Biotechnology of riboflavin

Facilitating stable gene integration expression and copy number amplification in Bacillus subtilis through a reversible homologous recombination switch

Strengthening the expression level of integrated genes on the genome is crucial for consistently expressing key enzymes in microbial cell factories for efficient bioproduction in synthetic biology. In comparison to plasmid-based multi-copy expression, the utilization of chromosomal multi-copy genes offers increased stability of expression level, diminishes the metabolic burden on host cells, and enhances overall genetic stability. In this study, we developed the "BacAmp", a stabilized gene integration expression and copy number amplification system for high-level expression in Bacillus subtilis, which was achieved by employing a combination of repressor and non-natural amino acids (ncAA)-dependent expression system to create a reversible switch to control the key gene recA for homologous recombination. When the reversible switch is turned on, genome editing and gene amplification can be achieved. Subsequently, the reversible switch was turned off therefore stabilizing the gene copy number. The stabilized gene amplification system marked by green fluorescent protein, achieved a 3-fold increase in gene expression by gene amplification and maintained the average gene copy number at 10 after 110 generations. When we implemented the gene amplification system for the regulation of N-acetylneuraminic acid (NeuAc) synthesis, the copy number of the critical gene increased to an average of 7.7, which yielded a 1.3-fold NeuAc titer. Our research provides a new avenue for gene expression in synthetic biology and can be applied in metabolic engineering in B. subtilis.

Self-association as a solubility limiting factor of riboflavin in aqueous media

Single crystal X-ray diffraction analysis of needle shaped riboflavin (RF) crystals revealed π-stacking of RF's isoalloxazine units (distance: 3.643-3.313 Å) with syn-orientated ribityl chains. In line with this, classical molecular dynamics (MD) (400 ns) using an isobaric-isothermal (NPT) ensemble of eight RF in a water box (〈V〉 ∼ 508.62 nm3, 〈p〉 = 1.11 bar) revealed anti-aligned aggregation of RF in water (COM-distance: 4 Å). Comparing umbrella sampling for the separation of two RF molecules to the separation of two lumichrome molecules, the similar mean potential force for the separation of RF and lumichrome (22.8 kJ mol-1; 24.4 kJ mol-1) proved dispersive interactions as the origin of RF's aggregation. Though stacking of RF is the major water-solubility limiting factor, the conformation of RF's ribityl chain may alter the solubility in water. Both, MD (in water) and COSMO-RS (in water continuum) predicted that conformations of RF with an extended ribityl chain are thermodynamically preferred over any conformations with internal hydrogen bonds between hydroxyl groups and nitrogen/oxygen atoms of the pyrimidine moiety of the flavin ring. Interestingly, COSMO-RS predicted the solubility of the extended conformation to be significantly lower than the latter leading to the very low average solubility of RF. Nuclear Overhauser effect measurements (NOESY) of the structurally related sodium riboflavin 5'-monophosphate (RF-PO4) in deuterium oxide confirmed π-stacking of the isoalloxazine rings. In conformity with the 350 times higher water-solubility of RF-PO4, NOESY also indicated a contorted conformation of the ribityl phosphate chain, whereas, for RF, indications for a contorted chain were not observed.

De novo engineering riboflavin production Bacillus subtilis by overexpressing the downstream genes in the purine biosynthesis pathway

BACKGROUND

Bacillus subtilis is widely used in industrial-scale riboflavin production. Previous studies have shown that targeted mutagenesis of the ribulose 5-phosphate 3-epimerase in B. subtilis can significantly enhance riboflavin production. This modification also leads to an increase in purine intermediate concentrations in the medium. Interestingly, B. subtilis exhibits remarkable efficiency in purine nucleoside synthesis, often exceeding riboflavin yields. These observations highlight the importance of the conversion steps from inosine-5'-monophosphate (IMP) to 2,5-diamino-6-ribosylamino-4(3 H)-pyrimidinone-5'-phosphate (DARPP) in riboflavin production by B. subtilis. However, research elucidating the specific impact of these reactions on riboflavin production remains limited.

RESULT

We expressed the genes encoding enzymes involved in these reactions (guaB, guaA, gmk, ndk, ribA) using a synthetic operon. Introduction of the plasmid carrying this synthetic operon led to a 3.09-fold increase in riboflavin production compared to the control strain. Exclusion of gmk from the synthetic operon resulted in a 36% decrease in riboflavin production, which was further reduced when guaB and guaA were not co-expressed. By integrating the synthetic operon into the genome and employing additional engineering strategies, we achieved riboflavin production levels of 2702 mg/L. Medium optimization further increased production to 3477 mg/L, with a yield of 0.0869 g riboflavin per g of sucrose.

CONCLUSION

The conversion steps from IMP to DARPP play a critical role in riboflavin production by B. subtilis. Our overexpression strategies have demonstrated their effectiveness in overcoming these limiting factors and enhancing riboflavin production.

Chiral Supramolecular Organogel Constructed Using Riboflavin and Melamine: Its Application in Photo-Catalyzed Colorimetric Chiral Sensing and Enantioselective Adsorption

The synthesis of a chiral supramolecular organogel via the hierarchical helical self-assembly of optically active riboflavin and melamine derivatives is described herein. Owing to the photocatalysis of riboflavin and the supramolecular chirality induced in the helically stacked riboflavin/melamine complex, the gel is observed to act as a light-stimulated chiral sensor of optically active alcohols by detecting the change in color from yellow to green. The gel also served as an efficient chiral adsorbent, enabling optical resolution of a racemic compound with high chiral recognition ability.

Fabrication of multifunctional bio-macromolecule organic-inorganic hybrid system for protein silk: Photochromic, UV protection, fire-proof and super durability

Nowadays, high value-added and multifunctional textiles have attracted widespread attention due to the changing demands of modern life. This study focused on the fabrication of silk with photochromism, flame retardancy, UV resistance and durability using riboflavin sodium phosphate (RSP) and various metal ions (Fe2+, Fe3+, Al3+, and Ti4+). Attractively, the photochromic performance was one of the most distinctive features of the modified silk, and the yellow silk fabric turned into fluorescent green under UV lamp. After a detailed comparison, it was determined that RSP/Fe3+ hybrid system was most effective in improving anti-UV performance of the silk with a high UPF of 25.8, achieving a "Good" level of UV protection. Specifically, it achieved a B1 fire protection with a low damaged-length of 9.4 cm and a high LOI of 28.3 %. Additionally, the modified silk showed the lowest smoke density, reducing by approximately 84.1 % versus that of pristine silk. Moreover, the modified silk was able to meet the B1 classification and the "Good" UV protection requirements even after 75 washing cycles, making it more durable than most functional textiles reported. The further analysis indicated that RSP and metal ions can synergistically enhance the condensed-phase action, thereby improving the fire resistance of silk.

