Engineering Bacillus subtilis for the conversion of the antimetabolite 4-hydroxy-l-threonine to pyridoxine

Control of three-carbon amino acid homeostasis by promiscuous importers and exporters in Bacillus subtilis: role of the "sleeping beauty" amino acid exporters

The Gram-positive model bacterium Bacillus subtilis can acquire amino acids by import, de novo biosynthesis, or degradation of proteins and peptides. The accumulation of several amino acids inhibits the growth of B. subtilis, probably due to misincorporation into cellular macromolecules such as proteins or peptidoglycan or due to interference with other amino acid biosynthetic pathways. Here, we studied the adaptation of B. subtilis to toxic concentrations of the three-carbon amino acids L-alanine, β-alanine, and 2,3-diaminopropionic acid, as well as the two-carbon amino acid glycine. Resistance to the non-proteinogenic amino acid β-alanine, which is a precursor for coenzyme A biosynthesis, is achieved by mutations that either activate a cryptic amino acid exporter, AexA (previously YdeD), or inactivate the amino acid importers AimA, AimB (previously YbxG), and BcaP. The aexA gene is very poorly expressed under most conditions studied. However, mutations affecting the transcription factor AerA (previously YdeC) can result in strong constitutive aexA expression. AexA is the first characterized member of a group of amino acid exporters in B. subtilis, which are all very poorly expressed. Therefore, we suggest to call this group "sleeping beauty amino acid exporters." 2,3-Diaminopropionic acid can also be exported by AexA, and this amino acid also seems to be a natural substrate of AerA/AexA, as it can cause a slight but significant induction of aexA expression, and AexA also provides some natural resistance toward 2,3-diaminopropionic acid. Moreover, our work shows how low-specificity amino acid transporters contribute to amino acid homeostasis in B. subtilis.IMPORTANCEEven though Bacillus subtilis is one of the most-studied bacteria, amino acid homeostasis in this organism is not fully understood. We have identified import and export systems for the C2 and C3 amino acids. Our work demonstrates that the responsible amino acid permeases contribute in a rather promiscuitive way to amino acid uptake. In addition, we have discovered AexA, the first member of a group of very poorly expressed amino acid exporters in B. subtilis that we call "sleeping beauty amino acid exporters." The expression of these transporters is typically triggered by mutations in corresponding regulator genes that are acquired upon exposure to toxic amino acids. These exporters are ubiquitous in all domains of life. It is tempting to speculate that many of them are not expressed until the cells experience selective pressure by toxic compounds, and they protect the cells from rare but potentially dangerous encounters with such compounds.

Control of asparagine homeostasis in Bacillus subtilis: identification of promiscuous amino acid importers and exporters

The Gram-positive model bacterium B. subtilis is able to import all proteinogenic amino acids from the environment as well as to synthesize them. However, the players involved in the acquisition of asparagine have not yet been identified for this bacterium. In this work, we used d-asparagine as a toxic analog of l-asparagine to identify asparagine transporters. This revealed that d- but not l-asparagine is taken up by the malate/lactate antiporter MleN. Specific strains that are sensitive to the presence of l-asparagine due to the lack of the second messenger cyclic di-AMP or due to the intracellular accumulation of this amino acid were used to isolate and characterize suppressor mutants that were resistant to the presence of otherwise growth-inhibiting concentrations of l-asparagine. These screens identified the broad-spectrum amino acid importers AimA and BcaP as responsible for the acquisition of l-asparagine. The amino acid exporter AzlCD allows detoxification of l-asparagine in addition to 4-azaleucine and histidine. This work supports the idea that amino acids are often transported by promiscuous importers and exporters. However, our work also shows that even stereo-enantiomeric amino acids do not necessarily use the same transport systems.IMPORTANCETransport of amino acid is a poorly studied function in many bacteria, including the model organism Bacillus subtilis. The identification of transporters is hampered by the redundancy of transport systems for most amino acids as well as by the poor specificity of the transporters. Here, we apply several strategies to use the growth-inhibitive effect of many amino acids under defined conditions to isolate suppressor mutants that exhibit either reduced uptake or enhanced export of asparagine, resulting in the identification of uptake and export systems for l-asparagine. The approaches used here may be useful for the identification of transporters for other amino acids both in B. subtilis and in other bacteria.

