Rewiring the native methanol assimilation metabolism by incorporating the heterologous ribulose monophosphate cycle into Methylorubrum extorquens

Interspecific cross-talk: The catalyst driving microbial biosynthesis of secondary metabolites

Microorganisms co-exist and co-evolve in nature, forming intricate ecological communities. The interspecies cross-talk within these communities creates and sustains their great biosynthetic potential, making them an important source of natural medicines and high-value-added chemicals. However, conventional investigations into microbial metabolites are typically carried out in pure cultures, resulting in the absence of specific activating factors and consequently causing a substantial number of biosynthetic gene clusters to remain silent. This, in turn, hampers the in-depth exploration of microbial biosynthetic potential and frequently presents researchers with the challenge of rediscovering compounds. In response to this challenge, the coculture strategy has emerged to explore microbial biosynthetic capabilities and has shed light on the study of cross-talk mechanisms. These elucidated mechanisms will contribute to a better understanding of complex biosynthetic regulations and offer valuable insights to guide the mining of secondary metabolites. This review summarizes the research advances in microbial cross-talk mechanisms, with a particular focus on the mechanisms that activate the biosynthesis of secondary metabolites. Additionally, the instructive value of these mechanisms for developing strategies to activate biosynthetic pathways is discussed. Moreover, challenges and recommendations for conducting in-depth studies on the cross-talk mechanisms are presented.

Industrializing methanotrophs and other methylotrophic bacteria: from bioengineering to product recovery

Microbes that use the single-carbon substrates methanol and methane offer great promise to bioindustry along with substantial environmental benefits. Methanotrophs and other methylotrophs can be engineered and optimized to produce a wide range of products, from biopolymers to biofuels and beyond. While significant limitations remain, including delivery of single-carbon feedstock to bioreactors, efficient growth, and scale-up, these challenges are being addressed and notable improvements have been rapid. Development of expression chassis, use of genome-scale and regulatory models based on omics data, improvements in bioreactor design and operation, and development of green product recovery schemes are enabling the rapid development of single-carbon bioconversion in the industrial space.

Phosphoribosylpyrophosphate synthetase as a metabolic valve advances Methylobacterium/Methylorubrum phyllosphere colonization and plant growth

The proficiency of phyllosphere microbiomes in efficiently utilizing plant-provided nutrients is pivotal for their successful colonization of plants. The methylotrophic capabilities of Methylobacterium/Methylorubrum play a crucial role in this process. However, the precise mechanisms facilitating efficient colonization remain elusive. In the present study, we investigate the significance of methanol assimilation in shaping the success of mutualistic relationships between methylotrophs and plants. A set of strains originating from Methylorubrum extorquens AM1 are subjected to evolutionary pressures to thrive under low methanol conditions. A mutation in the phosphoribosylpyrophosphate synthetase gene is identified, which converts it into a metabolic valve. This valve redirects limited C1-carbon resources towards the synthesis of biomass by up-regulating a non-essential phosphoketolase pathway. These newly acquired bacterial traits demonstrate superior colonization capabilities, even at low abundance, leading to increased growth of inoculated plants. This function is prevalent in Methylobacterium/Methylorubrum strains. In summary, our findings offer insights that could guide the selection of Methylobacterium/Methylorubrum strains for advantageous agricultural applications.

Engineering photo-methylotrophic Methylobacterium for enhanced 3-hydroxypropionic acid production during non-growth stage fermentation

This study explored the potential of methanol as a sustainable feedstock for biomanufacturing, focusing on Methylobacterium extorquens, a well-established representative of methylotrophic cell factories. Despite this bacterium's long history, its untapped photosynthetic capabilities for production enhancement have remained unreported. Using genome-scale flux balance analysis, it was hypothesized that introducing photon fluxes could boost the yield of 3-hydroxypropionic acid (3-HP), an energy- and reducing equivalent-consuming chemicals. To realize this, M. extorquens was genetically modified by eliminating the negative regulator of photosynthesis, leading to improved ATP levels and metabolic activity in non-growth cells during a two-stage fermentation process. This modification resulted in a remarkable 3.0-fold increase in 3-HP titer and a 2.1-fold increase in its yield during stage (II). Transcriptomics revealed that enhanced light-driven methanol oxidation, NADH transhydrogenation, ATP generation, and fatty acid degradation were key factors. This development of photo-methylotrophy as a platform technology introduced novel opportunities for future production enhancements.