Biomaterial composed of chitosan, riboflavin, and hydroxyapatite for bone tissue regeneration

Biomaterial engineering approaches involve using a combination of miscellaneous bioactive molecules which may promote cell proliferation and, thus, form a scaffold with the environment that favors the regeneration process. Chitosan, a naturally occurring biodegradable polymer, possess some essential features, i.e., biodegradability, biocompatibility, and in the solid phase good porosity, which may contribute to promote cell adhesion. Moreover, doping of the materials with other biocompounds will create a unique and multifunctional scaffold that will be useful in regenerative medicine. This study is focused on the manufacturing and characterization of composite materials based on chitosan, hydroxyapatite, and riboflavin. The resulting films were fabricated by the casting/solvent evaporation method. Morphological and spectroscopy analyses of the films revealed a porous structure and an interconnection between chitosan and apatite. The composite material showed an inhibitory effect on Staphylococcus aureus and exhibited higher antioxidant activity compared to pure chitosan. In vitro studies on riboflavin showed increased cell proliferation and migration of fibroblasts and osteosarcoma cells, thus demonstrating their potential for bone tissue engineering applications.

Bacillus subtilis, a Swiss Army Knife in Science and Biotechnology

Next to Escherichia coli, Bacillus subtilis is the most studied and best understood organism that also serves as a model for many important pathogens. Due to its ability to form heat-resistant spores that can germinate even after very long periods of time, B. subtilis has attracted much scientific interest. Another feature of B. subtilis is its genetic competence, a developmental state in which B. subtilis actively takes up exogenous DNA. This makes B. subtilis amenable to genetic manipulation and investigation. The bacterium was one of the first with a fully sequenced genome, and it has been subject to a wide variety of genome- and proteome-wide studies that give important insights into many aspects of the biology of B. subtilis. Due to its ability to secrete large amounts of proteins and to produce a wide range of commercially interesting compounds, B. subtilis has become a major workhorse in biotechnology. Here, we review the development of important aspects of the research on B. subtilis with a specific focus on its cell biology and biotechnological and practical applications from vitamin production to concrete healing. The intriguing complexity of the developmental programs of B. subtilis, paired with the availability of sophisticated tools for genetic manipulation, positions it at the leading edge for discovering new biological concepts and deepening our understanding of the organization of bacterial cells.

Involvement of a flavoprotein, acetohydroxyacid synthase, in growth and riboflavin production in riboflavin-overproducing Ashbya gossypii mutant

BACKGROUND

Previously, we isolated a riboflavin-overproducing Ashbya gossypii mutant (MT strain) and discovered some mutations in genes encoding flavoproteins. Here, we analyzed the riboflavin production in the MT strain, in view of flavoproteins, which are localized in the mitochondria.

RESULTS

In the MT strain, mitochondrial membrane potential was decreased compared with that in the wild type (WT) strain, resulting in increased reactive oxygen species. Additionally, diphenyleneiodonium (DPI), a universal flavoprotein inhibitor, inhibited riboflavin production in the WT and MT strains at 50 µM, indicating that some flavoproteins may be involved in riboflavin production. The specific activities of NADH and succinate dehydrogenases were significantly reduced in the MT strain, but those of glutathione reductase and acetohydroxyacid synthase were increased by 4.9- and 25-fold, respectively. By contrast, the expression of AgGLR1 gene encoding glutathione reductase was increased by 32-fold in the MT strain. However, that of AgILV2 gene encoding the catalytic subunit of acetohydroxyacid synthase was increased by only 2.1-fold. These results suggest that in the MT strain, acetohydroxyacid synthase, which catalyzes the first reaction of branched-chain amino acid biosynthesis, is vital for riboflavin production. The addition of valine, which is a feedback inhibitor of acetohydroxyacid synthase, to a minimal medium inhibited the growth of the MT strain and its riboflavin production. In addition, the addition of branched-chain amino acids enhanced the growth and riboflavin production in the MT strain.

CONCLUSION

The significance of branched-chain amino acids for riboflavin production in A. gossypii is reported and this study opens a novel approach for the effective production of riboflavin in A. gossypii.

Metabolic engineering of thermophilic Bacillus methanolicus for riboflavin overproduction from methanol

The growing need of next generation feedstocks for biotechnology spurs an intensification of research on the utilization of methanol as carbon and energy source for biotechnological processes. In this paper, we introduced the methanol-based overproduction of riboflavin into metabolically engineered Bacillus methanolicus MGA3. First, we showed that B. methanolicus naturally produces small amounts of riboflavin. Then, we created B. methanolicus strains overexpressing either homologous or heterologous gene clusters encoding the riboflavin biosynthesis pathway, resulting in riboflavin overproduction. Our results revealed that the supplementation of growth media with sublethal levels of chloramphenicol contributes to a higher plasmid-based riboflavin production titre, presumably due to an increase in plasmid copy number and thus biosynthetic gene dosage. Based on this, we proved that riboflavin production can be increased by exchanging a low copy number plasmid with a high copy number plasmid leading to a final riboflavin titre of about 523 mg L^-1 in methanol fed-batch fermentation. The findings of this study showcase the potential of B. methanolicus as a promising host for methanol-based overproduction of extracellular riboflavin and serve as basis for metabolic engineering of next generations of riboflavin overproducing strains.

Enhancing the biosynthesis of riboflavin in the recombinant Escherichia coli BL21 strain by metabolic engineering

In this study, to construct the riboflavin-producing strain R1, five key genes, ribA, ribB, ribC, ribD, and ribE, were cloned and ligated to generate the plasmid pET-AE, which was overexpressed in Escherichia coli BL21. The R1 strain accumulated 182.65 ± 9.04 mg/l riboflavin. Subsequently, the R2 strain was constructed by the overexpression of zwf harboring the constructed plasmid pAC-Z in the R1 strain. Thus, the level of riboflavin in the R2 strain increased to 319.01 ± 20.65 mg/l (74.66% increase). To further enhance ribB transcript levels and riboflavin production, the FMN riboswitch was deleted from E. coli BL21 with CRISPR/Cas9 to generate the R3 strain. The R4 strain was constructed by cotransforming pET-AE and pAC-Z into the R3 strain. Compared to those of E. coli BL21, the ribB transcript levels of R2 and R4 improved 2.78 and 3.05-fold, respectively. The R4 strain accumulated 437.58 ± 14.36 mg/l riboflavin, increasing by 37.17% compared to the R2 strain. These results suggest that the deletion of the FMN riboswitch can improve the transcript level of ribB and facilitate riboflavin production. A riboflavin titer of 611.22 ± 11.25 mg/l was achieved under the optimal fermentation conditions. Ultimately, 1574.60 ± 109.32 mg/l riboflavin was produced through fed-batch fermentation with 40 g/l glucose. This study contributes to the industrial production of riboflavin by the recombinant E. coli BL21.

Metabolic Engineering of Bacillus subtilis for Riboflavin Production: A Review

Riboflavin (vitamin B₂) is one of the essential vitamins that the human body needs to maintain normal metabolism. Its biosynthesis has become one of the successful models for gradual replacement of traditional chemical production routes. B. subtilis is characterized by its short fermentation time and high yield, which shows a huge competitive advantage in microbial fermentation for production of riboflavin. This review summarized the advancements of regulation on riboflavin production as well as the synthesis of two precursors of ribulose-5-phosphate riboflavin (Ru5P) and guanosine 5'-triphosphate (GTP) in B. subtilis. The different strategies to improve production of riboflavin by metabolic engineering were also reviewed.

