5 mM and 113 ± 3 mM α-acetolactate could be made from glucose or lactose in dairy waste, respectively. The whole-cell biocatalyst was robust and fully active in crude fermentation broth containing pyruvate.

Conclusions

An efficient approach for converting sugar into α-acetolactate, via pyruvate, was developed and tested successfully. Due to the anaerobic conditions used for the biotransformation, little diacetyl was generated, and this allowed for efficient biotransformation of pyruvate into α-acetolactate, with the highest titers reported to date. The use of a two-step procedure for producing α-acetolactate, where non-toxic pyruvate first is formed, and subsequently converted into α-acetolactate, also simplified the process optimization. We conclude that whole cell catalysis is suitable for converting lactose in dairy waste into α-acetolactate, which favors resource utilization.

Analysis of the CodY RNome reveals RsaD as a stress-responsive riboregulator of overflow metabolism in Staphylococcus aureus

In Staphylococcus aureus, the transcription factor CodY modulates the expression of hundreds of genes, including most virulence factors, in response to the availability of key nutrients like GTP and branched-chain amino acids. Despite numerous studies examining how CodY controls gene expression directly or indirectly, virtually nothing is known about the extent to which CodY exerts its effect through small regulatory RNAs (sRNAs). Herein, we report the first set of sRNAs under the control of CodY. We reveal that staphylococcal sRNA RsaD is overexpressed >20-fold in a CodY-deficient strain in three S. aureus clinical isolates and in S. epidermidis. We validated the CodY-dependent regulation of rsaD and demonstrated that CodY directly represses rsaD expression by binding the promoter. Using a combination of molecular techniques, we show that RsaD posttranscriptionally regulates alsS (acetolactate synthase) mRNA and enzyme levels. We further show that RsaD redirects carbon overflow metabolism, contributing to stationary phase cell death during exposure to weak acid stress. Taken together, our data delineate a role for CodY in controlling sRNA expression in a major human pathogen and indicate that RsaD may integrate nutrient depletion and other signals to mount a response to physiological stress experienced by S. aureus in diverse environments.

Fluorogenic Assay for Acetohydroxyacid Synthase: Design and Applications

Acetohydroxyacid synthase (AHAS) exists in plants and many microorganisms (including gut flora) but not in mammals, making it an attractive drug target. Fluorescent-based methods should be practical for high-throughput screening of inhibitors. Herein, we describe the development of the first AHAS fluorogenic assay based on an intramolecular charge transfer (ICT)-based fluorescent probe. The assay is facile, sensitive, and continuous and can be applied toward various AHASs from different species, AHAS mutants, and crude cell lysates. The fluorogenic assay was successfully applied for (1) high-throughput screening of commerical herbicides toward different AHASs for choosing matching herbicides, (2) identification of a Soybean AHAS gene with broad-spectrum herbicide resistance, and (3) identification of selective inhibitors toward intestinal-bacterial AHASs. Among the AHAS inhibitors, an active agent was found for selective inhibition of obesity-associated Ruminococcus torques growth, implying the possibility of AHAS inhibitors for the ultimate goal toward antiobesity therapeutics. The fluorogenic assay opens the door for high-throughput programs in AHAS-related fields, and the design principle might be applied for development of fluorogenic assays of other synthases.

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.

Effect of Ahasl1-1 and Ahasl1-4 alleles on herbicide resistance and its associated dominance in sunflower

BACKGROUND

Acetohydroxyacid synthase large subunit 1 (Ahasl1) is a multiallelic locus involved in herbicide resistance in sunflower. Ahasl1-1 and Ahasl1-4 alleles harbor different point mutations that lead to different amino acid substitutions (Ala205Val and Trp574Leu, respectively). The objectives of this work were to evaluate the effect of these alleles at the enzymatic and whole-plant levels, and to determine the dominance relationships for imazapyr and metsulfuron-methyl herbicides.

RESULTS

Resistant near-isogenic lines showed significantly lower specific AHAS activity than susceptible near-isoline. However, kinetic studies indicated that mutations did not change AHAS pyruvate affinity. Dose-response for six near-isolines carrying different combinations of Ahasl1-1 and Ahasl1-4 alleles and two herbicides (imazapyr and metsulfuron-methyl) were evaluated at whole-plant and enzymatic levels. Ahasl1-1 allele conferred moderate resistance to imazapyr and low resistance to metsulfuron-methyl. Conversely, Ahasl1-4 allele endowed high levels of resistance for both herbicides. Dominance of resistance at whole-plant level showed a semi-dominant behavior among the alleles for both herbicides.

CONCLUSION

Ahasl1-4 allele confers higher resistance levels than Ahasl1-1 when evaluated with imazapyr and metsulfuron-methyl. Dominance estimations suggested that both parental lines should carry a resistance trait when developing hybrids. © 2018 Society of Chemical Industry.