The low mutational flexibility of the EPSP synthase in Bacillus subtilis is due to a higher demand for shikimate pathway intermediates

Glyphosate (GS) inhibits the 5-enolpyruvyl-shikimate-3-phosphate (EPSP) synthase that is required for aromatic amino acid, folate and quinone biosynthesis in Bacillus subtilis and Escherichia coli. The inhibition of the EPSP synthase by GS depletes the cell of these metabolites, resulting in cell death. Here, we show that like the laboratory B. subtilis strains also environmental and undomesticated isolates adapt to GS by reducing herbicide uptake. Although B. subtilis possesses a GS-insensitive EPSP synthase, the enzyme is strongly inhibited by GS in the native environment. Moreover, the B. subtilis EPSP synthase mutant was only viable in rich medium containing menaquinone, indicating that the bacteria require a catalytically efficient EPSP synthase under nutrient-poor conditions. The dependency of B. subtilis on the EPSP synthase probably limits its evolvability. In contrast, E. coli rapidly acquires GS resistance by target modification. However, the evolution of a GS-resistant EPSP synthase under non-selective growth conditions indicates that GS resistance causes fitness costs. Therefore, in both model organisms, the proper function of the EPSP synthase is critical for the cellular viability. This study also revealed that the uptake systems for folate precursors, phenylalanine and tyrosine need to be identified and characterized in B. subtilis.

Protein engineering and iterative multimodule optimization for vitamin B6 production in Escherichia coli

Vitamin B6 is an essential nutrient with extensive applications in the medicine, food, animal feed, and cosmetics industries. Pyridoxine (PN), the most common commercial form of vitamin B6, is currently chemically synthesized using expensive and toxic chemicals. However, the low catalytic efficiencies of natural enzymes and the tight regulation of the metabolic pathway have hindered PN production by the microbial fermentation process. Here, we report an engineered Escherichia coli strain for PN production. Parallel pathway engineering is performed to decouple PN production and cell growth. Further, protein engineering is rationally designed including the inefficient enzymes PdxA, PdxJ, and the initial enzymes Epd and Dxs. By the iterative multimodule optimization strategy, the final strain produces 1.4 g/L of PN with productivity of 29.16 mg/L/h by fed-batch fermentation. The strategies reported here will be useful for developing microbial strains for the production of vitamins and other bioproducts having inherently low metabolic fluxes.

Pyridoxal 5'-phosphate synthesis and salvage in Bacteria and Archaea: predicting pathway variant distributions and holes

Pyridoxal 5’-phosphate or PLP is a cofactor derived from B6 vitamers and essential for growth in all known organisms. PLP synthesis and salvage pathways are well characterized in a few model species even though key components, such as the vitamin B6 transporters, are still to be identified in many organisms including the model bacteria Escherichia coli or Bacillus subtilis . Using a comparative genomic approach, PLP synthesis and salvage pathways were predicted in 5840 bacterial and archaeal species with complete genomes. The distribution of the two known de novo biosynthesis pathways and previously identified cases of non-orthologous displacements were surveyed in the process. This analysis revealed that several PLP de novo pathway genes remain to be identified in many organisms, either because sequence similarity alone cannot be used to discriminate among several homologous candidates or due to non-orthologous displacements. Candidates for some of these pathway holes were identified using published TnSeq data, but many remain. We find that ~10 % of the analysed organisms rely on salvage but further analyses will be required to identify potential transporters. This work is a starting point to model the exchanges of B6 vitamers in communities, predict the sensitivity of a given organism to drugs targeting PLP synthesis enzymes, and identify numerous gaps in knowledge that will need to be tackled in the years to come.