Photoantimicrobials in agriculture

Classical approaches for controlling plant pathogens may be impaired by the development of pathogen resistance to chemical pesticides and by limited availability of effective antimicrobial agents. Recent increases in consumer awareness of and/or legislation regarding environmental and human health, and the urgent need to improve food security, are driving increased demand for safer antimicrobial strategies. Therefore, there is a need for a step change in the approaches used for controlling pre- and post-harvest diseases and foodborne human pathogens. The use of light-activated antimicrobial substances for the so-called antimicrobial photodynamic treatment is known to be effective not only in a clinical context, but also for use in agriculture to control plant-pathogenic fungi and bacteria, and to eliminate foodborne human pathogens from seeds, sprouted seeds, fruits, and vegetables. Here, we take a holistic approach to review and re-evaluate recent findings on: (i) the ecology of naturally-occurring photoantimicrobials, (ii) photodynamic processes including the light-activated antimicrobial activities of some plant metabolites, and (iii) fungus-induced photosensitization of plants. The inhibitory mechanisms of both natural and synthetic light-activated substances, known as photosensitizers, are discussed in the contexts of microbial stress biology and agricultural biotechnology. Their modes-of-antimicrobial action make them neither stressors nor toxins/toxicants (with specific modes of poisonous activity), but a hybrid/combination of both. We highlight the use of photoantimicrobials for the control of plant-pathogenic fungi and quantify their potential contribution to global food security.

Pigment produced from Arcopilus aureus isolated from grapevines: Promising natural yellow colorants for the food industry

Interest in the use of natural non-toxic pigments by the food industry has grown. Some filamentous fungi are producers of natural pigments that are more stable at temperature and pH than other pigments also classified as natural, such as those produced by plants. Production potential of natural pigments by endophytic fungi from grapevines was evaluated. Arcopilus aureus was selected as a potential source for a yellow pigment, which was characterized and tested for stability to variations in temperature and pH. Components, cochlioquinol II and riboflavin, were detected, which has not previously been reported in A. aureus. The pigment was stable and showed increased absorption at lower / acidic pH. These results provide information on the potential of this fungus and a yellow pigment for the first time, which can be used for further development and industrial application.

Riboflavin synthesis from gaseous nitrogen and carbon dioxide by a hybrid inorganic-biological system

Microbes can provide a more sustainable and energy-efficient method of food and nutrient production compared to plant and animal sources, but energy-intensive carbon (e.g., sugars) and nitrogen (e.g., ammonia) inputs are required. Gas-fixing microorganisms that can grow on H₂ from renewable water splitting and gaseous CO₂ and N₂ offer a renewable path to overcoming these limitations but confront challenges owing to the scarcity of genetic engineering in such organisms. Here, we demonstrate that the hydrogen-oxidizing carbon- and nitrogen-fixing microorganism Xanthobacter autotrophicus grown on a CO₂/N₂/H₂ gas mixture can overproduce the vitamin riboflavin (vitamin B₂). We identify plasmids and promoters for use in this bacterium and employ a constitutive promoter to overexpress riboflavin pathway enzymes. Riboflavin production is quantified at 15 times that of the wild-type organism. We demonstrate that riboflavin overproduction is maintained when the bacterium is grown under hybrid inorganic-biological conditions, in which H₂ from water splitting, along with CO₂ and N₂, is fed to the bacterium, establishing the viability of the approach to sustainably produce food and nutrients.

Ascomycota as a source of natural colorants

In the last few decades, there has been a great demand for natural colorants. Synthetic colorants are known to be easy to produce, are less expensive, and remain stable when subjected to chemical and physical factors. In addition, only small amounts are required to color any material, and unwanted flavors and aromas are not incorporated into the product. Natural colorants present in food, in addition to providing color, also have biological properties and effects that aid in the prevention and cure of many diseases. The main classes of colorants produced by phylum Ascomycota include polyketides and carotenoids. A promising producer of colorants should be able to assimilate a variety of sources of carbon and nitrogen and also exhibit relative stability. The strain should not be pathogenic, and its product should not be toxic. Production processes should also provide the expected color with a good yield through simple extraction methods. Research that seeks new sources of these compounds should continue to seek products of biotechnological origin in order to be competitive with products of synthetic and plant origin. In this review, we will focus on the recent studies on the main producing species, classes, and metabolic pathways of colorants produced by this phylum, historical background, impact of synthetic colorants on human health and the environment, social demand for natural colorants and also an in-depth approach to bioprocesses (influences on production, optimization of bioprocess, extraction, and identification), and limitations and perspectives for the use of fungal-based dyes.

Cheese whey supports high riboflavin synthesis by the engineered strains of the flavinogenic yeast Candida famata

Background

Riboflavin is a precursor of FMN and FAD which act as coenzymes of numerous enzymes. Riboflavin is an important biotechnological commodity with annual market sales exceeding nine billion US dollars. It is used primarily as a component of feed premixes, a food colorant, a component of multivitamin mixtures and medicines. Currently, industrial riboflavin production uses the bacterium, Bacillus subtilis, and the filamentous fungus, Ashbya gossypii, and utilizes glucose and/or oils as carbon substrates.

Results

We studied riboflavin biosynthesis in the flavinogenic yeast Candida famata that is a genetically stable riboflavin overproducer. Here it was found that the wild type C. famata is characterized by robust growth on lactose and cheese whey and the engineered strains also overproduce riboflavin on whey. The riboflavin synthesis on whey was close to that obtained on glucose. To further enhance riboflavin production on whey, the gene of the transcription activator SEF1 was expressed under control of the lactose-induced promoter of the native β-galactosidase gene LAC4. These transformants produced elevated amounts of riboflavin on lactose and especially on whey. The strain with additional overexpression of gene RIB6 involved in conversion of ribulose-5-phosphate to riboflavin precursor had the highest titer of accumulated riboflavin in flasks during cultivation on whey. Activation of riboflavin synthesis was also obtained after overexpression of the GND1 gene that is involved in the synthesis of the riboflavin precursor ribulose-5-phosphate. The best engineered strains accumulated 2.5 g of riboflavin/L on whey supplemented only with (NH₄)₂SO₄ during batch cultivation in bioreactor with high yield (more than 300 mg/g dry cell weight). The use of concentrated whey inhibited growth of wild-type and engineered strains of C. famata, so the mutants tolerant to concentrated whey were isolated.

Conclusions

Our data show that the waste of dairy industry is a promising substrate for riboflavin production by C. famata. Possibilities for using the engineered strains of C. famata to produce high-value commodity (riboflavin) from whey are discussed.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01888-0.

Dynamic Control Strategy to Produce Riboflavin with Lignocellulose Hydrolysate in the Thermophile Geobacillus thermoglucosidasius

Efficient utilization of both glucose and xylose, the two most abundant sugars in biomass hydrolysates, is one of the main objectives of biofermentation with lignocellulosic materials. The utilization of xylose is commonly inhibited by glucose, which is known as glucose catabolite repression (GCR). Here, we report a GCR-based dynamic control (GCR-DC) strategy aiming at better co-utilization of glucose and xylose, by decoupling the cell growth and biosynthesis of riboflavin as a product. Using the thermophilic strain Geobacillus thermoglucosidasius DSM 2542 as a host, we constructed additional riboflavin biosynthetic pathways that were activated by xylose but not glucose. The engineered strains showed a two-stage fermentation process. In the first stage, glucose was preferentially used for cell growth and no production of riboflavin was observed, while in the second stage where glucose was nearly depleted, xylose was effectively utilized for riboflavin biosynthesis. Using corn cob hydrolysate as a carbon source, the optimized riboflavin yields of strains DSM2542-DCall-MSS (full pathway dynamic control strategy) and DSM2542-DCrib (single-module dynamic control strategy) were 5.3- and 2.3-fold higher than that of the control strain DSM 2542 Rib-Gtg constitutively producing riboflavin, respectively. This GCR-DC strategy should also be applicable to the construction of cell factories that can efficiently use natural carbon sources with multiple sugar components for the production of high-value chemicals in future.

