Combinatorial optimization of synthetic operons for the microbial production of p-coumaryl alcohol with Escherichia coli

Whole-Cell Bioconversion Systems for Efficient Synthesis of Monolignols from L-Tyrosine in Escherichia coli

Monolignols and their derivatives exhibit various pharmaceutical and physiological characteristics, such as antioxidant and anti-inflammatory properties. However, they remain difficult to synthesize. In this study, we engineered several whole-cell bioconversion systems with carboxylate reductase (CAR)-mediated pathways for efficient synthesis of p-coumaryl, caffeyl, and coniferyl alcohols from l-tyrosine in Escherichia coli BL21 (DE3). By overexpressing the l-tyrosine ammonia lyase from Flavobacterium johnsoniae (FjTAL), carboxylate reductase from Segniliparus rugosus (SruCAR), alcohol dehydrogenase YqhD and hydroxylase HpaBC from E. coli, and caffeate 3-O-methyltransferase (COMT) from Arabidopsis thaliana, three enzyme cascades FjTAL-SruCAR-YqhD, FjTAL-SruCAR-YqhD-HpaBC, and FjTAL-SruCAR-YqhD-HpaBC-COMT were constructed to produce 1028.5 mg/L p-coumaryl alcohol, 1015.3 mg/L caffeyl alcohol, and 411.4 mg/L coniferyl alcohol from 1500, 1500, and 1000 mg/L l-tyrosine, with productivities of 257.1, 203.1, and 82.3 mg/L/h, respectively. This work provides an efficient strategy for the biosynthesis of p-coumaryl, caffeyl, and coniferyl alcohols from l-tyrosine.

Phytostilbenes as agrochemicals: biosynthesis, bioactivity, metabolic engineering and biotechnology

Covering: 1976 to 2020. Although constituting a limited chemical family, phytostilbenes represent an emblematic group of molecules among natural compounds. Ever since their discovery as antifungal compounds in plants and their ascribed role in human health and disease, phytostilbenes have never ceased to arouse interest for researchers, leading to a huge development of the literature in this field. Owing to this, the number of references to this class of compounds has reached the tens of thousands. The objective of this article is thus to offer an overview of the different aspects of these compounds through a large bibliography analysis of more than 500 articles. All the aspects regarding phytostilbenes will be covered including their chemistry and biochemistry, regulation of their biosynthesis, biological activities in plants, molecular engineering of stilbene pathways in plants and microbes as well as their biotechnological production by plant cell systems.

From Bugs to Bioplastics: Total (+)-Dihydrocarvide Biosynthesis by Engineered Escherichia coli

The monoterpenoid lactone derivative (+)-dihydrocarvide ((+)-DHCD) can be polymerised to form shape-memory polymers. Synthetic biology routes from simple, inexpensive carbon sources are an attractive, alternative route over chemical synthesis from (R)-carvone. We have demonstrated a proof-of-principle in vivo approach for the complete biosynthesis of (+)-DHCD from glucose in Escherichia coli (6.6 mg L-1 ). The pathway is based on the Mentha spicata route to (R)-carvone, with the addition of an 'ene'-reductase and Baeyer-Villiger cyclohexanone monooxygenase. Co-expression with a limonene synthesis pathway enzyme enables complete biocatalytic production within one microbial chassis. (+)-DHCD was successfully produced by screening multiple homologues of the pathway genes, combined with expression optimisation by selective promoter and/or ribosomal binding-site screening. This study demonstrates the potential application of synthetic biology approaches in the development of truly sustainable and renewable bioplastic monomers.

Engineering stilbene metabolic pathways in microbial cells

Numerous in vitro and in vivo studies on biological activities of phytostilbenes have brought to the fore the remarkable properties of these compounds and their derivatives, making them a top storyline in natural product research fields. However, getting stilbenes in sufficient amounts for routine biological activity studies and make them available for pharmaceutical and/or nutraceutical industry applications, is hampered by the difficulty to source them through synthetic chemistry-based pathways or extraction from the native plants. Hence, microbial cell cultures have rapidly became potent workhorse factories for stilbene production. In this review, we present the combined efforts made during the past 15 years to engineer stilbene metabolic pathways in microbial cells, mainly the Saccharomyces cerevisiae baker yeast, the Escherichia coli and the Corynebacterium glutamicum bacteria. Rationalized approaches to the heterologous expression of the partial or the entire stilbene biosynthetic routes are presented to allow the identification and/or bypassing of the major bottlenecks in the endogenous microbial cell metabolism as well as potential regulations of the genes involved in these metabolic pathways. The contributions of bioinformatics to synthetic biology are developed to highlight their tremendous help in predicting which target genes are likely to be up-regulated or deleted for controlling the dynamics of precursor flows in the tailored microbial cells. Further insight is given to the metabolic engineering of microbial cells with "decorating" enzymes, such as methyl and glycosyltransferases or hydroxylases, which can act sequentially on the stilbene core structure. Altogether, the cellular optimization of stilbene biosynthetic pathways integrating more and more complex constructs up to twelve genetic modifications has led to stilbene titers ranging from hundreds of milligrams to the gram-scale yields from various carbon sources. Through this review, the microbial production of stilbenes is analyzed, stressing both the engineering dynamic regulation of biosynthetic pathways and the endogenous control of stilbene precursors.