How To Deal with Toxic Amino Acids: the Bipartite AzlCD Complex Exports Histidine in Bacillus subtilis

The Gram-positive model bacterium Bacillus subtilis can use several amino acids as sources of carbon and nitrogen. However, some amino acids inhibit the growth of this bacterium. This amino acid toxicity is often enhanced in strains lacking the second messenger cyclic dimeric adenosine 3',5'-monophosphate (c-di-AMP). We observed that the presence of histidine is also toxic for a B. subtilis strain that lacks all three c-di-AMP synthesizing enzymes. However, suppressor mutants emerged, and whole-genome sequencing revealed mutations in the azlB gene that encode the repressor of the azl operon. This operon encodes an exporter and an importer for branched-chain amino acids. The suppressor mutations result in an overexpression of the azl operon. Deletion of the azlCD genes encoding the branched-chain amino acid exporter restored the toxicity of histidine, indicating that this exporter is required for histidine export and for resistance to otherwise toxic levels of the amino acid. The higher abundance of the amino acid exporter AzlCD increased the extracellular concentration of histidine, thus confirming the new function of AzlCD as a histidine exporter. Unexpectedly, the AzlB-mediated repression of the operon remains active even in the presence of amino acids, suggesting that the expression of the azl operon requires the mutational inactivation of AzlB. IMPORTANCE Amino acids are building blocks for protein biosynthesis in each living cell. However, due to their reactivity and the similarity between several amino acids, they may also be involved in harmful reactions or in noncognate interactions and thus may be toxic. Bacillus subtilis can deal with otherwise toxic histidine by overexpressing the bipartite amino acid exporter AzlCD. Although encoded in an operon that also contains a gene for an amino acid importer, the corresponding genes are not expressed, irrespective of the availability of amino acids in the medium. This suggests that the azl operon is a last resort by which to deal with histidine stress that can be expressed due to the mutational inactivation of the cognate repressor AzlB.

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.

Molecular mechanisms underlying glyphosate resistance in bacteria

Glyphosate is a nonselective herbicide that kills weeds and other plants competing with crops. Glyphosate specifically inhibits the 5-enolpyruvyl-shikimate-3-phosphate (EPSP) synthase, thereby depleting the cell of EPSP serving as a precursor for biosynthesis of aromatic amino acids. Glyphosate is considered to be toxicologically safe for animals and humans. Therefore, it became the most-important herbicide in agriculture. However, its intensive application in agriculture is a serious environmental issue because it may negatively affect the biodiversity. A few years after the discovery of the mode of action of glyphosate, it has been observed that bacteria evolve glyphosate resistance by acquiring mutations in the EPSP synthase gene, rendering the encoded enzyme less sensitive to the herbicide. The identification of glyphosate-resistant EPSP synthase variants paved the way for engineering crops tolerating increased amounts of the herbicide. This review intends to summarize the molecular mechanisms underlying glyphosate resistance in bacteria. Bacteria can evolve glyphosate resistance by (i) reducing glyphosate sensitivity or elevating production of the EPSP synthase, by (ii) degrading or (iii) detoxifying glyphosate and by (iv) decreasing the uptake or increasing the export of the herbicide. The variety of glyphosate resistance mechanisms illustrates the adaptability of bacteria to anthropogenic substances due to genomic alterations.

Production of proteins and commodity chemicals using engineered Bacillus subtilis platform strain

Currently, increasing demand of biochemicals produced from renewable resources has motivated researchers to seek microbial production strategies instead of traditional chemical methods. As a microbial platform, Bacillus subtilis possesses many advantages including the generally recognized safe status, clear metabolic networks, short growth cycle, mature genetic editing methods and efficient protein secretion systems. Engineered B. subtilis strains are being increasingly used in laboratory research and in industry for the production of valuable proteins and other chemicals. In this review, we first describe the recent advances of bioinformatics strategies during the research and applications of B. subtilis. Secondly, the applications of B. subtilis in enzymes and recombinant proteins production are summarized. Further, the recent progress in employing metabolic engineering and synthetic biology strategies in B. subtilis platform strain to produce commodity chemicals is systematically introduced and compared. Finally, the major limitations for the further development of B. subtilis platform strain and possible future directions for its research are also discussed.

Microbes in lung cancer initiation, treatment, and outcome: Boon or bane?

