Systems biology approach reveals that overflow metabolism of acetate in Escherichia coli is triggered by carbon catabolite repression of acetyl-CoA synthetase

Acetyl-CoA synthetase activity is enzymatically regulated by lysine acetylation using acetyl-CoA or acetyl-phosphate as donor molecule

The AMP-forming acetyl-CoA synthetase is regulated by lysine acetylation both in bacteria and eukaryotes. However, the underlying mechanism is poorly understood. The Bacillus subtilis acetyltransferase AcuA and the AMP-forming acetyl-CoA synthetase AcsA form an AcuA•AcsA complex, dissociating upon lysine acetylation of AcsA by AcuA. Crystal structures of AcsA from Chloroflexota bacterium in the apo form and in complex with acetyl-adenosine-5'-monophosphate (acetyl-AMP) support the flexible C-terminal domain adopting different conformations. AlphaFold2 predictions suggest binding of AcuA stabilizes AcsA in an undescribed conformation. We show the AcuA•AcsA complex dissociates upon acetyl-coenzyme A (acetyl-CoA) dependent acetylation of AcsA by AcuA. We discover an intrinsic phosphotransacetylase activity enabling AcuA•AcsA generating acetyl-CoA from acetyl-phosphate (AcP) and coenzyme A (CoA) used by AcuA to acetylate and inactivate AcsA. Here, we provide mechanistic insights into the regulation of AMP-forming acetyl-CoA synthetases by lysine acetylation and discover an intrinsic phosphotransacetylase allowing modulation of its activity based on AcP and CoA levels.

Microbial community-scale metabolic modelling predicts personalized short-chain fatty acid production profiles in the human gut

Microbially derived short-chain fatty acids (SCFAs) in the human gut are tightly coupled to host metabolism, immune regulation and integrity of the intestinal epithelium. However, the production of SCFAs can vary widely between individuals consuming the same diet, with lower levels often associated with disease. A systems-scale mechanistic understanding of this heterogeneity is lacking. Here we use a microbial community-scale metabolic modelling (MCMM) approach to predict individual-specific SCFA production profiles to assess the impact of different dietary, prebiotic and probiotic inputs. We evaluate the quantitative accuracy of our MCMMs using in vitro and ex vivo data, plus published human cohort data. We find that MCMM SCFA predictions are significantly associated with blood-derived clinical chemistries, including cardiometabolic and immunological health markers, across a large human cohort. Finally, we demonstrate how MCMMs can be leveraged to design personalized dietary, prebiotic and probiotic interventions aimed at optimizing SCFA production in the gut. Our model represents an approach to direct gut microbiome engineering for precision health and nutrition.

Transcriptional and Metabolic Response of a Strain of Escherichia coli PTS- to a Perturbation of the Energetic Level by Modification of [ATP]/[ADP] Ratio

The intracellular [ATP]/[ADP] ratio is crucial for Escherichia coli's cellular functions, impacting transport, phosphorylation, signaling, and stress responses. Overexpression of F1-ATPase genes in E. coli increases glucose consumption, lowers energy levels, and triggers transcriptional responses in central carbon metabolism genes, particularly glycolytic ones, enhancing carbon flux. In this contribution, we report the impact of the perturbation of the energetic level in a PTS- mutant of E. coli by modifying the [ATP]/[ADP] ratio by uncoupling the cytoplasmic activity of the F1 subunit of the ATP synthase. The disruption of [ATP]/[ADP] ratio in the evolved strain of E. coli PB12 (PTS-) was achieved by the expression of the atpAGD operon encoding the soluble portion of ATP synthase F1-ATPase (strain PB12AGD+). The analysis of the physiological and metabolic response of the PTS- strain to the ATP disruption was determined using RT-qPCR of 96 genes involved in glucose and acetate transport, glycolysis and gluconeogenesis, pentose phosphate pathway (PPP), TCA cycle and glyoxylate shunt, several anaplerotic, respiratory chain, and fermentative pathways genes, sigma factors, and global regulators. The apt mutant exhibited reduced growth despite increased glucose transport due to decreased energy levels. It heightened stress response capabilities under glucose-induced energetic starvation, suggesting that the carbon flux from glycolysis is distributed toward the pentose phosphate and the Entner-Duodoroff pathway with the concomitant. Increase acetate transport, production, and utilization in response to the reduction in the [ATP]/[ADP] ratio. Upregulation of several genes encoding the TCA cycle and the glyoxylate shunt as several respiratory genes indicates increased respiratory capabilities, coupled possibly with increased availability of electron donor compounds from the TCA cycle, as this mutant increased respiratory capability by 240% more than in the PB12. The reduction in the intracellular concentration of cAMP in the atp mutant resulted in a reduced number of upregulated genes compared to PB12, suggesting that the mutant remains a robust genetic background despite the severe disruption in its energetic level.