Heterologous production of 3-hydroxypropionic acid in Methylorubrum extorquens by introducing the mcr gene via a multi-round chromosomal integration system based on cre-lox71/lox66 and transposon

BACKGROUND AND AIM

Reprogramming microorganisms to enhance the production of metabolites is a part of contemporary synthetic biology, which relies on the availability of genetic tools to successfully manipulate the bacteria. Methylorubrum extorquens AM1 is a platform microorganism used to convert C1 compounds into various value-added products. However, the repertoire of available plasmids to conveniently and quickly fine-tune the expression of multiple genes in this strain is extremely limited compared with other model microorganisms such as Escherichia coli. Thus, this study aimed to integrate existing technologies, such as transposon-mediated chromosomal integration and cre-lox-mediated recombination, to achieve the diversified expression of target genes through multiple chromosomal insertions in M. extorquens AM1.

RESULTS

A single plasmid toolkit, pSL-TP-cre-km, containing a miniHimar1 transposon and an inducible cre-lox71/lox66 system, was constructed and characterized for its multiple chromosomal integration capacity. A co-transcribed mcr-egfp cassette [for the production of 3-hydroxypropionic acid (3-HP) and a reporting green fluorescent protein] was added to construct pTP-cre-mcr-egfp for evaluating its utility in mediating the expression of heterologous genes, resulting in the production of 3-HP with a titer of 34.7-55.2 mg/L by two chromosomal integration copies. Furthermore, in association with the expression of plasmid-based mcr, 3-HP production increased to 65.5-92.4 mg/L.

CONCLUSIONS

This study used a multi-round chromosomal integration system based on cre-lox71/lox66 and a transposon to construct a single constructed vector. A heterologous mcr gene was introduced through this vector, and high expression of 3-hydroxypropionic acid was achieved in M. extorquens. This study provided an efficient genetic tool for manipulating M. extorquens, which not only help increase the expression of heterologous genes in M. extorquens but also provide a reference for strains lacking genetic manipulation vectors.

Artificial carbon assimilation: From unnatural reactions and pathways to synthetic autotrophic systems

Synthetic biology is being increasingly used to establish novel carbon assimilation pathways and artificial autotrophic strains that can be used in low-carbon biomanufacturing. Currently, artificial pathway design has made significant progress from advocacy to practice within a relatively short span of just over ten years. However, there is still huge scope for exploration of pathway diversity, operational efficiency, and host suitability. The accelerated research process will bring greater opportunities and challenges. In this paper, we provide a comprehensive summary and interpretation of representative one-carbon assimilation pathway designs and artificial autotrophic strain construction work. In addition, we propose some feasible design solutions based on existing research results and patterns to promote the development and application of artificial autotrophy.

Metabolic engineering of Escherichia coli to utilize methanol as a co-substrate for the production of (R)-1,3-butanediol

Due to its abundance, cost-effectiveness, and high reducibility, methanol has gained considerable attention in the biomanufacturing industry as a nonfood feedstock for the production of value-added chemicals. The range of chemicals that can be derived from methanol, however, remains constrained and is currently in the concept validation phase. This study aimed to develop and evaluate a hybrid methanol assimilation pathway in Escherichia coli to improve the production of (R)-1,3-butanediol ((R)-1,3-BDO) by utilizing methanol and sugars as co-substrates. By combining the methanol dehydrogenase (MDH) from the prokaryotes with the dihydroxyacetone synthase (DAS) from the eukaryotes, the hybrid pathway facilitates methanol conversion into the central metabolism while generating NADH at the same time. Through pathway optimization and targeted gene deletions, we have successfully developed an E. coli strain capable of producing 5.79 g/L (R)-1,3-BDO in shake flask experiments and 13.71 g/L (R)-1,3-BDO with a yield of 0.35 C-mol/C-mol in batch fermentation using methanol and glucose as co-substrates. Our study also showed the incorporation of 13C-methanol into cellular intermediates and an increase in NAD(P)H concentration, confirming the role of methanol as a co-substrate and supplier of NADH. In addition, our study also demonstrated the co-utilization of methanol with xylose for the production of (R)-1,3-BDO, expanding the substrate spectrum for sustainable 1,3-BDO production.

