Genome shuffling and high-throughput screening of Brevibacterium flavum MDV1 for enhanced L-valine production

Multiple Functions of Compatible Solute Ectoine and Strategies for Constructing Overproducers for Biobased Production

Ectoine and its derivative 5-hydroxyectoine are compatible solutes initially found in the hyperhalophilic bacterium Ectothiorhodospira halochloris, which inhabits the desert in Egypt. The habitat of ectoine producers implies the primary function of ectoine as a cytoprotectant against harsh conditions such as high salinity, drought, and high radiation. More extensive and in-depth studies have revealed the multiple functions of ectoine in its native producer bacterial cells and other types of cells and its biomolecular components (such as proteins and DNA) as a general protective agent. Its chemical properties as a bio-based amino acid derivative make it attractive for basic scientific research and related industries, such as the food/agricultural industry, cosmetic manufacturing, biologics, and therapeutic agent preparation. This article first discusses the functions and applications of ectoine and 5-hydroxyectoine. Subsequently, more emphasis was placed on advances in bio-based ectoine and/or 5-hydroxyectoine production. Strategies for developing more robust cell factories for highly efficient ectoine and/or 5-hydroxyectoine production are further discussed. We hope this review will provide a valuable reference for studies on the bio-based production of ectoine and 5-hydroxyectoine.

High-level production of L-valine in Escherichia coli using multi-modular engineering

L-valine is a valuable amino acid in mammals that is used as the main component of feed additives. The low efficiency of the fermentation titer limits the industrial application of L-valine. Here, an L-valine-producing strain of Escherichia coli was obtained using a multi-modular strategy. Initially, a chassis strain was generated by mutagenesis and high-throughput screening. The L-valine biosynthetic pathway and transport module were modified to improve the L-valine titer. Subsequently, the transcription factors associated with L-valine biosynthesis were investigated. Overexpression of PdhR and inhibition of the expression of RpoS promoted L-valine synthesis. Finally, the NADPH supply was enhanced after the introduction of the heterologous Entner-Doudoroff (ED) pathway from Zymomonas mobilis. The strain VAL38 produced 92 g/L L-valine in a 5-L bioreactor with a yield of 0.34 g/g glucose. This strategy is provided as a reference for improving the production performance of cell factories for L-valine and its derivatives.

Engineering of microbial cells for L-valine production: challenges and opportunities

L-valine is an essential amino acid that has wide and expanding applications with a suspected growing market demand. Its applicability ranges from animal feed additive, ingredient in cosmetic and special nutrients in pharmaceutical and agriculture fields. Currently, fermentation with the aid of model organisms, is a major method for the production of L-valine. However, achieving the optimal production has often been limited because of the metabolic imbalance in recombinant strains. In this review, the constrains in L-valine biosynthesis are discussed first. Then, we summarize the current advances in engineering of microbial cell factories that have been developed to address and overcome major challenges in the L-valine production process. Future prospects for enhancing the current L-valine production strategies are also discussed.

Applications and research advance of genome shuffling for industrial microbial strains improvement

Genome shuffling, an efficient and practical strain improvement technology via recursive protoplasts fusion, can break through the limits of species even genus to accelerate the directed evolution of microbial strains, without requiring the comprehensively cognized genetic background and operable genetic system. Hence this technology has been widely used for many important strains to obtain the desirable industrial phenotypes. In this review, we introduce the procedure of genome shuffling, discuss the new aid strategies of genome shuffling, summarize the applications of genome shuffling for increasing metabolite yield, improving strain tolerance, enhancing substrate utilization, and put forward the outlook to the future development of this technology.

2,3-Dihydroxyisovalerate production by Klebsiella pneumoniae

2,3-Dihydroxyisovalerate is an intermediate of valine and leucine biosynthesis pathway; however, no natural microorganism has been found yet that can accumulate this compound. Klebsiella pneumoniae is a useful bacterium that can be used as a workhorse for the production of a range of industrially desirable chemicals. Dihydroxy acid dehydratase, encoded by the ilvD gene, catalyzes the reaction of 2-ketoisovalerate formation from 2,3-dihydroxyisovalerate. In this study, an ilvD disrupted strain was constructed which resulted in the inability to synthesize 2-ketoisovalerate, yet accumulate 2,3-dihydroxyisovalerate in its culture broth. 2,3-Butanediol is the main metabolite of K. pneumoniae and its synthesis pathway and the branched-chain amino acid synthesis pathway share the same step of the α-acetolactate synthesis. By knocking out the budA gene, carbon flow into the branched-chain amino acid synthesis pathway was upregulated, which resulted in a distinct increase in 2,3-dihydroxyisovalerate levels. Lactic acid was identified as a by-product of the process and by blocking the lactic acid synthesis pathway, a further increase in 2,3-dihydroxyisovalerate levels was obtained. The culture parameters of 2,3-dihydroxyisovalerate fermentation were optimized, which include acidic pH and medium level oxygen supplementation to favor 2,3-dihydroxyisovalerate synthesis. At optimal conditions (pH 6.5, 400 rpm), 36.5 g/L of 2,3-dihydroxyisovalerate was produced in fed-batch fermentation over 45 h, with a conversion ratio of 0.49 mol/mol glucose. Thus, a biological route of 2,3-dihydroxyisovalerate production with high conversion ratio and final titer was developed, providing a basis for an industrial process. Key Points • A biological route of 2,3-dihydroxyisovalerate production was setup. • Disruption of budA causes 2,3-dihydroxuisovalerate accumulation in K. pneumoniae. • Disruption of ilvD prevents 2,3-dihydroxyisovalerate reuse by the cell. • 36.5 g/L of 2,3-dihydroxyisovalerate was obtained in fed-batch fermentation.

Microbial Engineering for Production of N-Functionalized Amino Acids and Amines

N-functionalized amines play important roles in nature and occur, for example, in the antibiotic vancomycin, the immunosuppressant cyclosporine, the cytostatic actinomycin, the siderophore aerobactin, the cyanogenic glucoside linamarin, and the polyamine spermidine. In the pharmaceutical and fine-chemical industries N-functionalized amines are used as building blocks for the preparation of bioactive molecules. Processes based on fermentation and on enzyme catalysis have been developed to provide sustainable manufacturing routes to N-alkylated, N-hydroxylated, N-acylated, or other N-functionalized amines including polyamines. Metabolic engineering for provision of precursor metabolites is combined with heterologous N-functionalizing enzymes such as imine or ketimine reductases, opine or amino acid dehydrogenases, N-hydroxylases, N-acyltransferase, or polyamine synthetases. Recent progress and applications of fermentative processes using metabolically engineered bacteria and yeasts along with the employed enzymes are reviewed and the perspectives on developing new fermentative processes based on insight from enzyme catalysis are discussed.