Relative impact of three growth conditions on the Escherichia coli protein acetylome

Nε-lysine acetylation is a common posttranslational modification observed in Escherichia coli. In the present study, integrative analysis of the proteome and acetylome was performed using label-free quantitative mass spectrometry to analyze the relative influence of three factors affecting growth. The results revealed differences in the proteome, mainly owing to the type of culture medium used (defined or complex). In the acetylome, 7482 unique acetylation sites were identified. Acetylation is directly related to the abundance of proteins, and the level of acetylation in each type of culture is associated with extracellular acetate concentration. Furthermore, most acetylated lysines in the exponential phase remained in the stationary phase without dynamic turnover. Interestingly, unique acetylation sites were detected in proteins whose presence or abundance was linked to the type of culture medium. Finally, the biological function of the acetylation changes was demonstrated for three central metabolic proteins (GapA, Mdh, and AceA).

De Novo Synthesis of Dihydro-β-ionone through Metabolic Engineering and Bacterium-Yeast Coculture

Dihydro-β-ionone is a common type of ionone used in the flavor and fragrance industries because of its characteristic scent. The production of flavors in microbial cell factories offers a sustainable and environmentally friendly approach to accessing them, independent of extraction from natural sources. However, the native pathway of dihydro-β-ionone remains unclear, hindering heterologous biosynthesis in microbial hosts. Herein, we devised a microbial platform for de novo syntheses of dihydro-β-ionone from a simple carbon source with glycerol. The complete dihydro-β-ionone pathway was reconstructed in Escherichia coli with multiple metabolic engineering strategies to generate a strain capable of producing 8 mg/L of dihydro-β-ionone, although this was accompanied by a surplus precursor β-ionone in culture. To overcome this issue, Saccharomyces cerevisiae was identified as having a conversion rate for transforming β-ionone to dihydro-β-ionone that was higher than that of E. coli via whole-cell catalysis. Consequently, the titer of dihydro-β-ionone was increased using the E. coli-S. cerevisiae coculture to 27 mg/L. Our study offers an efficient platform for biobased dihydro-β-ionone production and extends coculture engineering to overproducing target molecules in extended metabolic pathways.

Efficient and Economic Utilization of Cellobiose for Glycosylation Modification by Regulating Carbon Source Supply and Metabolic Pathway In Vivo

Glycosylation, one of the most common and significant modifications in nature, has prompted the development of a cellobiose phosphorolysis route for glycosylation in vivo. However, the process of glycosylation is hampered by the notably low conversion rate of cellobiose. In this work, regulation of the carbon source supply by changing the ratio of glucose to cellobiose improved the conversion rate of cellobiose, resulting in enhancing the efficiency of glycosylation and the production of vitexin. Moreover, three genes (pgm, agp, and ushA) involved in the degradation of UDP-glucose were knocked out to relieve the degradation and diversion of the cellobiose phosphorolysis route. Finally, through the optimization of conversion conditions, we observed a continuous enhancement in cellobiose conversion rate and vitexin production in BL21ΔushAΔagp-TcCGT-CepA, corresponding to an increased concentration of added glucose. The maximum production of vitexin reached 2228 mg/L with the addition of 2 g/L cellobiose and 6 g/L glucose, which was 312% of that in BL21-TcCGT-CepA with the addition of 2 g/L cellobiose. The conversion rate of cellobiose in BL21ΔushAΔagp-TcCGT-CepA reached 88%, which was the highest conversion rate of cellobiose to date. Therefore, this study presents a cost-effective and efficient method to enhance the conversion rate of cellobiose during the glycosylation process.

Metabolic engineering of glycolysis in Escherichia coli for efficient production of patchoulol and τ-cadinol

Glucose metabolism suppresses the microbial synthesis of sesquiterpenes with a syndrome of "too much of a good thing can be bad". Here, patchoulol production in Escherichia coli was increased 2.02 times by engineering patchoulol synthase to obtain an initial strain. Knocking out the synthetic pathway for cyclic adenosine monophosphate relieved glucose repression and improved patchoulol titer and yield by 27.7 % and 43.1 %, respectively. A glycolysis regulation device mediated by pyruvate sensing was constructed which effectively alleviated overflow metabolism in a high-glucose environment with 10.2 % greater patchoulol titer in strain 070QA. Without fine-tuning the glucose-feeding rate, patchoulol titer further increased to 1675.1 mg/L in a 5-L bioreactor experiment, which was the highest level reported in E. coli. Using strain 070QA as a chassis, the τ-cadinol titer reached 15.2 g/L, representing the first report for microbial production of τ-cadinol. These findings will aid in the industrial production of sesquiterpene.