The potential of CO2-based production cycles in biotechnology to fight the climate crisis

Rising CO2 emissions have pushed scientists to develop new technologies for a more sustainable bio-based economy. Microbial conversion of CO2 and CO2-derived carbon substrates into valuable compounds can contribute to carbon neutrality and sustainability. Here, we discuss the potential of C1 carbon sources as raw materials to produce energy, materials, and food and feed using microbial cell factories. We provide an overview of potential microbes, natural and synthetic C1 utilization pathways, and compare their metabolic driving forces. Finally, we sketch a future in which C1 substrates replace traditional feedstocks and we evaluate the costs associated with such an endeavor.

Size-resolved ambient bioaerosols concentration, antibiotic resistance, and community composition during autumn and winter seasons in Qatar

This study investigates the size distribution, microbial composition, and antibiotic resistance (ABR) of airborne bioaerosols at a suburban location in Doha, Qatar between October 2021 and January 2022. Samples were collected using an Andersen six-stage viable cascade impactor and a liquid impinger. Findings showed that the mean bacteria concentration (464 CFU/m3) was significantly higher than that of fungi (242 CFU/m3) during the study period. Both bacteria and fungi were most abundant in the aerodynamic size fractions of 1.10-2.21 μm, with peak concentrations observed in the mornings and lowest concentrations in the afternoons across all size fractions. A total of 24 different culturable species were identified, with the most abundant ones being Pasteurella pneumotropica (9.71%), Pantoea spp. 1 (8.73%), and Proteus penneri (7.77%) spp. At the phylum level, the bacterial community configurations during the autumn and winter seasons were nearly identical as revealed by molecular genomics, with Proteobacteria being the most predominant, followed by Firmicutes, Bacteroidetes, Acidobacteriota, and Planctomycetota. However, there was a significant variation in dominant genera between autumn and winter. The most abundant genera included Sphingomonas, Paraburkholderia, Comamonas, Bacillus, and Lysinibacillus. Several bacterial genera identified in this study have important public health and ecological implications, including the risk of respiratory tract infections. Furthermore, the study found that ABR was highest in December, with bioaerosols exhibiting resistance to at least 5 out of 10 antibiotics, and 100% resistance to Metronidazole in all samples. Metagenomics analysis revealed the presence of various airborne bacteria that were not detected through culture-dependent methods. This study provides valuable insights into the airborne microbial composition, temporal variability and ABR in the Arabian Gulf region.

Characterization of C30 carotenoid and identification of its biosynthetic gene cluster in Methylobacterium extorquens AM1

Methylobacterium species, the representative bacteria distributed in phyllosphere region of plants, often synthesize carotenoids to resist harmful UV radiations. Methylobacterium extorquens is known to produce a carotenoid pigment and recent research revealed that this carotenoid has a C30 backbone. However, its exact structure remains unknown. In the present study, the carotenoid produced by M. extorquens AM1 was isolated and its structure was determined as 4-[2-O-11Z-octadecenoyl-β-glucopyranosyl]-4,4'-diapolycopenedioc acid (1), a glycosylated C30 carotenoid. Furthermore, the genes related to the C30 carotenoid synthesis were investigated. Squalene, the precursor of the C30 carotenoid, is synthesized by the co-occurrence of META1p1815, META1p1816 and META1p1817. Further overexpression of the genes related to squalene synthesis improved the titer of carotenoid 1. By using gene deletion and gene complementation experiments, the glycosyltransferase META1p3663 and acyltransferase META1p3664 were firstly confirmed to catalyze the tailoring steps from 4,4'-diapolycopene-4,4'-dioic acid to carotenoid 1. In conclusion, the structure and biosynthetic genes of carotenoid 1 produced by M. extorquens AM1 were firstly characterized in this work, which shed lights on engineering M. extorquens AM1 for producing carotenoid 1 in high yield.