Bacterial pigments: A colorful palette reservoir for biotechnological applications

Synthetic derivatives are currently used instead of pigments in many applicative fields, from food to feed, from pharmaceutical to diagnostic, from agronomy to industry. Progress in organic chemistry allowed to obtain rather cheap compounds covering the whole color spectrum. However, several concerns arise from this chemical approach, as it is mainly based on nonrenewable resources such as fossil oil, and the toxicity or carcinogenic properties of products and/or precursors may be harmful for personnel involved in the productive processes. In this scenario, microorganisms and their pigments represent a colorful world to discover and reconsider. Each living bacterial strain may be a source of secondary metabolites with peculiar functions. The aim of this review is to link the physiological role of bacterial pigments with their potential use in different biotechnological fields. This enormous potential supports the big challenge for the development of strategies useful to identify, produce, and purify the right pigment for the desired application. At the end of this ideal journey through the world of bacterial pigments, the attention will be focused on melanin compounds, whose production relies upon different techniques ranging from natural producers, heterologous hosts, or isolated enzymes. In a green workflow, the microorganisms represent the starting and final point of pigment production.

From Brown Seaweed to a Sustainable Microbial Feedstock for the Production of Riboflavin

The increasing global demand for food and energy production encourages the development of new production strategies focused on sustainability. Often, microbial bioprocesses rely on food or feed competitive feedstocks; hence, there is a trending need for green substrates. Here, we have proven the potential of brown seaweed biomass as microbial feedstock on account of its content of mannitol and the glucose polymer laminarin. Our host, Corynebacterium glutamicum, was engineered to enable access to mannitol as a carbon source through the heterologous expression of the mannitol-specific phosphotransferase system and the mannitol-1-phosphate-5-dehydrogenase from Bacillus subtilis. Overproduction of riboflavin was coupled with mannitol and glucose consumption via constitutive overexpression of the biosynthetic riboflavin operon ribGCAH from C. glutamicum. Brown seaweed extract and brown seaweed hydrolysate from Laminaria hyperborea, containing mannitol and glucose, were used as a carbon source for flask and bioreactor fermentations. In a seaweed-based fed-batch fermentation, the riboflavin final titer, yield, and volumetric productivity values of 1,291.2 mg L⁻¹, 66.1 mg g⁻¹, and 17.2 mg L⁻¹ h⁻¹, respectively, were achieved.

Co-Overexpression of RIB1 and RIB6 Increases Riboflavin Production in the Yeast Candida famata

Riboflavin or vitamin B2 is a water-soluble vitamin and a precursor of flavin coenzymes, flavin mononucleotide, and flavin adenine dinucleotide, which play a key role as enzyme cofactors in energy metabolism. Candida famata yeast is a promising producer of riboflavin, as it belongs to the group of so-called flavinogenic yeasts, capable of riboflavin oversynthesis under conditions of iron starvation. The role of the particular structural genes in the limitation of riboflavin oversynthesis is not known. To study the impact of overexpression of the structural genes of riboflavin synthesis on riboflavin production, a set of plasmids containing genes RIB1, RIB6, and RIB7 in different combinations was constructed. The transformants of the wild-type strain of C. famata, as well as riboflavin overproducer, were obtained, and the synthesis of riboflavin was studied. It was found that overexpression of RIB1 and RIB6 genes coding for enzymes GTP cyclohydrolase II and 3,4-dihydroxy-2-butanone-4-phosphate synthase, which catalase the initial steps of riboflavin synthesis, elevated riboflavin production by 13–28% relative to the parental riboflavin-overproducing strains.

Genome Sequence and Analysis of the Flavinogenic Yeast Candida membranifaciens IST 626

The ascomycetous yeast Candida membranifaciens has been isolated from diverse habitats, including humans, insects, and environmental sources, exhibiting a remarkable ability to use different carbon sources that include pentoses, melibiose, and inulin. In this study, we isolated four C. membranifaciens strains from soil and investigated their potential to overproduce riboflavin. C. membranifaciens IST 626 was found to produce the highest concentrations of riboflavin. The volumetric production of this vitamin was higher when C. membranifaciens IST 626 cells were cultured in a commercial medium without iron and when xylose was the available carbon source compared to the same basal medium with glucose. Supplementation of the growth medium with 2 g/L glycine favored the metabolization of xylose, leading to biomass increase and consequent enhancement of riboflavin volumetric production that reached 120 mg/L after 216 h of cultivation. To gain new insights into the molecular basis of riboflavin production and carbon source utilization in this species, the first annotated genome sequence of C. membranifaciens is reported in this article, as well as the result of a comparative genomic analysis with other relevant yeast species. A total of 5619 genes were predicted to be present in C. membranifaciens IST 626 genome sequence (11.5 Mbp). Among them are genes involved in riboflavin biosynthesis, iron homeostasis, and sugar uptake and metabolism. This work put forward C. membranifaciens IST 626 as a riboflavin overproducer and provides valuable molecular data for future development of superior producing strains capable of using the wide range of carbon sources, which is a characteristic trait of the species.

Biological Properties of Vitamins of the B-Complex, Part 1: Vitamins B1, B2, B3, and B5

This review summarizes the current knowledge on essential vitamins B₁, B₂, B₃, and B₅. These B-complex vitamins must be taken from diet, with the exception of vitamin B₃, that can also be synthetized from amino acid tryptophan. All of these vitamins are water soluble, which determines their main properties, namely: they are partly lost when food is washed or boiled since they migrate to the water; the requirement of membrane transporters for their permeation into the cells; and their safety since any excess is rapidly eliminated via the kidney. The therapeutic use of B-complex vitamins is mostly limited to hypovitaminoses or similar conditions, but, as they are generally very safe, they have also been examined in other pathological conditions. Nicotinic acid, a form of vitamin B₃, is the only exception because it is a known hypolipidemic agent in gram doses. The article also sums up: (i) the current methods for detection of the vitamins of the B-complex in biological fluids; (ii) the food and other sources of these vitamins including the effect of common processing and storage methods on their content; and (iii) their physiological function.

Enhancement of riboflavin production in Bacillus subtilis via in vitro and in vivo metabolic engineering of pentose phosphate pathway

OBJECTIVES

The production of riboflavin with Bacillus subtilis, is an established process, however it is yet to be fully optimized. The aim of this study was to explore how riboflavin yields can be improved via in vitro and in vivo metabolic engineering modification of the pentose phosphate pathway (PPP).