Lung cancer is the top reason for cancer-related deaths worldwide. The 5-year overall survival rate of lung cancer is approximately 20 % due to the delayed diagnosis and low response rate to regular treatments. Microbiota, both host-microbiota and alien pathogenic microbiota, have been investigated to be involved in a complicated and contradictory relationship with lung cancer initiation, treatments, and prognosis. Disorders of certain host-microbiota and pathogen infection are associated with the risk of lung cancers based on epidemiological evidence, and antibiotics (ATBs) could dramatically impair anti-cancer treatment efficacy, including chemotherapy and immunotherapy. Moreover, probiotics and microbe-mediated drugs are potential approaches to enhance regular anti-tumor treatments. Therefore, the knowledge of the complex dual effect of microbes on lung cancer is beneficial to take their essence and remove their dross. This review offers insight into the current trends and advancements in microbiota or microbial components related to lung cancer.

A Bacillus subtilis ΔpdxT mutant suppresses vitamin B6 limitation by acquiring mutations enhancing pdxS gene dosage and ammonium assimilation

Pyridoxal-5'-phosphate (PLP), the biologically active form of vitamin B6, serves as a cofactor for many enzymes. The Gram-positive model bacterium Bacillus subtilis synthesizes PLP via the PdxST enzyme complex, consisting of the PdxT glutaminase and the PdxS PLP synthase subunits, respectively. PdxT converts glutamine to glutamate and ammonia of which the latter is channelled to PdxS. At high extracellular ammonium concentrations, the PdxS PLP synthase subunit does not depend on PdxT. Here, we assessed the potential of a B. subtilis ΔpdxT mutant to adapt to PLP limitation at the genome level. The majority of ΔpdxT suppressors had amplified a genomic region containing the pdxS gene. We also identified mutants having acquired as yet undescribed mutations in ammonium assimilation genes, indicating that the overproduction of PdxS and the NrgA ammonium transporter partially relieve vitamin B6 limitation in a ΔpdxT mutant when extracellular ammonium is scarce. Furthermore, we found that PdxS positively affects complex colony formation in B. subtilis. The catalytic mechanism of the PdxS PLP synthase subunit could be the reason for the limited evolution of the enzyme and why we could not identify a PdxS variant producing PLP independently of PdxT at low ammonium concentrations.

Underground metabolism facilitates the evolution of novel pathways for vitamin B6 biosynthesis

Abstract

The term vitamin B6 is a designation for the vitamers pyridoxal, pyridoxamine, pyridoxine and the respective phosphate esters pyridoxal-5′-phosphate (PLP), pyridoxamine-5′-phosphate and pyridoxine-5′-phosphate. Animals and humans are unable to synthesise vitamin B6. These organisms have to take up vitamin B6 with their diet. Therefore, vitamin B6 is of commercial interest as a food additive and for applications in the pharmaceutical industry. As yet, two naturally occurring routes for de novo synthesis of PLP are known. Both routes have been genetically engineered to obtain bacteria overproducing vitamin B6. Still, major genetic engineering efforts using the existing pathways are required for developing fermentation processes that could outcompete the chemical synthesis of vitamin B6. Recent suppressor screens using mutants of the Gram-negative and Gram-positive model bacteria Escherichia coli and Bacillus subtilis, respectively, carrying mutations in the native pathways or heterologous genes uncovered novel routes for PLP biosynthesis. These pathways consist of promiscuous enzymes and enzymes that are already involved in vitamin B6 biosynthesis. Thus, E. coli and B. subtilis contain multiple promiscuous enzymes causing a so-called underground metabolism allowing the bacteria to bypass disrupted vitamin B6 biosynthetic pathways. The suppressor screens also show the genomic plasticity of the bacteria to suppress a genetic lesion. We discuss the potential of the serendipitous pathways to serve as a starting point for the development of bacteria overproducing vitamin B6.

Key points

• Known vitamin B6 routes have been genetically engineered.

• Underground metabolism facilitates the emergence of novel vitamin B6 biosynthetic pathways.

• These pathways may be suitable to engineer bacteria overproducing vitamin B6.

Temperature-controlled molecular weight of hyaluronic acid produced by engineered Bacillus subtilis

To produce high-, medium- and low-molecular-weight hyaluronic acid (HA) at different temperatures using engineered Bacillus subtilis expressing hyaluronidase (HAase) from leech. By overexpressing the HAase gene hya in the HA-producing strain WmB using temperature-sensitive plasmid pKSV7, the engineered strain WmB-PYh produced HA with different molecular weights (8.61 kDa at 32 °C, 0.615 MDa at 42 °C, and 6.19 MDa at 47 °C). In this study, the molecular weight of HA was regulated by using leech HAase expressed from a temperature-sensitive plasmid. We thus obtained different molecular weight HAs by using a single bacterial strain at different culture temperatures.