Simultaneous biosynthesis of (R)-acetoin and ethylene glycol from D-xylose through in vitro metabolic engineering

(R)-acetoin is a four-carbon platform compound used as the precursor for synthesizing novel optically active materials. Ethylene glycol (EG) is a large-volume two-carbon commodity chemical used as the anti-freezing agent and building-block molecule for various polymers. Currently established microbial fermentation processes for converting monosaccharides to either (R)-acetoin or EG are plagued by the formation of undesirable by-products. We show here that a cell-free bioreaction scheme can generate enantiomerically pure acetoin and EG as co-products from biomass-derived D-xylose. The seven-step, ATP-free system included in situ cofactor regeneration and recruited enzymes from Escherichia coli W3110, Bacillus subtilis shaijiu 32 and Caulobacter crescentus CB 2. Optimized in vitro biocatalytic conditions generated 3.2 mM (R)-acetoin with stereoisomeric purity of 99.5% from 10 mM D-xylose at 30 °C and pH 7.5 after 24 h, with an initial (R)-acetoin productivity of 1.0 mM/h. Concomitantly, EG was produced at 5.5 mM, with an initial productivity of 1.7 mM/h. This in vitro biocatalytic platform illustrates the potential for production of multiple value-added biomolecules from biomass-based sugars with no ATP requirement.

Heterologous Expression and Functional Characterization of Catalytic Subunit of Rice Acetohydroxyacid Synthase

BACKGROUND

Acetohydroxyacid Synthase (AHAS) is the first enzyme in the biosynthesis pathway of the branched chain amino acids. AHAS is the common target site of five herbicide chemical groups: sulfonylurea, imidazolinone, triazolopyrimidine, pyrimidinyl-thiobenzoates, and sulfonyl-aminocarbonyl-triazolinone.

OBJECTIVE

The purification of protein enabled us to study the physical and biochemical properties of the enzyme. In addition in vitro activity of this enzyme was tested in the presence of four different sulfonylureaherbicides and the feedback regulation of enzyme was analyzed in the presence of branched amino acids.

METHODS

The gene encoding catalytic subunit of rice AHAS (cOsAHAS) without part of the chloroplast transit sequence was cloned into the bacterial expression vector pET41a and heterologously expressed in Escherichia coli as carboxy-terminal extensions of glutathione-S-transferase (GST).The soluble protein was purified using affinity chromatography. The measurement of GSTOsAHAS activity was performed under optimized conditions at present of branched-chain amino acids and sulfonylurea herbicides independently.

RESULTS

The optimum pH and temperature for GST-cOsAHAS activity was 8.0 and 37 °C, respectively. The specific activity and Km value of this enzyme toward pyruvate were 0.08 U/mg and 30 mM, respectively.GST-cOsAHAS was inhibited by herbicides tribenuron, sulfosulfuron, nicosulfuron and bensulfuron while the enzyme was insensitivieto end products.

CONCLUSION

These results suggest that the recombinant form of GST-cOsAHAS is functionally active and carries the binding site for sulfynylurea herbicides. Furthermore, GST-cOsAHAS was insensitive to feedback inhibition by endproducts which indicates the existence of a regulator subunit in rice AHAS as previously has been described in other plant AHASs.

AHAS Trp574Leu substitution in Raphanus sativus L.: screening, enzyme activity and fitness cost

BACKGROUND

Feral radish (Raphanus sativus L.) is a problematic weed that has become resistant to acetohydroxyacid synthase (AHAS) inhibitor herbicides due to the Trp574Leu mutation. An AHAS gene mutation that causes herbicide resistance may have negative pleiotropic effects on plant fitness. This study reports the effects of the Trp574Leu mutation on AHAS activity and reproductive traits of R. sativus.

RESULTS

Eight of 17 feral radish accessions presented individuals resistant to metsulfuron-methyl at 0.5% to >90.0% and all the resistant individuals analyzed showed the Trp574Leu mutation. Without herbicide selection, the AHAS activity was 3.2-fold higher in the susceptible accession than in the resistant one. The resistant accession was >9000-fold more resistant to metsulfuron-methyl and imazethapyr than the susceptible accession. Under low intraspecific competition during two growing seasons, AHAS-resistant feral radish accessions showed 22-38% and 21-47% lower seed numbers and yield per plant than the susceptible accession.

CONCLUSION

This is the first report of fitness cost associated with the AHAS Trp574Leu mutation in R. sativus populations. This fitness cost could reduce frequency of the resistant allele without herbicide selection. © 2018 Society of Chemical Industry.