Methanol-based biomanufacturing of fuels and chemicals using native and synthetic methylotrophs

Methanol has recently gained significant attention as a potential carbon substrate for the production of fuels and chemicals, owing to its high degree of reduction, abundance, and low price. Native methylotrophic yeasts and bacteria have been investigated for the production of fuels and chemicals. Alternatively, synthetic methylotrophic strains are also being developed by reconstructing methanol utilization pathways in model microorganisms, such as Escherichia coli. Owing to the complex metabolic pathways, limited availability of genetic tools, and methanol/formaldehyde toxicity, the high-level production of target products for industrial applications are still under development to satisfy commercial feasibility. This article reviews the production of biofuels and chemicals by native and synthetic methylotrophic microorganisms. It also highlights the advantages and limitations of both types of methylotrophs and provides an overview of ways to improve their efficiency for the production of fuels and chemicals from methanol.

Comparative Genomic Analysis of a Methylorubrum rhodesianum MB200 Isolated from Biogas Digesters Provided New Insights into the Carbon Metabolism of Methylotrophic Bacteria

Methylotrophic bacteria are widely distributed in nature and can be applied in bioconversion because of their ability to use one-carbon source. The aim of this study was to investigate the mechanism underlying utilization of high methanol content and other carbon sources by Methylorubrum rhodesianum strain MB200 via comparative genomics and analysis of carbon metabolism pathway. The genomic analysis revealed that the strain MB200 had a genome size of 5.7 Mb and two plasmids. Its genome was presented and compared with that of the 25 fully sequenced strains of Methylobacterium genus. Comparative genomics revealed that the Methylorubrum strains had closer collinearity, more shared orthogroups, and more conservative MDH cluster. The transcriptome analysis of the strain MB200 in the presence of various carbon sources revealed that a battery of genes was involved in the methanol metabolism. These genes are involved in the following functions: carbon fixation, electron transfer chain, ATP energy release, and resistance to oxidation. Particularly, the central carbon metabolism pathway of the strain MB200 was reconstructed to reflect the possible reality of the carbon metabolism, including ethanol metabolism. Partial propionate metabolism involved in ethyl malonyl-CoA (EMC) pathway might help to relieve the restriction of the serine cycle. In addition, the glycine cleavage system (GCS) was observed to participate in the central carbon metabolism pathway. The study revealed the coordination of several metabolic pathways, where various carbon sources could induce associated metabolic pathways. To the best of our knowledge, this is the first study providing a more comprehensive understanding of the central carbon metabolism in Methylorubrum. This study provided a reference for potential synthetic and industrial applications of this genus and its use as chassis cells.

Embracing a low-carbon future by the production and marketing of C1 gas protein

Food scarcity and environmental deterioration are two major problems that human populations currently face. Fortunately, the disruptive innovation of raw food materials has been stimulated by the rapid evolution of biomanufacturing. Therefore, it is expected that the new trends in technology will not only alter the natural resource-dependent food production systems and the traditional way of life but also reduce and assimilate the greenhouse gases released into the atmosphere. This review article summarizes the metabolic pathways associated with C1 gas conversion and the production of single-cell protein for animal feed. Moreover, the protein function, worldwide authorization, market access, and methods to overcome challenges in C1 gas assimilation microbial cell factory construction are also provided. With widespread attention and increasing policy support, the production of C1 gas protein will bring more opportunities and make tremendous contributions to our sustainable future.

Engineering and adaptive laboratory evolution of Escherichia coli for improving methanol utilization based on a hybrid methanol assimilation pathway