RESULTS

In vitro, glucose was replaced with sodium gluconate to enhance PPP. Flask tests showed that the riboflavin titer increased from 0.64 to 0.87 g/L. The results revealed that the direct use of sodium gluconate could benefit riboflavin production. In vivo, gntP (encoding gluconate permease) was overexpressed to improve sodium gluconate uptake. The riboflavin titer reached 1.00 g/L with the mutant B. subtilis RF01. Ultimately, the fermentation verification of the engineered strain was carried out in a 7-L fermenter, with the increased riboflavin titer validating this approach.

CONCLUSIONS

The combination of metabolic engineering modifications in vitro and in vivo was confirmed to promote riboflavin production efficiently by increasing PPP and has great potential for industrial application. This work is aimed to explore how to improve the riboflavin yield by the rational renovation of the pentose phosphate pathway (PPP). In vitro, metabolic engineering mainly uses sodium gluconate as a carbon source instead of glucose, and in vivo, metabolic engineering mainly includes the overexpression of sodium gluconate utility-related genes. The effect of sodium gluconate on cell growth, riboflavin production was investigated in the flasks and fermenter scale.

New Promoters for Metabolic Engineering of Ashbya gossypii

Ashbya gossypii is a filamentous fungus that is currently exploited for the industrial production of riboflavin. In addition, metabolically engineered strains of A. gossypii have also been described as valuable biocatalysts for the production of different metabolites such as folic acid, nucleosides, and biolipids. Hence, bioproduction in A. gossypii relies on the availability of well-performing gene expression systems both for endogenous and heterologous genes. In this regard, the identification of novel promoters, which are critical elements for gene expression, decisively helps to expand the A. gossypii molecular toolbox. In this work, we present an adaptation of the Dual Luciferase Reporter (DLR) Assay for promoter analysis in A. gossypii using integrative cassettes. We demonstrate the efficiency of the analysis through the identification of 10 new promoters with different features, including carbon source-regulatable abilities, that will highly improve the gene expression platforms used in A. gossypii. Three novel strong promoters (P_CCW12, P_SED1, and P_TSA1) and seven medium/weak promoters (P_HSP26, P_AGL366C, P_TMA10, P_CWP1, P_AFR038W, P_PFS1, and P_CDA2) are presented. The functionality of the promoters was further evaluated both for the overexpression and for the underexpression of the A. gossypii MSN2 gene, which induced significant changes in the sporulation ability of the mutant strains.

Improving riboflavin production by modifying related metabolic pathways in Bacillus subtilis

Riboflavin is a feed additive, food additive and clinical drug, with a significant annual demand of nearly 8000 t. Fermentation using recombinant Bacillus subtilis is currently one of the most important industrial production method for riboflavin. First, a suitable medium was selected and the expression of the ureABC operon was modified. The ykgB gene was overexpressed in B. subtilis RX10, the production of the derivative strain RX20 was increased to 4·61 g l^-1 riboflavin, and the yield was increased to 52 mg riboflavin g^-1 glucose. The relative transcription level of pyr operon in RX20 was reduced to 71%, the production of the derivative strain RX21 was increased to 5·82 g l^-1 riboflavin, and the yield was 76 mg riboflavin g^-1 glucose. The start codon of the pyrE gene in RX21 was modified to 'TTG', the production of the derivative strain RX22 was increased to 7·01 g l^-1 riboflavin, and the yield was 89 mg riboflavin g^-1 glucose. These results indicated that overexpression of the ykgB gene and reduction of the metabolic flux of de novo synthesis of pyrimidine nucleotides were beneficial to the synthesis of riboflavin.

Rational Engineering of Escherichia coli for High-Level Production of Riboflavin

Riboflavin is widely used as a food additive. Here, multiple strategies were used to increase riboflavin production in Escherichia coli LS31T. First, purR deletion and co-overexpression of fbp, purF, prs, gmk, and ndk genes resulted in an increase of 18.6% in riboflavin titer (reaching 729.7 mg/L). Second, optimization of reduced nicotinamide adenine dinucleotide phosphate/nicotinamide adenine dinucleotide ratio and respiratory chain activity in LS31T increased the titer up to 1020.2 mg/L. Third, the expression level of the guaC gene in LS31T was downregulated by ribosome binding site replacement, and the riboflavin production was increased by 10.6% to 658.5 mg/L. Then, all the favorable modifications were integrated together, and the resulting strain LS72T produced 1339 mg/L of riboflavin. Moreover, the riboflavin titer of LS72T reached 21 g/L in fed-batch cultivation, with a yield of 110 mg riboflavin/g glucose. To our knowledge, both the riboflavin titer and yield obtained in fed-batch fermentation are the highest ones among all the rationally engineered strains.

Microbial 5'-nucleotidases: their characteristics, roles in cellular metabolism, and possible practical applications

5′-Nucleotidases (EC 3.1.3.5) are enzymes that catalyze the hydrolytic dephosphorylation of 5′-ribonucleotides and 5′-deoxyribonucleotides to their respective nucleosides and phosphate. Most 5′-nucleotidases have broad substrate specificity and are multifunctional enzymes capable of cleaving phosphorus from not only mononucleotide phosphate molecules but also a variety of other phosphorylated metabolites. 5′-Nucleotidases are widely distributed throughout all kingdoms of life and found in different cellular locations. The well-studied vertebrate 5′-nucleotidases play an important role in cellular metabolism. These enzymes are involved in purine and pyrimidine salvage pathways, nucleic acid repair, cell-to-cell communication, signal transduction, control of the ribo- and deoxyribonucleotide pools, etc. Although the first evidence of microbial 5′-nucleotidases was obtained almost 60 years ago, active studies of genetic control and the functions of microbial 5′-nucleotidases started relatively recently. The present review summarizes the current knowledge about microbial 5′-nucleotidases with a focus on their diversity, cellular localizations, molecular structures, mechanisms of catalysis, physiological roles, and activity regulation and approaches to identify new 5′-nucleotidases. The possible applications of these enzymes in biotechnology are also discussed.

Key points

• Microbial 5′-nucleotidases differ in molecular structure, hydrolytic mechanism, and cellular localization.

• 5′-Nucleotidases play important and multifaceted roles in microbial cells.

• Microbial 5′-nucleotidases have wide range of practical applications.