Resistance to serine in Bacillus subtilis: identification of the serine transporter YbeC and of a metabolic network that links serine and threonine metabolism

The Gram‐positive bacterium Bacillus subtilis uses serine not only as a building block for proteins but also as an important precursor in many anabolic reactions. Moreover, a lack of serine results in the initiation of biofilm formation. However, excess serine inhibits the growth of B. subtilis. To unravel the underlying mechanisms, we isolated suppressor mutants that can tolerate toxic serine concentrations by three targeted and non‐targeted genome‐wide screens. All screens as well as genetic complementation in Escherichia coli identified the so far uncharacterized permease YbeC as the major serine transporter of B. subtilis. In addition to YbeC, the threonine transporters BcaP and YbxG make minor contributions to serine uptake. A strain lacking these three transporters was able to tolerate 100 mM serine whereas the wild type strain was already inhibited by 1 mM of the amino acid. The screen for serine‐resistant mutants also identified mutations that result in increased serine degradation and in increased expression of threonine biosynthetic enzymes suggesting that serine toxicity results from interference with threonine biosynthesis.

Complete Genome Sequence of the Prototrophic Bacillus subtilis subsp. subtilis Strain SP1

Here, we present the complete genome sequence of the Bacillus subtilis strain SP1. This strain is a descendant of the laboratory strain 168. The strain is suitable for biotechnological applications because the prototrophy for tryptophan has been restored. Due to laboratory cultivation, the strain has acquired 24 additional sequence variations.

Here, we present the complete genome sequence of the Bacillus subtilis strain SP1. This strain is a descendant of the laboratory strain 168. The strain is suitable for biotechnological applications because the prototrophy for tryptophan has been restored. Due to laboratory cultivation, the strain has acquired 24 additional sequence variations.

A Survey of Pyridoxal 5'-Phosphate-Dependent Proteins in the Gram-Positive Model Bacterium Bacillus subtilis

The B6 vitamer pyridoxal 5′-phosphate (PLP) is a co-factor for proteins and enzymes that are involved in diverse cellular processes. Therefore, PLP is essential for organisms from all kingdoms of life. Here we provide an overview about the PLP-dependent proteins from the Gram-positive soil bacterium Bacillus subtilis. Since B. subtilis serves as a model system in basic research and as a production host in industry, knowledge about the PLP-dependent proteins could facilitate engineering the bacteria for biotechnological applications. The survey revealed that the majority of the PLP-dependent proteins are involved in metabolic pathways like amino acid biosynthesis and degradation, biosynthesis of antibacterial compounds, utilization of nucleotides as well as in iron and carbon metabolism. Many PLP-dependent proteins participate in de novo synthesis of the co-factors biotin, folate, heme, and NAD⁺ as well as in cell wall metabolism, tRNA modification, regulation of gene expression, sporulation, and biofilm formation. A surprisingly large group of PLP-dependent proteins (29%) belong to the group of poorly characterized proteins. This review underpins the need to characterize the PLP-dependent proteins of unknown function to fully understand the “PLP-ome” of B. subtilis.

Identification of the first glyphosate transporter by genomic adaptation

The soil bacterium Bacillus subtilis can get into contact with growth-inhibiting substances, which may be of anthropogenic origin. Glyphosate is such a substance serving as a nonselective herbicide. Glyphosate specifically inhibits the 5-enolpyruvyl-shikimate-3-phosphate (EPSP) synthase, which generates an essential precursor for de novo synthesis of aromatic amino acids in plants, fungi, bacteria and archaea. Inhibition of the EPSP synthase by glyphosate results in depletion of the cellular levels of aromatic amino acids unless the environment provides them. Here, we have assessed the potential of B. subtilis to adapt to glyphosate at the genome level. In contrast to Escherichia coli, which evolves glyphosate resistance by elevating the production and decreasing the glyphosate sensitivity of the EPSP synthase, B. subtilis primarily inactivates the gltT gene encoding the high-affinity glutamate/aspartate symporter GltT. Further adaptation of the gltT mutants to glyphosate led to the inactivation of the gltP gene encoding the glutamate transporter GltP. Metabolome analyses confirmed that GltT is the major entryway of glyphosate into B. subtilis. GltP, the GltT homologue of E. coli also transports glyphosate into B. subtilis. Finally, we found that GltT is involved in uptake of the herbicide glufosinate, which inhibits the glutamine synthetase.