Development of a Bacterial Biosensor for Rapid Screening of Yeast p-Coumaric Acid Production

Transcription factor-based biosensors are used to identify producer strains, a critical bottleneck in cell factory engineering. Here, we address two challenges with this methodology: transplantation of heterologous transcriptional regulators into new hosts to generate functional biosensors and biosensing of the extracellular product concentration that accurately reflects the effective cell factory production capacity. We describe the effects of different translation initiation rates on the dynamic range of a p-coumaric acid biosensor based on the Bacillus subtilis transcriptional repressor PadR by varying its ribosomal binding site. Furthermore, we demonstrate the functionality of this p-coumaric acid biosensor in Escherichia coli and Corynebacterium glutamicum. Finally, we encapsulate yeast p-coumaric acid-producing cells with E. coli-biosensing cells in picoliter droplets and, in a microfluidic device, rapidly sort droplets containing yeast cells producing high amounts of extracellular p-coumaric acid using the fluorescent E. coli biosensor signal. As additional biosensors become available, such approaches will find broad applications for screening of an extracellular product.

Reversal of β-oxidative pathways for the microbial production of chemicals and polymer building blocks

β-Oxidation is the ubiquitous metabolic strategy to break down fatty acids. In the course of this four-step process, two carbon atoms are liberated per cycle from the fatty acid chain in the form of acetyl-CoA. However, typical β-oxidative strategies are not restricted to monocarboxylic (fatty) acid degradation only, but can also be involved in the utilization of aromatic compounds, amino acids and dicarboxylic acids. Each enzymatic step of a typical β-oxidation cycle is reversible, offering the possibility to also take advantage of reversed metabolic pathways for applied purposes. In such cases, 3-oxoacyl-CoA thiolases, which catalyze the final chain-shortening step in the catabolic direction, mediate the condensation of an acyl-CoA starter molecule with acetyl-CoA in the anabolic direction. Subsequently, the carbonyl-group at C3 is stepwise reduced and dehydrated yielding a chain-elongated product. In the last years, several β-oxidation pathways have been studied in detail and reversal of these pathways already proved to be a promising strategy for the production of chemicals and polymer building blocks in several industrially relevant microorganisms. This review covers recent advancements in this field and discusses constraints and bottlenecks of this metabolic strategy in comparison to alternative production pathways.

Production of Cinnamyl Alcohol Glucoside from Glucose in Escherichia coli

Rosin, a cinnamyl alcohol glucoside, is one of the important ingredients in Rhodiola rosea, which is a valuable medicinal herb used for centuries. Rosin displayed multiple biological activities. The traditional method for producing rosin and derivatives is direct extraction from R. rosea, which suffers from limited availability of natural resources and complicated purification procedure. This work achieved de novo biosynthesis of rosin in Escherichia coli. First, a biosynthetic pathway of aglycon cinnamyl alcohol from phenylalanine was constructed. Subsequently, the UGT genes from Rhodiola sachalinensis (UGT73B6) or Arabidopsis thaliana (UGT73C5) were introduced into the above recombinant E. coli strain to produce rosin. Then the phenylalanine metabolic pathway of E. coli was optimized by genetic manipulation, and the production of rosin by the engineered E. coli reached 258.5 ± 8.8 mg/L. This study lays a significant foundation for microbial production of rosin and its derivatives using glucose as the renewable carbon source.

Functional expression of plant-derived O-methyltransferase, flavanone 3-hydroxylase, and flavonol synthase in Corynebacterium glutamicum for production of pterostilbene, kaempferol, and quercetin

Plant polyphenols receive significant attention due to their anti-oxidative and health-promoting properties, and several microorganisms are currently engineered towards producing these valuable compounds. Previously, Corynebacterium glutamicum has been engineered for synthesizing polyphenol core structures such as the stilbene resveratrol and the (2S)-flavanone naringenin. Decoration of these compounds by O-methylation or hydroxylation would provide access to polyphenols of even higher commercial interest. In this study, introduction of a heterologous O-methyltransferase into a resveratrol-producing C. glutamicum strain allowed synthesis of 42mg/L (0.16mM) of the di-O-methylated pterostilbene from p-coumaric acid. A prerequisite for reaching this product titer was a fusion of O-methyltransferase with the maltose-binding protein of Escherichia coli lacking its signal peptide, thereby increasing the solubility of the O-methyltransferase. Furthermore, expression of heterologous dioxygenase genes in (2S)-flavanone-producing C. glutamicum strains enabled the production of flavanonols and flavonols starting from the phenylpropanoids p-coumaric acid and caffeic acid. For the flavonols kaempferol and quercetin, maximum product titers of 23mg/L (0.08mM) and 10mg/L (0.03mM) could be achieved, respectively. The obtained results demonstrate that C. glutamicum is a suitable host organism for the production of more complex plant polyphenols.