Microbial community-scale metabolic modeling predicts personalized short chain fatty acid production profiles in the human gut

Microbially-derived short chain fatty acids (SCFAs) in the human gut are tightly coupled to host metabolism, immune regulation, and integrity of the intestinal epithelium. However, the production of SCFAs can vary widely between individuals consuming the same diet, with lower levels often associated with disease. A systems-scale mechanistic understanding of this heterogeneity is lacking. We present a microbial community-scale metabolic modeling (MCMM) approach to predict individual-specific SCFA production profiles. We assess the quantitative accuracy of our MCMMs using in vitro, ex vivo, and in vivo data. Next, we show how MCMM SCFA predictions are significantly associated with blood-derived clinical chemistries, including cardiometabolic and immunological health markers, across a large human cohort. Finally, we demonstrate how MCMMs can be leveraged to design personalized dietary, prebiotic, and probiotic interventions that optimize SCFA production in the gut. Our results represent an important advance in engineering gut microbiome functional outputs for precision health and nutrition.

Biosensor-based high-throughput screening enabled efficient adipic acid production

Adipic acid is an industrially important chemical, but the current approach to synthesize it can be of serious pollution to the environment. Rencently, bio-based production of adipic acid has significantly advanced with the development of metabolic engineering and synthetic biology. However, genetic heterogeneity-caused decrease of product titer has largely limited the industrialization of chemicals like adipic acid. Therefore, in the attempt to overcome this challenge, we constitutively expressed the reverse adipate degradation pathway, designed and optimized an adipic acid biosensor, and established a high-throughput screening platform to screen for high-performance strains based on the optimized biosensor. Using this platform, we successfully screened a strain with an adipic acid titer of 188.08 mg·L^-1. Coupling the screening platform with fermentation optimization, the titer of adipic acid reached 531.88 mg·L^-1 under shake flask fermentation, which achieved an 18.78-fold improvement comparing to the initial strain. Scale-up fermentation in a 5-L fermenter utilizing the screened high-performance strain was eventually conducted, in which the adipic acid titer reached 3.62 g·L^-1. Overall, strategies developed in this study proved to be a potentially efficient method in reducing the genetic heterogeneity and was expected to provide guidance in helping to build a more efficient industrial screening process. KEY POINTS: • Developed a fine-tuned adipic acid biosensor. • Established a high-throughput screening platform to screen high-performance strains. • The titer of adipic acid reached 3.62 g·L^-1 in a 5-L fermenter.

Model-Based Characterization of E. coli Strains with Impaired Glucose Uptake

The bacterium Escherichia coli is a widely used organism in biotechnology. For high space-time yields, glucose-limited fed-batch technology is the industry standard; this is because an overflow metabolism of acetate occurs at high glucose concentrations. As an interesting alternative, various strains with limited glucose uptake have been developed. However, these have not yet been characterized under process conditions. To demonstrate the efficiency of our previously developed high-throughput robotic platform, in the present work, we characterized three different exemplary E. coli knockout (KO) strains with limited glucose uptake capacities at three different scales (microtiter plates, 10 mL bioreactor system and 100 mL bioreactor system) under excess glucose conditions with different initial glucose concentrations. The extensive measurements of growth behavior, substrate consumption, respiration, and overflow metabolism were then used to determine the appropriate growth parameters using a mechanistic mathematical model, which allowed for a comprehensive comparative analysis of the strains. The analysis was performed coherently with these different reactor configurations and the results could be successfully transferred from one platform to another. Single and double KO mutants showed reduced specific rates for substrate uptake q_Smax and acetate production q_Apmax; meanwhile, higher glucose concentrations had adverse effects on the biomass yield coefficient Y_XSem. Additional parameters compared to previous studies for the oxygen uptake rate and carbon dioxide production rate indicated differences in the specific oxygen uptake rate q_Omax. This study is an example of how automated robotic equipment, together with mathematical model-based approaches, can be successfully used to characterize strains and obtain comprehensive information more quickly, with a trade-off between throughput and analytical capacity.

A quantitative interpretation of oxidative protein folding activity in Escherichia coli

BACKGROUND

Escherichia coli is of central interest to biotechnological research and a widely used organism for producing proteins at both lab and industrial scales. However, many proteins remain difficult to produce efficiently in E. coli. This is particularly true for proteins that require post translational modifications such as disulfide bonds.