Wiring cell growth to product formation

Microbial cell factories offer new and sustainable production routes for high-value chemicals. However, identification of high producers within a library of clones remains a challenge. When product formation is coupled to growth, millions of metabolic variants can be effectively interrogated by growth selection, dramatically increasing the throughput of strain evaluation. While growth-coupled selections for cell factories have a long history of success based on metabolite auxotrophies and toxic antimetabolites, such methods are generally restricted to molecules native to their host metabolism. New synthetic biology tools offer the opportunity to rewire cellular metabolism to depend on specific and non-native products for growth.

Microbial cell factories for the sustainable manufacturing of B vitamins

Vitamins are essential compounds in human and animal diets. Their demand is increasing globally in food, feed, cosmetics, chemical and pharmaceutical industries. Most current production methods are unsustainable because they use non-renewable sources and often generate hazardous waste. Many microorganisms produce vitamins naturally, but their corresponding metabolic pathways are tightly regulated since vitamins are needed only in catalytic amounts. Metabolic engineering is accelerating the development of microbial cell factories for vitamins that could compete with chemical methods that have been optimized over decades, but scientific hurdles remain. Additional technological and regulatory issues need to be overcome for innovative bioprocesses to reach the market. Here, we review the current state of development and challenges for fermentative processes for the B vitamin group.

A two-step evolutionary process establishes a non-native vitamin B6 pathway in Bacillus subtilis

Pyridoxal 5'-phosphate (PLP), the most important form of vitamin B6 serves as a cofactor for many proteins. Two alternative pathways for de novo PLP biosynthesis are known: the short deoxy-xylulose-5-phosphate (DXP)-independent pathway, which is present in the Gram-positive model bacterium Bacillus subtilis and the longer DXP-dependent pathway, which has been intensively studied in the Gram-negative model bacterium Escherichia coli. Previous studies revealed that bacteria contain many promiscuous enzymes causing a so-called 'underground metabolism', which can be important for the evolution of novel pathways. Here, we evaluated the potential of B. subtilis to use a truncated non-native DXP-dependent PLP pathway from E. coli for PLP synthesis. Adaptive laboratory evolution experiments revealed that two non-native enzymes catalysing the last steps of the DXP-dependent PLP pathway and two genomic alterations are sufficient to allow growth of vitamin B6 auxotrophic bacteria as rapid as the wild type. Thus, the existence of an underground metabolism in B. subtilis facilitates the generation of a pathway for synthesis of PLP using parts of a non-native vitamin B6 pathway. The introduction of non-native enzymes into a metabolic network and rewiring of native metabolism could be helpful to generate pathways that might be optimized for producing valuable substances.

The contribution of bacterial genome engineering to sustainable development

The United Nations’ Sustainable Development Goals define important challenges for the prosperous development of mankind. To reach several of these goals, among them the production of value‐added compounds, improved economic and ecologic balance of production processes, prevention of climate change and protection of ecosystems, the use of engineered bacteria can make valuable contributions. We discuss the strategies for genome engineering and how they can be applied to meet the United Nations’ goals for sustainable development.