NADH/NADPH bi-cofactor-utilizing and thermoactive ketol-acid reductoisomerase from Sulfolobus acidocaldarius

Ketol-acid reductoisomerase (KARI) is a bifunctional enzyme in the second step of branched-chain amino acids biosynthetic pathway. Most KARIs prefer NADPH as a cofactor. However, KARI with a preference for NADH is desirable in industrial applications including anaerobic fermentation for the production of branched-chain amino acids or biofuels. Here, we characterize a thermoacidophilic archaeal Sac-KARI from Sulfolobus acidocaldarius and present its crystal structure at a 1.75-Å resolution. By comparison with other holo-KARI structures, one sulphate ion is observed in each binding site for the 2′-phosphate of NADPH, implicating its NADPH preference. Sac-KARI has very high affinity for NADPH and NADH, with K_M values of 0.4 μM for NADPH and 6.0 μM for NADH, suggesting that both are good cofactors at low concentrations although NADPH is favoured over NADH. Furthermore, Sac-KARI can catalyze 2(S)-acetolactate (2S-AL) with either cofactor from 25 to 60 °C, but the enzyme has higher activity by using NADPH. In addition, the catalytic activity of Sac-KARI increases significantly with elevated temperatures and reaches an optimum at 60 °C. Bi-cofactor utilization and the thermoactivity of Sac-KARI make it a potential candidate for use in metabolic engineering or industrial applications under anaerobic or harsh conditions.

Genotypic and phenotypic characterization of the food spoilage bacterium Brochothrix thermosphacta

Microbial food spoilage is responsible for significant economic losses. Brochothrix thermosphacta is one of the major bacteria involved in the spoilage of meat and seafood. Its growth and metabolic activities during food storage result in the production of metabolites associated with off-odors. In this study, we evaluated the genotypic and phenotypic diversity of this species. A collection of 161 B. thermosphacta strains isolated from different foods, spoiled or not, and from a slaughterhouse environment was constituted from various laboratory collections and completed with new isolates. A PCR test based on the rpoB gene was developed for a fast screening of B. thermosphacta isolates. Strains were typed by MALDI-TOF MS, rep-PCR, and PFGE. Each typing method separated strains into distinct groups, revealing significant intra-species diversity. These classifications did not correlate with the ecological origin of strains. The ability to produce acetoin and diacetyl, two molecules associated with B. thermosphacta spoilage, was evaluated in meat and shrimp juices. The production level was variable between strains and the spoilage ability on meat or shrimp juice did not correlate with the substrate origin of strains. Although the B. thermosphacta species encompasses ubiquitous strains, spoiling ability is both strain- and environment-dependent.

Efficient production of 2,3-butanediol by recombinant Saccharomyces cerevisiae through modulation of gene expression by cocktail δ-integration

In this study, the expression of 4 genes encoding α-acetolactate synthase, α-acetolactate decarboxylase, 2,3-butanediol dehydrogenase, and NADH oxidase was modulated using a previously developed cocktail δ-integration strategy. The resultant strain, YPH499/dPdAdG/BD6-10, was used in a fed-batch cultivation for the production of 2,3-butanediol. The concentration, production rate, and yield obtained were 80.0g/L, 4.00g/L/h, and 41.7%, respectively. The production rate and yield of the compound obtained are higher for this strain compared to reports published for Saccharomyces cerevisiae so far. The cocktail δ-integration strategy allows for modulation of multiple gene expression, without the exact knowledge of rate-limiting steps, and therefore, could be used as a promising strategy for the production of bio-based chemicals in recombinant S. cerevisiae.

Systematic Approaches to Efficiently Produce 2,3-Butanediol in a Marine Cyanobacterium

Cyanobacteria have attracted significant interest as a platform for renewable production of fuel and feedstock chemicals from abundant atmospheric carbon dioxide by way of photosynthesis. While great strides have been made in developing this technology in freshwater cyanobacteria, logistical issues remain in scale-up. Use of the cyanobacterium Synechococcus sp. PCC 7002 (7002) as a chemical production chassis could address a number of these issues given the higher tolerance to salt, light, and heat as well as the fast growth rate of 7002 in comparison to traditional model cyanobacteria such as Synechococcus elongatus PCC 7942 and Synechocystis sp. PCC 6803. However, despite growing interest, the development of genetic engineering tools for 7002 continues to lag behind those available for model cyanobacterial strains. In this work we demonstrate the systematic development of a 7002 production strain for the feedstock chemical 2,3-butanediol (23BD). We expand the range of tools available for use in 7002 by identifying and utilizing new integration sites for homologous recombination, demonstrating the inducibility of theophylline riboswitches, and screening a set of isopropyl β-d-1-thiogalactopyranoside (IPTG) inducible promoters. We then demonstrate improvements of 23BD production with the systematic screening of different conditions including: operon arrangement and copy number, light strength, inducer concentration, cell density at the time of induction, and nutrient concentration. Final production tests yielded titers of 1.6 g/L 23BD after 16 days at a rate of 100 mg/L/day. This work represents great strides in the development of 7002 as an industrially relevant production host.

Conversion of cellulose and hemicellulose of biomass simultaneously to acetoin by thermophilic simultaneous saccharification and fermentation

BACKGROUND

Acetoin (3-hydroxy-2-butanone), the precursor of biofuel 2,3-butanediol, is an important bio-based platform chemical with wide applications. Fermenting the low-cost and renewable plant biomass is undoubtedly a promising strategy for acetoin production. Isothermal simultaneous saccharification and fermentation (SSF) is regarded as an efficient method for bioconversion of lignocellulosic biomass, in which the temperature optima fitting for both lignocellulose-degrading enzymes and microbial strains.