Engineering Escherichia coli for efficient methanol assimilation is important for developing methanol as an emerging next-generation feedstock for industrial biotechnology. While recent attempts to engineer E. coli as a synthetic methylotroph have achieved great success, most of these works are based on the engineering of the prokaryotic ribulose monophosphate (RuMP) pathway. In this study, we introduced a hybrid methanol assimilation pathway which consists of prokaryotic methanol dehydrogenase (Mdh) and eukaryotic xylulose monophosphate (XuMP) pathway enzyme dihydroxyacetone synthase (Das) into E. coli and reprogrammed E. coli metabolism to improve methanol assimilation by combining rational design and adaptive laboratory evolution. By deletion and down-regulation of key genes in the TCA cycle and glycolysis to increase the flux toward the cyclic XuMP pathway, methanol consumption and the assimilation of methanol to biomass were significantly improved. Further improvements in methanol utilization and cell growth were achieved via adaptive laboratory evolution and a final evolved strain can grow on methanol with only 0.1 g/L yeast extract as co-substrate. ¹³C-methanol labeling assay demonstrated significantly higher labeling in intracellular metabolites in glycolysis, TCA cycle, pentose phosphate pathway, and amino acids. Transcriptomics analysis showed that the expression of fba, dhak, and part of pentose phosphate pathway genes were highly up-regulated, suggesting that the rational engineering strategies and adaptive evolution are effective for activating the cyclic XuMP pathway. This study demonstrated the feasibility and provided new strategies to construct synthetic methylotrophy of E. coli based on the hybrid methanol assimilation pathway with Mdh and Das.

C1-based biomanufacturing: Advances, challenges and perspectives

One-carbon (C1) compounds have emerged as a key research focus due to the growth of metabolic engineering and synthetic biology as affordable and sustainable nonfood sugar feedstocks for energy-efficient and environmentally friendly biomanufacturing. This paper summarizes and discusses current developments in C1 compounds for biomanufacturing. First, two primary groups of microbes that use C1 compounds (native and synthetic) are introduced, and the traits, categorization, and functions of C1 microbes are summarized. Second, engineering strategies for C1 utilization are compiled and reviewed, including reconstruction of C1-utilization pathway, enzyme engineering, cofactor engineering, genome-scale modeling, and adaptive laboratory evolution. Third, a review of C1 compounds' uses in the synthesis of biofuels and high-value compounds is presented. Finally, potential obstacles to C1-based biomanufacturing are highlighted along with future research initiatives.

Developing methylotrophic microbial platforms for a methanol-based bioindustry

Methanol, a relatively cheap and renewable single-carbon feedstock, has gained considerable attention as a substrate for the bio-production of commodity chemicals. Conventionally produced from syngas, along with emerging possibilities of generation from methane and CO2, this C1 substrate can serve as a pool for sequestering greenhouse gases while supporting a sustainable bio-economy. Methylotrophic organisms, with the inherent ability to use methanol as the sole carbon and energy source, are competent candidates as platform organisms. Accordingly, methanol bioconversion pathways have been an attractive target for biotechnological and bioengineering interventions in developing microbial cell factories. This review summarizes the recent advances in methanol-based production of various bulk and value-added chemicals exploiting the native and synthetic methylotrophic organisms. Finally, the current challenges and prospects of streamlining these methylotrophic platforms are discussed.

Efficient fatty acid synthesis from methanol in methylotrophic yeast

Methanol is an attractive C1 feedstock with high abundance and low cost in bio-manufacturing. However, the metabolic construction of cell factories to utilize methanol for chemicals production remains a challenge due to the toxic intermediates and complicated metabolic pathways. The group of Zhou rescued methylotrophic yeast from cell death and achieved high-level production of free fatty acids from methanol through a combination of adaptive laboratory evolution, rational metabolic engineering and multi-omics analysis.

Rescuing yeast from cell death enables overproduction of fatty acids from sole methanol

Methanol is an ideal feedstock for biomanufacturing that can be beneficial for global carbon neutrality; however, the toxicity of methanol limits the efficiency of methanol metabolism toward biochemical production. We here show that engineering production of free fatty acids from sole methanol results in cell death with decreased cellular levels of phospholipids in the methylotrophic yeast Ogataea polymorpha, and cell growth is restored by adaptive laboratory evolution. Whole-genome sequencing of the adapted strains reveals that inactivation of LPL1 (encoding a putative lipase) and IZH3 (encoding a membrane protein related to zinc metabolism) preserve cell survival by restoring phospholipid metabolism. Engineering the pentose phosphate pathway and gluconeogenesis enables high-level production of free fatty acid (15.9 g l^-1) from sole methanol. Preventing methanol-associated toxicity underscores the link between lipid metabolism and methanol tolerance, which should contribute to enhancing methanol-based biomanufacturing.