Occurrence of the (2R,3S)-Isomer of 2-Amino-3,4-dihydroxybutanoic Acid in the Mushroom Hypsizygus marmoreus

Here, we report the occurrence of the (2R,3S)-isomer of 2-amino-3,4-dihydroxybutanoic acid (d-ADHB) in the fruiting body of an edible mushroom, Hypsizygus marmoreus. This is an unusual example of the accumulation of a d-amino acid whose enantiomer is not a proteinogenic amino acid. We show that d-ADHB occurs specifically in the mushroom H. marmoreus. Other edible mushrooms examined, including Pholiota microspora, Pleurotus eryngii, Mycena chlorophos, Sparassis crispa, Grifola frondosa, Pleurotus ostreatus, and Flammulina velutipes, do not contain detectable levels of d-ADHB. The concentration of d-ADHB in the fruiting body of H. marmoreus is relatively high (approximately 1.3 mg/g of fruiting body) and is comparable to the concentration of some of the most abundant free proteinogenic amino acids. Quantitative analysis of d-ADHB during fruiting body development demonstrated that the amino acid is synthesized during the fruiting body formation period. The absence of the putative precursors of d-ADHB, the (2S,3S)-isomer of ADHB and 2-oxo-tetronate, and the enzyme activities of d-ADHB racemase (2-epimerase) and transaminase suggested that d-ADHB is synthesized by a unique mechanism in this organism. Our data also suggested that the lack of or low expression of a d-ADHB degradation enzyme is a key determinant of d-ADHB accumulation in H. marmoreus.

Anticancer effects of the microbiome and its products

The human gut microbiome modulates many host processes, including metabolism, inflammation, and immune and cellular responses. It is becoming increasingly apparent that the microbiome can also influence the development of cancer. In preclinical models, the host response to cancer treatment has been improved by modulating the gut microbiome; this is known to have an altered composition in many diseases, including cancer. In addition, cancer treatment with microbial agents or their products has the potential to shrink tumours. However, the microbiome could also negatively influence cancer prognosis through the production of potentially oncogenic toxins and metabolites by bacteria. Thus, future antineoplastic treatments could combine the modulation of the microbiome and its products with immunotherapeutics and more conventional approaches that directly target malignant cells.

Vitamin B6 metabolism in microbes and approaches for fermentative production

Vitamin B6 is a designation for the six vitamers pyridoxal, pyridoxine, pyridoxamine, pyridoxal 5'-phosphate (PLP), pyridoxine 5'-phosphate, and pyridoxamine. PLP, being the most important B6 vitamer, serves as a cofactor for many proteins and enzymes. In contrast to other organisms, animals and humans have to ingest vitamin B6 with their food. Several disorders are associated with vitamin B6 deficiency. Moreover, pharmaceuticals interfere with metabolism of the cofactor, which also results in vitamin B6 deficiency. Therefore, vitamin B6 is a valuable compound for the pharmaceutical and the food industry. Although vitamin B6 is currently chemically synthesized, there is considerable interest on the industrial side to shift from chemical processes to sustainable fermentation technologies. Here, we review recent findings regarding biosynthesis and homeostasis of vitamin B6 and describe the approaches that have been made in the past to develop microbial production processes. Moreover, we will describe novel routes for vitamin B6 biosynthesis and discuss their potential for engineering bacteria that overproduce the commercially valuable substance. We also highlight bottlenecks of the vitamin B6 biosynthetic pathways and propose strategies to circumvent these limitations.

Members of a Novel Kinase Family (DUF1537) Can Recycle Toxic Intermediates into an Essential Metabolite

DUF1537 is a novel family of kinases identified by comparative genomic approaches. The family is widespread and found in all sequenced plant genomes and 16% of sequenced bacterial genomes. DUF1537 is not a monofunctional family and contains subgroups that can be separated by phylogenetic and genome neighborhood context analyses. A subset of the DUF1537 proteins is strongly associated by physical clustering and gene fusion with the PdxA2 family, demonstrated here to be a functional paralog of the 4-phosphohydroxy-l-threonine dehydrogenase enzyme (PdxA), a central enzyme in the synthesis of pyridoxal-5'-phosphate (PLP) in proteobacteria. Some members of this DUF1537 subgroup phosphorylate l-4-hydroxythreonine (4HT) into 4-phosphohydroxy-l-threonine (4PHT), the substrate of PdxA, in vitro and in vivo. This provides an alternative route to PLP from the toxic antimetabolite 4HT that can be directly generated from the toxic intermediate glycolaldehyde. Although the kinetic and physical clustering data indicate that these functions in PLP synthesis are not the main roles of the DUF1537-PdxA2 enzymes, genetic and physiological data suggest these side activities function has been maintained in diverse sets of organisms.