RESULTS

A thermotolerant (up to 52 °C) acetoin producer Bacillus subtilis IPE5-4 which simultaneously consumed glucose and xylose was isolated and identified. By compound mutagenesis, the mutant IPE5-4-UD-4 with higher acetoin productivity was selected. When fermenting at 50 °C in a 5-L bioreactor using glucose as the feedstock by strain IPE5-4-UD-4, the acetoin concentration reached 28.83 ± 0.91 g L^-1 with the acetoin yield and productivity of 0.34 g g^-1 glucose and 0.60 g L^-1 h^-1, respectively. Furthermore, an optimized and thermophilic SSF process operating at 50 °C was conducted for acetoin production from alkali-pretreated corncob (APC). An acetoin concentration of 12.55 ± 0.28 g L^-1 was achieved by strain IPE5-4-UD-4 in shake flask SSF, with the acetoin yield and productivity of 0.25 g g^-1 APC and 0.17 g L^-1 h^-1. Meanwhile, the utilization of cellulose and hemicellulose in the SSF approach reached 96.34 and 93.29%, respectively. When further fermented at 50 °C in a 5-L bioreactor, the concentration of acetoin reached the maximum of 22.76 ± 1.16 g L^-1, with the acetoin yield and productivity reaching, respectively, 0.46 g g^-1 APC and 0.38 g L^-1 h^-1. This was by far the highest acetoin yield in SSF from lignocellulosic biomass.

CONCLUSIONS

This thermophilic SSF process provided an efficient and economical route for acetoin production from lignocellulosic biomass at ideal temperature for both enzymatic hydrolysis and microbial fermentation.

Occurrence of Different Resistance Mechanisms to Acetolactate Synthase Inhibitors in European Sorghum halepense

Four Hungarian and two Italian Sorghum halepense populations harvested in maize fields were investigated to elucidate the levels and mechanisms underlying acetolactate synthase (ALS) inhibitors resistance. The two Italian populations were highly cross-resistant to all ALS inhibitors tested, and the variant ALS allele Leu₅₇₄ was identified in most of the plants; no differences were observed when the plants were treated with herbicide plus malathion. This suggests that the main resistance mechanism is target-site mediated. The Hungarian populations proved to be controlled by imazamox, while they were resistant to sulfonylureas and bispyribac-Na. All Hungarian populations, but not all plants of population 12-49H, presented the variant allele Glu₃₇₆. This is the first documented occurrence of the Asp-376-Glu substitution in S. halepense. ALS enzyme bioassay and treatment with malathion confirmed that at least in plants of two populations the resistance is very likely due to both target-site and enhanced metabolism of P450 enzymes.

Understanding the Differential Response of Setaria viridis L. (green foxtail) and Setaria pumila Poir. (yellow foxtail) to Pyroxsulam

Green foxtail [Setaria viridis (L) Beauv.] and yellow foxtail [Setaria pumila (Poir.) Roem. & Schult.] are among the most abundant and troublesome annual grass weeds in cereal crops in the Northern Plains of the United States and the Prairie Provinces of Canada. Greenhouse and laboratory experiments were conducted to examine the differential responses of both weed species to foliar applications of the new triazolopyrimidine sulfonamide acetolactate synthase-inhibiting herbicide, pyroxsulam, and to determine the mechanism(s) of differential weed control. Foliar applications of pyroxsulam resulted in >90% control of yellow foxtail at rates between 7.5 and 15 g ai ha^-1, whereas the same rates resulted in a reduced efficacy on green foxtail (≤81%). The absorption and translocation of [¹⁴C]pyroxsulam in green and yellow foxtail were similar and could not explain the differential whole-plant efficacy. Studies with [¹⁴C]pyroxsulam revealed a higher percentage of absorbed pyroxsulam was metabolized into an inactive metabolite in the treated leaf of green foxtail than in the treated leaf of yellow foxtail. Metabolism studies demonstrated that, 48 h after application, 50 and 35% of pyroxsulam in the treated leaf was converted to 5-hydroxy-pyroxsulam in green and yellow foxtail, respectively. The acetolactate synthase (ALS) inhibition assay showed that ALS extracted from green foxtail was more tolerant to pyroxsulam than the enzyme extracted from yellow foxtail was. The in vitro ALS assay showed IC₅₀ values of 8.39 and 0.26 μM pyroxsulam for green and yellow foxtail, respectively. The ALS genes from both green and yellow foxtail were sequenced and revealed amino acid differences; however, the changes are not associated with known resistance-inducing mutations. The differential control of green and yellow foxtail following foliar applications of pyroxsulam was attributed to differences in both metabolism and ALS sensitivity.

