Citrate Stimulates Oligosaccharide Synthesis in Metabolically Engineered Agrobacterium sp

Biosynthesis and applications of curdlan

Curdlan is widely applied in the food and pharmaceutical industries. This review focuses on the biosynthetic pathways, regulatory mechanisms and metabolic engineering strategies for curdlan production. Firstly, curdlan biosynthesis is discussed. Furthermore, various strategies to increase curdlan production are summarized from four aspects, including the overexpression of genes for curdlan biosynthesis, weakening/knockdown of genes from competing pathways, increasing the supply of curdlan precursors, and optimization of fermentation conditions. Moreover, the emerging and advanced applications of curdlan are introduced. Finally, the challenges that are frequently encountered during curdlan biosynthesis are noted with a discussion of directions for curdlan production.

Improvement of glycerol catabolism in Bacillus licheniformis for production of poly-γ-glutamic acid

Bacillus licheniformis WX-02 is a well-studied strain to produce poly-γ-glutamic acid (γ-PGA) with numerous applications. This study is to improve WX-02 strain's capability of assimilating glycerol, a major byproduct of biofuels industries, through metabolic manipulation. Through gene knockout, the GlpK pathway was identified as the sole functional glycerol catabolism pathway, while the DhaK pathway was inactive for this strain under either aerobic or anaerobic conditions. The enhancement of glycerol utilization was attempted by substituting the native glpFK promoter with the constitutive promoter (P43), ytzE promoter (PytzE), and bacABC operon promoter (PbacA), respectively. The glycerol consumptions of the corresponding mutant strains WX02-P43glpFK, WX02-PytzEglpFK, and WX02-PbacAglpFK were 30.9, 26.42, and 18.8% higher than that of the WX-02 strain, respectively. The γ-PGA concentrations produced by the three mutant strains were 33.71, 23.39, and 30.05% higher than that of WX-02 strain, respectively. When biodiesel-derived crude glycerol was used as the carbon source, the mutant WX02-P43glpFK produced 16.63 g L^-1 of γ-PGA, with a productivity of 0.35 g L^-1 h^-1. Collectively, this study demonstrated that glycerol can be used as an effective substrate for producing γ-PGA by metabolic engineering B. licheniformis strains.

Identification and High-level Production of Pulcherrimin in Bacillus licheniformis DW2

Pulcherrimin, a potential biocontrol agent produced by microorganisms, has the promising applications in the agricultural, medical, and food areas, and the low yield of pulcherrimin has hindered its applications. In this study, the red pigment produced by Bacillus licheniformis DW2 was identified as pulcherrimin through the spectrometry analysis and genetic manipulation, and the component of the medium used for pulcherrimin production was optimized. Based on our results, the addition of 1.0 g L^-1 Tween 80 could improve the yield of pulcherrimin, and glucose and (NH₄)₂SO₄ were served as the optimal carbon and nitrogen sources for pulcherrimin synthesis, respectively. Furthermore, an orthogonal array design was applied for optimization of the medium. Under optimized condition, the maximum yield of pulcherrimin was 331.17 mg L^-1, 5.30-fold higher than that of the initial condition, which was the maximum yield reported for pulcherrimin production. Collectively, this study provided a promising strain and a feasible approach to achieve the high-level production of antimicrobial pulcherrimin.

Metabolic engineering of Agrobacterium sp. ATCC31749 for curdlan production from cellobiose

Curdlan is a commercial polysaccharide made by fermentation of Agrobacterium sp. Its anticipated expansion to larger volume markets demands improvement in its production efficiency. Metabolic engineering for strain improvement has so far been limited due to the lack of genetic tools. This research aimed to identify strong promoters and to engineer a strain that converts cellobiose efficiently to curdlan. Three strong promoters were identified and were used to install an energy-efficient cellobiose phosphorolysis mechanism in a curdlan-producing strain. The engineered strains were shown with enhanced ability to utilize cellobiose, resulting in a 2.5-fold increase in titer. The availability of metabolically engineered strain capable of producing β-glucan from cellobiose paves the way for its production from cellulose. The identified native promoters from Agrobacterium open up opportunities for further metabolic engineering for improved production of curdlan and other products. The success shown here marks the first such metabolic engineering effort in this microbe.

The sweet branch of metabolic engineering: cherry-picking the low-hanging sugary fruits

In the first science review on the then nascent Metabolic Engineering field in 1991, Dr. James E. Bailey described how improving erythropoietin (EPO) glycosylation can be achieved via metabolic engineering of Chinese hamster ovary (CHO) cells. In the intervening decades, metabolic engineering has brought sweet successes in glycoprotein engineering, including antibodies, vaccines, and other human therapeutics. Today, not only eukaryotes (CHO, plant, insect, yeast) are being used for manufacturing protein therapeutics with human-like glycosylation, newly elucidated bacterial glycosylation systems are enthusiastically embraced as potential breakthrough to revolutionize the biopharmaceutical industry. Notwithstanding these excitement in glycoprotein, the sweet metabolic engineering reaches far beyond glycoproteins. Many different types of oligo- and poly-saccharides are synthesized with metabolically engineered cells. For example, several recombinant hyaluronan bioprocesses are now in commercial production, and the titer of 2′-fucosyllactose, the most abundant fucosylated trisaccharide in human milk, reaches over 20 g/L with engineered E. coli cells. These successes represent only the first low hanging fruits, which have been appreciated scientifically, medically and fortunately, commercially as well. As one of the four building blocks of life, sugar molecules permeate almost all aspects of life. They are also unique in being intimately associated with all major types of biopolymers (including DNA/RNA, proteins, lipids) meanwhile they stand alone as bioactive polysaccharides, or free soluble oligosaccharides. As such, all sugar moieties in biological components, small or big and free or bound, are important targets for metabolic engineering. Opportunities abound at the interface of glycosciences and metabolic engineering. Continued investment and successes in this branch of metabolic engineering will make vastly diverse sugar-containing molecules (a.k.a. glycoconjugates) available for biomedical applications, sustainable technology development, and as invaluable tools for basic scientific research. This short review focuses on the most recent development in the field, with emphasis on the synthesis technology for glycoprotein, polysaccharide, and oligosaccharide.