Effects of sirtuins on the riboflavin production in Ashbya gossypii

This study focuses on sirtuins, which catalyze the reaction of NAD⁺-dependent protein deacetylase, for riboflavin production in A. gossypii. Nicotinamide, a known inhibitor of sirtuin, made the color of A. gossypii colonies appear a deeper yellow at 5 mM. A. gossypii has 4 sirtuin genes (AgHST1, AgHST2, AgHST3, AgHST4) and these were disrupted to investigate the role of sirtuins in riboflavin production in A. gossypii. AgHST1∆, AgHST3∆, and AgHST4∆ strains were obtained, but AgHST2∆ was not. The AgHST1∆ and AgHST3∆ strains produced approximately 4.3- and 2.9-fold higher amounts of riboflavin than the WT strain. The AgHST3∆ strain showed a lower human sirtuin 6 (SIRT6)-like activity than the WT strain and only in the AgHST3∆ strain was a higher amount of acetylation of histone H3 K9 and K56 (H3K9ac and H3K56ac) observed compared to the WT strain. These results indicate that AgHst3 is SIRT6-like sirtuin in A. gossypii and the activity has an influence on the riboflavin production in A. gossypii. In the presence of 5 mM hydroxyurea and 50 µM camptothecin, which causes DNA damage, especially double-strand DNA breaks, the color of the WT strain colonies turned a deeper yellow. Additionally, hydroxyurea significantly led to the production of approximately 1.5 higher amounts of riboflavin and camptothecin also enhanced the riboflavin production even through the significant difference was not detected. Camptothecin tended to increase the amount of H3K56ac, but the amount of H3K56ac was not increased by hydroxyurea treatment. This study revealed that AgHst1 and AgHst3 are involved in the riboflavin production in A. gossypii through NAD metabolism and the acetylation of H3, respectively. This new finding is a step toward clarifying the role of sirtuins in riboflavin over-production by A. gossypii.Key points• Nicotinamide enhanced the riboflavin production in Ashbya gossypii.• Disruption of AgHST1 or AgHST3 gene also enhanced the riboflavin production in Ashbya gossypii.• Acetylation of H3K56 led to the enhancement of the riboflavin production in Ashbya gossypii.

Effects of a proteasome inhibitor on the riboflavin production in Ashbya gossypii

AIMS

Effects of a proteasome inhibitor, MG-132, on the riboflavin production in Ashbya gossypii were investigated to elucidate the relationship of the riboflavin production with flavoprotein homeostasis.

METHODS AND RESULTS

The addition of MG-132 to the liquid medium reduced the specific riboflavin production by 79% in A. gossypii at 25 μM after 24 h. The addition of the inhibitor also caused the accumulation of reactive oxygen species and ubiquitinated proteins. These results indicated that MG-132 works in A. gossypii without any genetic engineering and reduces riboflavin production. In the presence of 25 μM MG-132, specific NADH dehydrogenase activity was increased by 1.4-fold compared to DMSO, but specific succinate dehydrogenase (SDH) activity was decreased to 52% compared to DMSO. Additionally, the amount of AgSdh1p (ACR052Wp) was also reduced. Specific riboflavin production was reduced to 22% when 20 mM malonate, a SDH inhibitor, was added to the culture medium. The riboflavin production in heterozygous AgSDH1 gene-disrupted mutant (AgSDH1^-/+ ) was reduced to 63% compared to that in wild type.

CONCLUSIONS

MG-132 suppresses the riboflavin production and SDH activity in A. gossypii. SDH is one of the flavoproteins involved in the riboflavin production in A. gossypii.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study shows that MG-132 has a negative influence on the riboflavin production and SDH activity in A. gossypii and leads to the elucidation of the connection of the riboflavin production with flavoproteins.

Microbial production of riboflavin: Biotechnological advances and perspectives

Riboflavin is an essential nutrient for humans and animals, and its derivatives flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) are cofactors in the cells. Therefore, riboflavin and its derivatives are widely used in the food, pharmaceutical, nutraceutical and cosmetic industries. Advances in biotechnology have led to a complete shift in the commercial production of riboflavin from chemical synthesis to microbial fermentation. In this review, we provide a comprehensive review of biotechnologies that enhance riboflavin production in microorganisms, as well as representative examples. Firstly, the synthesis pathways and metabolic regulatory processes of riboflavin in microorganisms; and the current strategies and methods of metabolic engineering for riboflavin production are systematically summarized and compared. Secondly, the using of systematic metabolic engineering strategies to enhance riboflavin production is discussed, including laboratory evolution, histological analysis and high-throughput screening. Finally, the challenges for efficient microbial production of riboflavin and the strategies to overcome these challenges are prospected.

Adaptive laboratory evolution of microbial co-cultures for improved metabolite secretion

Adaptive laboratory evolution has proven highly effective for obtaining microorganisms with enhanced capabilities. Yet, this method is inherently restricted to the traits that are positively linked to cell fitness, such as nutrient utilization. Here, we introduce coevolution of obligatory mutualistic communities for improving secretion of fitness‐costly metabolites through natural selection. In this strategy, metabolic cross‐feeding connects secretion of the target metabolite, despite its cost to the secretor, to the survival and proliferation of the entire community. We thus co‐evolved wild‐type lactic acid bacteria and engineered auxotrophic Saccharomyces cerevisiae in a synthetic growth medium leading to bacterial isolates with enhanced secretion of two B‐group vitamins, viz., riboflavin and folate. The increased production was specific to the targeted vitamin, and evident also in milk, a more complex nutrient environment that naturally contains vitamins. Genomic, proteomic and metabolomic analyses of the evolved lactic acid bacteria, in combination with flux balance analysis, showed altered metabolic regulation towards increased supply of the vitamin precursors. Together, our findings demonstrate how microbial metabolism adapts to mutualistic lifestyle through enhanced metabolite exchange.

Improving the Production of Riboflavin by Introducing a Mutant Ribulose 5-Phosphate 3-Epimerase Gene in Bacillus subtilis

Ribulose 5-phosphate (Ru5P) and guanosine 5′-triphosphate (GTP) are two key precursors of riboflavin, whereby Ru5P is also a precursor of GTP. Ribulose 5-phosphate 3-epimerase (Rpe) catalyzes the conversion of ribulose 5-phosphate into xylulose 5-phosphate. Inactivation of Rpe can reduce the consumption of Ru5P, enhancing the carbon flux toward riboflavin biosynthesis. Here we investigated the effect of mutation of rpe and other related genes on riboflavin production, physiological and metabolic phenotypes in Bacillus subtilis LY (BSLY). Introducing single nucleotide deletion (generated BSR) or nonsense mutation (generated BSRN) on the genomic copy of rpe, resulting in more than fivefold increase of riboflavin production over the parental strain. BSR process 62% Rpe activity, while BSRN lost the entire Rpe activity and had a growth defect compared with the parent strain. BSR and BSRN exhibited increases of the inosine and guanine titers, in addition, BSRN exhibited an increase of inosine 5′-monophosphate titer in fermentation. The transcription levels of most oxidative pentose phosphate pathway and purine synthesis genes were unchanged in BSR, except for the levels of zwf and ndk, which were higher than in BSLY. The production of riboflavin was increased to 479.90 ± 33.21 mg/L when ribA was overexpressed in BSR. The overexpression of zwf, gntZ, prs, and purF also enhanced the riboflavin production. Finally, overexpression of the rib operon by the pMX45 plasmid and mutant gnd by pHP03 plasmid in BSR led to a 3.05-fold increase of the riboflavin production (977.29 ± 63.44 mg/L), showing the potential for further engineering of this strain.

Strategies to Increase the Production of Biosynthetic Riboflavin

Riboflavin is widely regarded as an essential nutrient that is involved in biological oxidation in vivo. In addition to preventing and treating acyl-CoA dehydrogenase deficiency in patients with keratitis, stomatitis, and glossitis, riboflavin is also closely related to the treatment of radiation mucositis and cardiovascular disease. Chemical synthesis has been the dominant method for producing riboflavin for approximately 50 years. Nevertheless, due to the intricate synthesis process, relatively high cost, and high risk of pollution, alternative methods of chemical syntheses, such as the fermentation method, began to develop and eventually became the main methods for producing riboflavin. At present, there are three types of strains used in industrial riboflavin production: Ashbya gossypii, Candida famata, and Bacillus subtilis. Additionally, many recent studies have been conducted on Escherichia coli and Lactobacillus. Fermentation increases the yield of riboflavin using genetic engineering technology to modify and induce riboflavin production in the strain, as well as to regulate the metabolic flux of the purine pathway and pentose phosphate pathway (PP pathway), thereby optimizing the culture process. This article briefly introduces recent progress in the fermentation of riboflavin.