Computational design of novel inhibitors to overcome weed resistance associated with acetohydroxyacid synthase (AHAS) P197L mutant

BACKGOUND

Acetohydroxyacid synthase (AHAS; EC 2.2.1.6) is the first common enzyme in the biosynthetic pathway leading to the branched-chain amino acids in plants and a wide range of microorganisms. With the long-term and wide application of AHAS inhibitors, weed resistance is becoming a global problem, which leads to an urgent demand for novel inhibitors to antagonize both wild-type and resistant AHAS.

RESULTS

Pyrimidinyl salicylic acid derivatives, as one of the main classes of commercial AHAS herbicides, show potential anti-resistant bioactivity to wild-type and P197L mutant. In current work, a series of novel 2-benzoyloxy-6-pyrimidinyl salicylic acid derivatives were designed through fragment-based drug discovery. Fortunately, the newly synthesized compounds showed good inhibitory activity against both wild-type and P197L mutant. Some compounds not only had a lower resistance factor value but also showed excellent inhibitory activity against wild-type AHAS and P197L mutant. Furthermore, greenhouse experiments showed compound 11m displayed almost 100% inhibition against both wild-type and high-resistant Descurainia sophia at a dosage of 150 g a.i. ha^-1 .

CONCLUSION

The present work indicated that the 2-benzoyloxy-6-pyrimidinyl salicylic acid motif was well worth further optimization. Also, compound 11m could be used as a potential anti-resistant AHAS herbicide, which requires further research. © 2016 Society of Chemical Industry.

A New Ala-122-Asn Amino Acid Change Confers Decreased Fitness to ALS-Resistant Echinochloa crus-galli

Gene mutations conferring herbicide resistance may cause pleiotropic effects on plant fitness. Knowledge of these effects is important for managing the evolution of herbicide-resistant weeds. An Echinochloa crus-galli population resistant to acetolactate synthase (ALS) herbicides was collected in a maize field in north-eastern Italy and the cross-resistance pattern, resistance mechanism and fitness costs associated to mutant-resistant plants under field conditions in the presence or absence of intra-specific competition were determined. The study reports for the first time the Ala-122-Asn amino-acid change in the ALS gene that confers high levels of cross-resistance to all ALS inhibitors tested. Results of 3-year growth analysis showed that mutant resistant E. crus-galli plants had a delayed development in comparison with susceptible plants and this was registered in both competitive (3, 7, and 20 plants m^-2) and non-competitive (spaced plants) situations. The number of panicles produced by resistant plants was also lower (about 40% fewer panicles) than susceptible plants under no-intraspecific competition. Instead, with the increasing competition level, the difference in panicle production at harvest time decreased until it became negligible at 20 plants m^-2. Evaluation of total dry biomass as well as biomass allocation in vegetative parts did not highlight any difference between resistant and susceptible plants. Instead, panicle dry weight was higher in susceptible plants indicating that they allocated more biomass than resistant ones to the reproductive organs, especially in no-competition and in competition situations at lower plant densities. The different fitness between resistant and susceptible phenotypes suggests that keeping the infestation density as low as possible can increase the reproduction success of the susceptible phenotype and therefore contribute to lowering the ratio between resistant and susceptible alleles. If adequately embedded in a medium or long-term integrated weed management strategy, the presence of R plants with a fitness penalty provides an opportunity to minimize or reverse herbicide resistance evolution through the implementation of integrated weed management, i.e., all possible control tools available.

The role of budABC on 1,3-propanediol and 2,3-butanediol production from glycerol in Klebsiella pneumoniae CICIM B0057

1,3-propanediol (1,3-PD) is an important compound from which many others can be synthesized. 2,3-butanediol (BDO) is the key by-product in the biosynthesis of 1,3-PD from glycerol, but it impedes its downstream purification. In Klebsiella, the budA, budB and budC genes encode enzymes that are responsible for the synthesis of BDO. In this study, 3 individual antisense RNAs were designed to repress the expression and hence activity of BudA-C. Compared with the parent strains, the activities of BudB and BudC were reduced by 60.5% and 70.5%, respectively, and the mRNA level of budA was reduced by 70%. Decreased BudC activity had no effect on cell growth or carbon distribution. However, reduced BudA and BudB activity decreased the BDO concentration by 35% and led to a 10% increase in the yield of 1,3-PD. This result suggests the activities of BudA and BudB could be key factors in the production of BDO from glycerol in Klebsiella. This study provides a deeper understanding of the role of budABC in glycerol metabolism in Klebsiella.

Genome Engineering of the 2,3-Butanediol Biosynthetic Pathway for Tight Regulation in Cyanobacteria

Cyanobacteria have gained popularity among the metabolic engineering community as a tractable photosynthetic host for renewable chemical production. However, though a number of successfully engineered production systems have been reported, long-term genetic stability remains an issue for cyanobacterial systems. The genetic engineering toolbox for cyanobacteria is largely lacking inducible systems for expression control. The characterization of tight regulation systems for use in cyanobacteria may help to alleviate this problem. In this work we explore the function of the IPTG inducible promoter P(L)lacO1 in the model cyanobacterium Synechococcus elongatus PCC 7942 as well as the effect of gene order within an operon on pathway expression. According to our experiments, P(L)lacO1 functions well as an inducible promoter in S. elongatus. Additionally, we found that gene order within an operon can strongly influence control of expression of each gene.