RESULTS

In this study we develop a novel approach for quantitatively investigating the ability of E. coli to produce disulfide bonds in its own proteome. We summarise the existing knowledge of the E. coli disulfide proteome and use this information to investigate the demand on this organism's quantitative oxidative folding apparatus under different growth conditions. Furthermore, we built an ordinary differential equation-based model describing the cells oxidative folding capabilities. We use the model to infer the kinetic parameters required by the cell to achieve the observed oxidative folding requirements. We find that the cellular requirement for disulfide bonded proteins changes significantly between growth conditions. Fast growing cells require most of their oxidative folding capabilities to keep up their proteome while cells growing in chemostats appear limited by their disulfide bond isomerisation capacities.

CONCLUSION

This study establishes a novel approach for investigating the oxidative folding capacities of an organism. We show the capabilities and limitations of E. coli for producing disulfide bonds under different growth conditions and predict under what conditions excess capability is available for recombinant protein production.

A synthetic C2 auxotroph of Pseudomonas putida for evolutionary engineering of alternative sugar catabolic routes

Acetyl-coenzyme A (AcCoA) is a metabolic hub in virtually all living cells, serving as both a key precursor of essential biomass components and a metabolic sink for catabolic pathways for a large variety of substrates. Owing to this dual role, tight growth-production coupling schemes can be implemented around the AcCoA node. Building on this concept, a synthetic C2 auxotrophy was implemented in the platform bacterium Pseudomonas putida through an in silico-informed engineering approach. A growth-coupling strategy, driven by AcCoA demand, allowed for direct selection of an alternative sugar assimilation route-the phosphoketolase (PKT) shunt from bifidobacteria. Adaptive laboratory evolution forced the synthetic P. putida auxotroph to rewire its metabolic network to restore C2 prototrophy via the PKT shunt. Large-scale structural chromosome rearrangements were identified as possible mechanisms for adjusting the network-wide proteome profile, resulting in improved PKT-dependent growth phenotypes. ¹³C-based metabolic flux analysis revealed an even split between the native Entner-Doudoroff pathway and the synthetic PKT bypass for glucose processing, leading to enhanced carbon conservation. These results demonstrate that the P. putida metabolism can be radically rewired to incorporate a synthetic C2 metabolism, creating novel network connectivities and highlighting the importance of unconventional engineering strategies to support efficient microbial production.

Glucose consumption rate-dependent transcriptome profiling of Escherichia coli provides insight on performance as microbial factories

Background

The modification of glucose import capacity is an engineering strategy that has been shown to improve the characteristics of Escherichia coli as a microbial factory. A reduction in glucose import capacity can have a positive effect on production strain performance, however, this is not always the case. In this study, E. coli W3110 and a group of four isogenic derivative strains, harboring single or multiple deletions of genes encoding phosphoenolpyruvate:sugar phosphotransferase system (PTS)-dependent transporters as well as non-PTS transporters were characterized by determining their transcriptomic response to reduced glucose import capacity.

Results

These strains were grown in bioreactors with M9 mineral salts medium containing 20 g/L of glucose, where they displayed specific growth rates ranging from 0.67 to 0.27 h⁻¹, and specific glucose consumption rates (qs) ranging from 1.78 to 0.37 g/g h. RNA-seq analysis revealed a transcriptional response consistent with carbon source limitation among all the mutant strains, involving functions related to transport and metabolism of alternate carbon sources and characterized by a decrease in genes encoding glycolytic enzymes and an increase in gluconeogenic functions. A total of 107 and 185 genes displayed positive and negative correlations with qs, respectively. Functions displaying positive correlation included energy generation, amino acid biosynthesis, and sugar import.

Conclusion

Changes in gene expression of E. coli strains with impaired glucose import capacity could be correlated with qs values and this allowed an inference of the physiological state of each mutant. In strains with lower qs values, a gene expression pattern is consistent with energy limitation and entry into the stationary phase. This physiological state could explain why these strains display a lower capacity to produce recombinant protein, even when they show very low rates of acetate production. The comparison of the transcriptomes of the engineered strains employed as microbial factories is an effective approach for identifying favorable phenotypes with the potential to improve the synthesis of biotechnological products.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01909-y.

Regulation of Translation by Lysine Acetylation in Escherichia coli

Numerous cellular processes are regulated in response to the metabolic state of the cell. One such regulatory mechanism involves lysine acetylation, a covalent modification involving the transfer of an acetyl group from central metabolite acetyl-coenzyme A or acetyl phosphate to a lysine residue in a protein.