Riboflavin Inhibits Histamine-Dependent Itch by Modulating Transient Receptor Potential Vanilloid 1 (TRPV1)

Riboflavin, also known as vitamin B₂, isfound in foods and is used as a dietary supplement. Its deficiency (also called ariboflavinosis) results in some skin lesions and inflammations, such as stomatitis, cheilosis, oily scaly skin rashes, and itchy, watery eyes. Various therapeutic effects of riboflavin, such as anticancer, antioxidant, anti-inflammatory, and anti-nociceptive effects, are well known. Although some studies have identified the clinical effect of riboflavin on skin problems, including itch and inflammation, its underlying mechanism of action remains unknown. In this study, we investigated the molecular mechanism of the effects of riboflavin on histamine-dependent itch based on behavioral tests and electrophysiological experiments. Riboflavin significantly reduced histamine-induced scratching behaviors in mice and histamine-induced discharges in single-nerve fiber recordings, while it did not alter motor function in the rotarod test. In cultured dorsal root ganglion (DRG) neurons, riboflavin showed a dose-dependent inhibitory effect on the histamine- and capsaicin-induced inward current. Further tests wereconducted to determine whether two endogenous metabolites of riboflavin, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), have similar effects to those of riboflavin. Here, FMN, but not FAD, significantly inhibited capsaicin-induced currents and itching responses caused by histamine. In addition, in transient receptor potential vanilloid 1 (TRPV1)-transfected HEK293 cells, both riboflavin and FMN blocked capsaicin-induced currents, whereas FAD did not. These results revealed that riboflavin inhibits histamine-dependent itch by modulating TRPV1 activity. This study will be helpful in understanding how riboflavin exerts antipruritic effects and suggests that it might be a useful drug for the treatment of histamine-dependent itch.

Microbial Cell Factories for Green Production of Vitamins

Vitamins are a group of essential nutrients that are necessary to maintain normal metabolic activities and optimal health. There are wide applications of different vitamins in food, cosmetics, feed, medicine, and other areas. The increase in the global demand for vitamins has inspired great interest in novel production strategies. Chemical synthesis methods often require high temperatures or pressurized reactors and use non-renewable chemicals or toxic solvents that cause product safety concerns, pollution, and hazardous waste. Microbial cell factories for the production of vitamins are green and sustainable from both environmental and economic standpoints. In this review, we summarized the vitamins which can potentially be produced using microbial cell factories or are already being produced in commercial fermentation processes. They include water-soluble vitamins (vitamin B complex and vitamin C) as well as fat-soluble vitamins (vitamin A/D/E and vitamin K). Furthermore, metabolic engineering is discussed to provide a reference for the construction of microbial cell factories. We also highlight the current state and problems encountered in the fermentative production of vitamins.

Recent Advances in Construction of the Efficient Producers of Riboflavin and Flavin Nucleotides (FMN, FAD) in the Yeast Candida famata

The approaches used by the authors to design the Candida famata strains capable to overproduce riboflavin, flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD) are described. The metabolic engineering approaches include overexpression of SEF1 gene encoding positive regulator of riboflavin biosynthesis, IMH3 (coding for IMP dehydrogenase) orthologs from another species of flavinogenic yeast Debaryomyces hansenii, and the homologous genes RIB1 and RIB7 encoding GTP cyclohydrolase II and riboflavin synthase, the first and the last enzymes of riboflavin biosynthesis pathway, respectively. Overexpression of the above mentioned genes in the genetically stable riboflavin overproducer AF-4 obtained by classical selection resulted in fourfold increase of riboflavin production in shake flask experiments.Overexpression of engineered enzymes phosphoribosyl pyrophosphate synthetase and phosphoribosyl pyrophosphate amidotransferase catalyzing the initial steps of purine nucleotide biosynthesis enhances riboflavin synthesis in the flavinogenic yeast C. famata even more.Recombinant strains of C. famata containing FMN1 gene from D. hansenii encoding riboflavin kinase under control of the strong constitutive TEF1 promoter were constructed. Overexpression of the FMN1 gene in the riboflavin-producing mutant led to the 30-fold increase of the riboflavin kinase activity and 400-fold increase of FMN production in the resulting recombinant strains which reached maximally 318.2 mg/L.FAD overproducing strains of C. famata were also constructed. This was achieved by overexpression of FAD1 gene from D. hansenii in C. famata FMN overproducing strain. The 7- to 15-fold increase in FAD synthetase activity as compared to the wild-type strain and FAD accumulation into cultural medium were observed. The maximal FAD titer 451.5 mg/L was achieved.

Overexpression of Riboflavin Excretase Enhances Riboflavin Production in the Yeast Candida famata

Many microorganisms are capable of riboflavin oversynthesis and accumulation in a medium, suggesting that they efficiently excrete riboflavin. The mechanisms of riboflavin efflux in microorganisms remain elusive. Candida famata are representatives of a group of so-called flavinogenic yeast species that overproduce riboflavin (vitamin B₂) in response to iron limitation. The riboflavin overproducers of this yeast species have been obtained by classical mutagenesis and metabolic engineering. Overproduced riboflavin accumulates in the cultural medium rather than in the cells suggesting existence of the special mechanisms involved in riboflavin excretion. The appropriate protein and gene have not been identified in yeasts till recently. At the same time, the gene BCRP (breast cancer resistance protein) has been identified in mammal mammary glands. Several homologs of the mammal BCRP gene encoding putative riboflavin efflux protein (excretase) were identified in the flavinogenic yeasts Debaryomyces hansenii and C. famata. Here we evaluate the yeast homologs of BCRP with respect to improvement of a riboflavin production by C. famata. The closest homologs from D. hansenii or C. famata were expressed under the control of TEF1 promoter of these yeasts in the wild-type and riboflavin-overproducing strains of C. famata. Resulted transformants overexpressed the corresponding genes (designated as DhRFE and CfRFE) and produced 1.4- to 6-fold more riboflavin as compared to the corresponding parental strains. They also were characterized by overexpression of RIB1 and RIB6 genes which encode the first and the last structural enzymes of riboflavin synthesis and exhibited elevated specific activity of GTP cyclohydrolase II. Thus, overexpression of yeast homolog of mammal gene BCRP may be useful to increase the riboflavin yield in a riboflavin production process using a recombinant overproducing C. famata strain or other flavinogenic microorganisms.

Multiple copies of the oxytetracycline gene cluster in selected Streptomyces rimosus strains can provide significantly increased titers

Background

Natural products are a valuable source of biologically active compounds that have applications in medicine and agriculture. One disadvantage with natural products is the slow, time-consuming strain improvement regimes that are necessary to ensure sufficient quantities of target compounds for commercial production. Although great efforts have been invested in strain selection methods, many of these technologies have not been improved in decades, which might pose a serious threat to the economic and industrial viability of such important bioprocesses.