Characterization of acetohydroxyacid synthase from the hyperthermophilic bacterium Thermotoga maritima

Acetohydroxyacid synthase (AHAS) is the key enzyme in branched chain amino acid biosynthesis pathway. The enzyme activity and properties of a highly thermostable AHAS from the hyperthermophilic bacterium Thermotoga maritima is being reported. The catalytic and regulatory subunits of AHAS from T. maritima were over-expressed in Escherichia coli. The recombinant subunits were purified using a simplified procedure including a heat-treatment step followed by chromatography. A discontinuous colorimetric assay method was optimized and used to determine the kinetic parameters. AHAS activity was determined to be present in several Thermotogales including T. maritima. The catalytic subunit of T. maritima AHAS was purified approximately 30-fold, with an AHAS activity of approximately 160±27 U/mg and native molecular mass of 156±6 kDa. The regulatory subunit was purified to homogeneity and showed no catalytic activity as expected. The optimum pH and temperature for AHAS activity were 7.0 and 85 °C, respectively. The apparent K_m and V_max for pyruvate were 16.4±2 mM and 246±7 U/mg, respectively. Reconstitution of the catalytic and regulatory subunits led to increased AHAS activity. This is the first report on characterization of an isoleucine, leucine, and valine operon (ilv operon) enzyme from a hyperthermophilic microorganism and may contribute to our understanding of the physiological pathways in Thermotogales. The enzyme represents the most active and thermostable AHAS reported so far.

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First report of AHAS from a hyperthermophilic bacterium.

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Catalytic and regulatory subunits of AHAS of T. maritima was expressed in E. coli.

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Recombinant proteins were purified using a simplified procedure.

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Enzyme represents the most active and thermostable AHAS reported so far.

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Kinetic parameters were determined for the purified recombinant enzyme

New insight into the photoheterotrophic growth of the isocytrate lyase-lacking purple bacterium Rhodospirillum rubrum on acetate

Purple non-sulfur bacteria are well known for their metabolic versatility. One of these bacteria, Rhodospirillum rubrum S1H, has been selected by the European Space Agency to ensure the photoheterotrophic assimilation of volatile fatty acids in its regenerative life support system, MELiSSA. Here, we combined proteomic analysis with bacterial growth analysis and enzymatic activity assays in order to better understand acetate photoassimilation. In this isocitrate lyase-lacking organism, the assimilation of two-carbon compounds cannot occur through the glyoxylate shunt, and the citramalate cycle has been proposed to fill this role, while, in Rhodobacter sphaeroides, the ethylmalonyl-CoA pathway is used for acetate assimilation. Using proteomic analysis, we were able to identify and quantify more than 1700 unique proteins, representing almost one-half of the theoretical proteome of the strain. Our data reveal that a pyruvate : ferredoxin oxidoreductase (NifJ) could be used for the direct assimilation of acetyl-CoA through pyruvate, potentially representing a new redox-balancing reaction. We additionally propose that the ethylmalonyl-CoA pathway could also be involved in acetate assimilation by the examined strain, since specific enzymes of this pathway were all upregulated and activity of crotonyl-CoA reductase/carboxylase was increased in acetate conditions. Surprisingly, we also observed marked upregulation of glutaryl-CoA dehydrogenase, which could be a component of a new pathway for acetate photoassimilation. Finally, our data suggest that citramalate could be an intermediate of the branched-chain amino acid biosynthesis pathway, which is activated during acetate assimilation, rather than a metabolite of the so-called citramalate cycle.

Overexpression of ribosome elongation factor G and recycling factor increases L-isoleucine production in Corynebacterium glutamicum

Ribosome elongation factor G encoded by fusA promotes the translocation step of protein synthesis in bacteria; ribosome recycling factor encoded by frr, together with the elongation factor G, dissociates ribosomes from messenger RNA after the termination of translation. Both factors play important roles during protein synthesis in bacteria. In this study, we found that overexpression of fusA and/or frr led to the increase of L-isoleucine production in Corynebacterium glutamicum IWJ001, an L-isoleucine production strain generated by random mutagenesis. Reverse transcription polymerase chain reaction analysis showed that transcriptional levels of genes lysC, hom, thrB, ilvA, ilvBN, and ilvE encoding the key enzymes in the biosynthetic pathway of L-isoleucine increased in C. glutamicum IWJ001 when fusA and/or frr were overexpressed. Co-overexpression of fusA and frr, together with genes ilvA, ilvB, ilvN, and ppnk in C. glutamicum IWJ001, led to 76.5 % increase of L-isoleucine production in flask cultivation and produced 28.5 g/L L-isoleucine in 72-h fed-batch fermentation. The results demonstrate that overexpressing ribosome elongation factor G and ribosome recycling factor is an efficient approach to enhance L-isoleucine production in C. glutamicum.