Screening through the PLICable promoter toolbox enhances protein production in Escherichia coli

Escherichia coli is a common host for recombinant protein production in which production titers are highly dependent on the employed expression system. Promoters are thereby a key element to control gene expression levels. In this study, a novel PLICable promoter toolbox was developed which enables in a single cloning step and after a screening experiment to identify out of ten IPTG-inducible promoters (T7, A3, lpp, tac, pac, Sp6, lac, npr, trc and syn) the most suitable one for high level protein production. The target gene is cloned under the control of different promoters in a single and efficient cloning step using the ligase-free cloning method PLICing (phosphorothioate-based ligase-independent gene cloning). The promoter toolbox was firstly validated using three well producible proteins (a cellulase from a metagenome library, a phytase from Yersinia mollaretii and an alcohol dehydrogenase from Pseudomonas putida) and then applied to two enzymes (3D1 DNA polymerase and glutamate dehydrogenase mutant) which are poorly produced in E. coli. By applying our PLICable pET-promoter toolbox, the authors were able to increase production by two-fold for 3D1 DNA polymerase (lac promoter) and 29-fold for glutamate dehydrogenase mutant H52Y (trc promoter).

Exploiting members of the BAHD acyltransferase family to synthesize multiple hydroxycinnamate and benzoate conjugates in yeast

BACKGROUND

BAHD acyltransferases, named after the first four biochemically characterized enzymes of the group, are plant-specific enzymes that catalyze the transfer of coenzyme A-activated donors onto various acceptor molecules. They are responsible for the synthesis in plants of a myriad of secondary metabolites, some of which are beneficial for humans either as therapeutics or as specialty chemicals such as flavors and fragrances. The production of pharmaceutical, nutraceutical and commodity chemicals using engineered microbes is an alternative, green route to energy-intensive chemical syntheses that consume petroleum-based precursors. However, identification of appropriate enzymes and validation of their functional expression in heterologous hosts is a prerequisite for the design and implementation of metabolic pathways in microbes for the synthesis of such target chemicals.

RESULTS

For the synthesis of valuable metabolites in the yeast Saccharomyces cerevisiae, we selected BAHD acyltransferases based on their preferred donor and acceptor substrates. In particular, BAHDs that use hydroxycinnamoyl-CoAs and/or benzoyl-CoA as donors were targeted because a large number of molecules beneficial to humans belong to this family of hydroxycinnamate and benzoate conjugates. The selected BAHD coding sequences were synthesized and cloned individually on a vector containing the Arabidopsis gene At4CL5, which encodes a promiscuous 4-coumarate:CoA ligase active on hydroxycinnamates and benzoates. The various S. cerevisiae strains obtained for co-expression of At4CL5 with the different BAHDs effectively produced a wide array of valuable hydroxycinnamate and benzoate conjugates upon addition of adequate combinations of donors and acceptor molecules. In particular, we report here for the first time the production in yeast of rosmarinic acid and its derivatives, quinate hydroxycinnamate esters such as chlorogenic acid, and glycerol hydroxycinnamate esters. Similarly, we achieved for the first time the microbial production of polyamine hydroxycinnamate amides; monolignol, malate and fatty alcohol hydroxycinnamate esters; tropane alkaloids; and benzoate/caffeate alcohol esters. In some instances, the additional expression of Flavobacterium johnsoniae tyrosine ammonia-lyase (FjTAL) allowed the synthesis of p-coumarate conjugates and eliminated the need to supplement the culture media with 4-hydroxycinnamate.

CONCLUSION

We demonstrate in this study the effectiveness of expressing members of the plant BAHD acyltransferase family in yeast for the synthesis of numerous valuable hydroxycinnamate and benzoate conjugates.

Radicals from the gas-phase pyrolysis of a lignin model compound: p-coumaryl alcohol

The intermediate radicals produced in the gas-phase pyrolysis of one of the main building blocks of lignin - p-coumaryl alcohol (p-CMA) - were investigated using the low temperature matrix isolation technique interfaced with electron paramagnetic resonance spectroscopy (LTMI-EPR). An anisotropic EPR spectrum characterized by a high g-value (>2.0080) and a relatively low saturation coefficient (∼1.40) throughout the high pyrolytic temperature region (700 to 1000 °C) was observed. Theoretical calculations revealed plausible decomposition pathways for p-CMA comprising highly delocalized aromatic radicals. The results provide evidence for a dominant role of oxygen-centered radicals during the pyrolysis of p-CMA.