Results

In recent years, introduction of extra copies of an entire biosynthetic pathway that encodes a target product in a single microbial host has become a technically feasible approach. However, this often results in minor to moderate increases in target titers. Strain stability and process reproducibility are the other critical factors in the industrial setting. Industrial Streptomyces rimosus strains for production of oxytetracycline are one of the most economically efficient strains ever developed, and thus these represent a very good industrial case. To evaluate the applicability of amplification of an entire gene cluster in a single host strain, we developed and evaluated various gene tools to introduce multiple copies of the entire oxytetracycline gene cluster into three different Streptomyces rimosus strains: wild-type, and medium and high oxytetracycline-producing strains. We evaluated the production levels of these engineered S. rimosus strains with extra copies of the oxytetracycline gene cluster and their stability, and the oxytetracycline gene cluster expression profiles; we also identified the chromosomal integration sites.

Conclusions

This study shows that stable and reproducible increases in target secondary metabolite titers can be achieved in wild-type and in high oxytetracycline-producing strains, which always reflects the metabolic background of each independent S. rimosus strain. Although this approach is technically very demanding and requires systematic effort, when combined with modern strain selection methods, it might constitute a very valuable approach in industrial process development.

Unlocking Nature's Biosynthetic Power-Metabolic Engineering for the Fermentative Production of Chemicals

Fermentation as a production method for chemicals is especially attractive, as it is based on cheap renewable raw materials and often exhibits advantages in terms of costs and sustainability. The tremendous development of technology in bioscience has resulted in an exponentially increasing knowledge about biological systems and has become the main driver for innovations in the field of metabolic engineering. Progress in recombinant DNA technology, genomics, and computational methods open new, cheaper, and faster ways to metabolically engineer microorganisms. Existing biosynthetic pathways for natural products, such as vitamins, organic acids, amino acids, or secondary metabolites, can be discovered and optimized efficiently, thereby enabling competitive commercial production processes. Novel biosynthetic routes can now be designed by the rearrangement of nature's unlimited number of enzymes and metabolic pathways in microbial strains. This expands the range of chemicals accessible by biotechnology and has yielded the first commercial products, while new fermentation technologies targeting novel active ingredients, commodity chemicals, and CO₂ -fixation methods are on the horizon.

Reactive crystallization: a review

Reactive crystallization is not new, but there has been recent growth in its use as a means of improving performance and sustainability of industrial processes.

Bio-synthesis of food additives and colorants-a growing trend in future food

Food additives and colorants are extensively used in the food industry to improve food quality and safety during processing, storage and packing. Sourcing of these molecules is predominately through three means: extraction from natural sources, chemical synthesis, and bio-production, with the first two being the most utilized. However, growing demands for sustainability, safety and "natural" products have renewed interest in using bio-based production methods. Likewise, the move to more cultured foods and meat alternatives requires the production of new additives and colorants. The production of bio-based food additives and colorants is an interdisciplinary research endeavor and represents a growing trend in future food. To highlight the potential of microbial hosts for food additive and colorant production, we focus on current advances for example molecules based on their utilization stage and bio-production yield as follows: (I) approved and industrially produced with high titers; (II) approved and produced with decent titers (in the g/L range), but requiring further engineering to reduce production costs; (III) approved and produced with very early stage titers (in the mg/L range); and (IV) new/potential candidates that have not been approved but can be sourced through microbes. Promising approaches, as well as current challenges and future directions will also be thoroughly discussed for the bioproduction of these food additives and colorants.

Pentose metabolism and conversion to biofuels and high-value chemicals in yeasts

Pentose sugars are widespread in nature and two of them, D-xylose and L-arabinose belong to the most abundant sugars being the second and third by abundance sugars in dry plant biomass (lignocellulose) and in general on planet. Therefore, it is not surprising that metabolism and bioconversion of these pentoses attract much attention. Several different pathways of D-xylose and L-arabinose catabolism in bacteria and yeasts are known. There are even more common and really ubiquitous though not so abundant pentoses, D-ribose and 2-deoxy-D-ribose, the constituents of all living cells. Thus, ribose metabolism is example of endogenous metabolism whereas metabolism of other pentoses, including xylose and L-arabinose, represents examples of the metabolism of foreign exogenous compounds which normally are not constituents of yeast cells. As a rule, pentose degradation by the wild-type strains of microorganisms does not lead to accumulation of high amounts of valuable substances; however, productive strains have been obtained by random selection and metabolic engineering. There are numerous reviews on xylose and (less) L-arabinose metabolism and conversion to high value substances; however, they mostly are devoted to bacteria or the yeast Saccharomyces cerevisiae. This review is devoted to reviewing pentose metabolism and bioconversion mostly in non-conventional yeasts, which naturally metabolize xylose. Pentose metabolism in the recombinant strains of S. cerevisiae is also considered for comparison. The available data on ribose, xylose, L-arabinose transport, metabolism, regulation of these processes, interaction with glucose catabolism and construction of the productive strains of high-value chemicals or pentose (ribose) itself are described. In addition, genome studies of the natural xylose metabolizing yeasts and available tools for their molecular research are reviewed. Metabolism of other pentoses (2-deoxyribose, D-arabinose, lyxose) is briefly reviewed.

Manipulation of Purine Metabolic Networks for Riboflavin Production in Bacillus subtilis

Guanosine monophosphate, the precursor for riboflavin biosynthesis, can be converted to or generated from other purine compounds in purine metabolic networks. In this study, genes in these networks were manipulated in a riboflavin producer, Bacillus subtilis R, to test their contribution to riboflavin biosynthesis. Knocking out adenine phosphoribosyltransferase (apt), xanthine phosphoribosyltransferase (xpt), and adenine deaminase (adeC) increased the riboflavin production by 14.02, 6.78, and 41.50%, respectively, while other deletions in the salvage pathway, interconversion pathway, and nucleoside decomposition genes have no positive effects. The enhancement of riboflavin production in apt and adeC deletion mutants is dependent on the purine biosynthesis regulator PurR. Repression of ribonucleotide reductases (RNRs) led to a 13.12% increase of riboflavin production, which also increased in two RNR regulator mutants PerR and NrdR by 37.52 and 8.09%, respectively. The generation of deoxyribonucleoside competed for precursors with riboflavin biosynthesis, while other pathways do not contribute to the supply of precursors; nevertheless, they have regulatory effects.

Production of Vitamin B2 (Riboflavin) by Microorganisms: An Overview

Riboflavin is a crucial micronutrient that is a precursor to coenzymes flavin mononucleotide and flavin adenine dinucleotide, and it is required for biochemical reactions in all living cells. For decades, one of the most important applications of riboflavin has been its global use as an animal and human nutritional supplement. Being well-informed of the latest research on riboflavin production via the fermentation process is necessary for the development of new and improved microbial strains using biotechnology and metabolic engineering techniques to increase vitamin B2 yield. In this review, we describe well-known industrial microbial producers, namely, Ashbya gossypii, Bacillus subtilis, and Candida spp. and summarize their biosynthetic pathway optimizations through genetic and metabolic engineering, combined with random chemical mutagenesis and rational medium components to increase riboflavin production.