Biochemical Markers and Enzyme Assays for Herbicide Mode of Action and Resistance Studies

Herbicides inhibit biochemical and physiological processes or both with lethal consequences. The target sites of these small molecules are usually enzymes involved in primary metabolic pathways or proteins carrying out essential physiological functions. Herbicides tend to be highly specific for their respective target sites and have served as tools to study these physiological and biochemical processes in plants (Dayan et al. 2010b).

Generation and characterization of tribenuron-methyl herbicide-resistant rapeseed (Brasscia napus) for hybrid seed production using chemically induced male sterility

Identification and molecular analysis of four tribenuron-methyl resistant mutants in Brassica napus , which would be very useful in hybrid production using a Chemically induced male sterility system. Chemically induced male sterility (CIMS) systems dependent on chemical hybridization agents (CHAs) like tribenuron-methyl (TBM) represent an important approach for practical utilization of heterosis in rapeseed. However, when spraying the female parents with TBM to induce male sterility the male parents must be protected with a shield to avoid injury to the stamens, which would otherwise complicate the seed production protocol and increase the cost of hybrid seed production. Here we report the first proposed application of a herbicide-resistant cultivar in hybrid production, using a CIMS system based on identifying four TBM-resistant mutants in Brassica napus. Genetic analysis indicated that the TBM resistance was controlled by a single dominant nuclear gene. An in vitro enzyme activity assay for acetohydroxyacid synthase (AHAS) suggested that the herbicide resistance is caused by a gain-of-function mutation in a copy of AHAS genes. Comparative sequencing of the mutants and wild type BnaA.AHAS.a coding sequences identified a C-to-T transition at either position 535 or 536 from the translation start site, which resulted in a substitution of proline with serine or leucine at position 197 according to the Arabidopsis thaliana protein sequence. An allele-specific dCAPS marker developed from the C536T variation co-segregated with the herbicide resistance. Transgenic A. thaliana plants expressing BnaA.ahas3.a conferred herbicide resistance, which confirmed that the P197 substitution in BnaA.AHAS.a was responsible for the herbicide resistance. Moreover, the TBM-resistant lines maintain normal male fertility under TBM treatment and can be of practical value in hybrid seed production using CIMS.

Mutational analysis of critical residues of FAD-independent catabolic acetolactate synthase from Enterococcus faecalis V583

Catabolic acetolactate synthase (cALS) from Enterococcus faecalis is a FAD-independent enzyme, which catalyzes the condensation of two molecules of pyruvate to produce acetolactate. Mutational and kinetic analyses of variants suggested the importance of H111, Q112, and Q411 residues for catalysis in cALS. The wild-type and variants were expressed as equally soluble proteins and co-migrated to a size of 60 kDa on SDS-PAGE. Importantly, H111 in cALS, which is widely present as phenylalanine in many other ThDP-dependent enzymes, plays a crucial role in substrate binding. Interestingly, the H111 variants, H111R and H111F, demonstrated altered specific activity of H111 variants with 17- and 26-fold increases in Km, respectively, compared to wild-type cALS. Furthermore, Q112 variants, Q112E, Q112N, and Q112V, exhibited significantly lower specific activity with 70-, 15-, and 10-fold higher Ks for ThDP, respectively. In the case of Q411, the variant Q411E showed a 10-fold rise in Km and a 20-fold increase in Ks for ThDP. Further, the molecular docking results indicated that the binding mode of ThDP was slightly affected in the variants of cALS. Based on these results, we suggest that H111 plays a role in substrate binding, and further suggest that Q112 and Q411 might be involved in ThDP binding of cALS.

Altered superoxide dismutase activity by carbohydrate utilization in a Lactococcus lactis strain

Reactive oxygen species, such as superoxide, can damage cellular components, such as proteins, lipids, and DNA. Superoxide dismutase (SOD) enzymes catalyze the conversion of superoxide anions to hydrogen peroxide and dioxygen. SOD is present in most lactococcal bacteria, which are commonly used as starters for manufacturing fermented dairy products and may have health benefits when taken orally. We assessed the effects of carbohydrate use on SOD activity in lactococci. In Lactococcus lactis ssp. lactis G50, the SOD activity of cells grown on lactose and galactose was higher than that on glucose; in Lactococcus lactis ssp. cremoris H61, SOD activity was independent of the type of carbohydrate used. We also investigated the activity of NADH oxidase, which is related to the production of superoxide in strains G50 and H61. Activity was highest in G50 cells grown on lactose, lower on galactose, and lowest on glucose, whereas activity in H61 cells did not differ with the carbohydrate source used. The SOD and NADH oxidase activities of strain G50 in three carbohydrates were linked. Strain G50 fermented lactose and galactose to lactate, acetate, formate, and ethanol (mixed-acid fermentation) and fermented glucose to mainly lactate (homolactic fermentation). Strain H61 fermented glucose, lactose, and galactose to mainly lactate (homolactic fermentation). In strain G50, when growth efficiency was reduced by adding a metabolic inhibitor to the growth medium, SOD activity was higher than in the control; however, the metabolism was homofermentative. Aerobic conditions, but not glucose-limited conditions, increased SOD activity, and mixed-acid fermentation occurred. We conclude that the effect of carbohydrate on SOD activity in lactococci is strain dependent and that the activity of commercial lactococci can be enhanced through carbohydrate selection for mixed-acid fermentation or by changing the energy distribution, thus enhancing the value of the starter and the resulting dairy products.

Acetohydroxyacid synthase activity and transcripts profiling reveal tissue-specific regulation of ahas genes in sunflower

Acetohydroxyacid synthase (AHAS) is the target site of several herbicides and catalyses the first step in the biosynthesis of branched chain amino acid. Three genes coding for AHAS catalytic subunit (ahas1, ahas2 and ahas3) have been reported for sunflower. The aim of this work was to study the expression pattern of ahas genes family and AHAS activity in sunflower (Helianthus annuus L.). Different organs (leaves, hypocotyls, roots, flowers and embryos) were evaluated at several developmental stages. The transcriptional profile was studied through RT-qPCR. The highest expression for ahas1 was shown in leaves, where all the induced and natural gene mutations conferring herbicide resistance were found. The maximal expression of ahas2 and ahas3 occurred in immature flowers and embryos. The highest AHAS activity was found in leaves and immature embryos. Correlation analysis among ahas gene expression and AHAS activity was discussed. Our results show that differences in ahas genes expression are tissue-specific and temporally regulated. Moreover, the conservation of multiple AHAS isoforms in sunflower seems to result from different expression requirements controlled by tissue-specific regulatory mechanisms at different developmental stages.

Comparative Assessment of Factors Involved in Acetoin Synthesis by Bacillus subtilis 168

Acetoin is widely used as flavor agent and serves as a precursor for chemical synthesis. Here we focused on identifying the best physiological conditions (initial substrate concentrations, pH, temperature, and agitation) for enhanced acetoin accumulation by Bacillus subtilis 168. The optimal physiological conditions support maximum acetoin accumulation by minimizing byproduct (acetate and butanediol) synthesis and a maximum of 75% enhancement in acetoin yield could be achieved. Additionally, the effect of change in ALS (acetolactate synthase) and ALDC (acetolactate decarboxylase) activities was evaluated on acetoin accumulation. Increasing ALS and ALDC enzyme activities led to efficient utilization of pyruvate towards acetoin accumulation and about 80% enhancement in acetoin accumulation was observed.

Isolation of acetoin-producing Bacillus strains from Japanese traditional food-natto

In this study, Bacillus strains with an ability to produce acetoin were isolated from a Japanese traditional food, natto, on the basis of the Voges-Proskauer (VP) reaction, and strain SF4-3 was shown to be a predominant strain in acetoin production. Based on a variety of morphological, physiological, and biochemical characteristics as well as the nucleotide sequence analysis of 16S rDNA, the strain SF4-3 was identified as Bacillus subtilis. When it was incubated at 37°C with a speed of 180 rpm for 96 hr in the flasks, the maximum acetoin concentration was up to 33.90 g/L. The fermentation broths were determined by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) analyses; the results showed that the major metabolite was acetoin, and the purity could reach more than 95% without butanedione and 2,3-butanediol, which were usually produced together with acetoin in other strains. A novel aqueous two-phase system (ATPS) composed of hydrophilic solvents and inorganic salts was developed for the extraction of acetoin from fermentation broths. The ethanol and dipotassium hydrogen phosphate system could be used to extract acetoin from fermentation broths. The influences of phase composition on partition of acetoin were investigated. The maximum partition coefficient (9.68) and recovery (94.6%) of acetoin were obtained, when 25% (w/w) dipotassium hydrogen phosphate and 24% (w/w) ethanol were used.

Enantioselective effects of herbicide imazapyr on Arabidopsis thaliana

The enantioselective toxicity of chiral herbicides in the environment is of increasing concern. To investigate the enantioselective effects of the chiral herbicide imazapyr on target organisms, we exposed Arabidopsis thaliana to imazapyr enantiomers and racemate. The results show that imazapyr was enantioselectively toxic to A. thaliana. The total chlorophyll content in A. thaliana was affected more by (+)-imazapyr than (±)-imazapyr and (-)-imazapyr. Concentrations of proline and malondialdehyde reflected a toxic effect in the order of (+)-imazapyr > (±)-imazapyr > (-)-imazapyr at every concentration. Acetolactate synthase (ALS) activity was inhibited more by (+)-imazapyr than (±)-imazapyr or (-)-imazapyr. At 100 mg L(-1) of imazapyr, ALS activity was 78%, 43%, and 19% with (-)-, (±)-, and (+)-imazapyr, respectively. The results suggest the significant enantioselective toxicity of imazapyr in A. thaliana for greater toxicity with (+)-imazapyr than (±)-imazapyr and (-)-imazapyr, which suggests that (+)-imazapyr has more herbicidal effect.