Assessing the growth kinetics and stoichiometry of Escherichia coli at the single-cell level

Microfluidic cultivation and single-cell analysis are inherent parts of modern microbial biotechnology and microbiology. However, implementing biochemical engineering principles based on the kinetics and stoichiometry of growth in microscopic spaces remained unattained. We here present a novel integrated framework that utilizes distinct microfluidic cultivation technologies and single-cell analytics to make the fundamental math of process-oriented biochemical engineering applicable at the single-cell level. A combination of non-invasive optical cell mass determination with sub-pg sensitivity, microfluidic perfusion cultivations for establishing physiological steady-states, and picoliter batch reactors, enabled the quantification of all physiological parameters relevant to approximate a material balance in microfluidic reaction environments. We determined state variables (biomass concentration based on single-cell dry weight and mass density), biomass synthesis rates, and substrate affinities of cells grown in microfluidic environments. Based on this data, we mathematically derived the specific kinetics of substrate uptake and growth stoichiometry in glucose-grown Escherichia coli with single-cell resolution. This framework may initiate microscale material balancing beyond the averaged values obtained from populations as a basis for integrating heterogeneous kinetic and stoichiometric single-cell data into generalized bioprocess models and descriptions.

Faster Growth Enhances Low Carbon Fuel and Chemical Production Through Gas Fermentation

Gas fermentation offers both fossil carbon-free sustainable production of fuels and chemicals and recycling of gaseous and solid waste using gas-fermenting microbes. Bioprocess development, systems-level analysis of biocatalyst metabolism, and engineering of cell factories are advancing the widespread deployment of the commercialised technology. Acetogens are particularly attractive biocatalysts but effects of the key physiological parameter–specific growth rate (μ)—on acetogen metabolism and the gas fermentation bioprocess have not been established yet. Here, we investigate the μ-dependent bioprocess performance of the model-acetogen Clostridium autoethanogenum in CO and syngas (CO + CO₂+H₂) grown chemostat cultures and assess systems-level metabolic responses using gas analysis, metabolomics, transcriptomics, and metabolic modelling. We were able to obtain steady-states up to μ ∼2.8 day⁻¹ (∼0.12 h⁻¹) and show that faster growth supports both higher yields and productivities for reduced by-products ethanol and 2,3-butanediol. Transcriptomics data revealed differential expression of 1,337 genes with increasing μ and suggest that C. autoethanogenum uses transcriptional regulation to a large extent for facilitating faster growth. Metabolic modelling showed significantly increased fluxes for faster growing cells that were, however, not accompanied by gene expression changes in key catabolic pathways for CO and H₂ metabolism. Cells thus seem to maintain sufficient “baseline” gene expression to rapidly respond to CO and H₂ availability without delays to kick-start metabolism. Our work advances understanding of transcriptional regulation in acetogens and shows that faster growth of the biocatalyst improves the gas fermentation bioprocess.

Synergetic Fermentation of Glucose and Glycerol for High-Yield N-Acetylglucosamine Production in Escherichia coli

N-acetylglucosamine (GlcNAc) is an amino sugar that has been widely used in the nutraceutical and pharmaceutical industries. Recently, microbial production of GlcNAc has been developed. One major challenge for efficient biosynthesis of GlcNAc is to achieve appropriate carbon flux distribution between growth and production. Here, a synergistic substrate co-utilization strategy was used to address this challenge. Specifically, glycerol was utilized to support cell growth and generate glutamine and acetyl-CoA, which are amino and acetyl donors, respectively, for GlcNAc biosynthesis, while glucose was retained for GlcNAc production. Thanks to deletion of the 6-phosphofructokinase (PfkA and PfkB) and glucose-6-phosphate dehydrogenase (ZWF) genes, the main glucose catabolism pathways of Escherichia coli were blocked. The resultant mutant showed a severe defect in glucose consumption. Then, the GlcNAc production module containing glucosamine-6-phosphate synthase (GlmS*), glucosamine-6-phosphate N-acetyltransferase (GNA1*) and GlcNAc-6-phosphate phosphatase (YqaB) expression cassettes was introduced into the mutant, to drive the carbon flux from glucose to GlcNAc. Furthermore, co-utilization of glucose and glycerol was achieved by overexpression of glycerol kinase (GlpK) gene. Using the optimized fermentation medium, the final strain produced GlcNAc with a high stoichiometric yield of 0.64 mol/mol glucose. This study offers a promising strategy to address the challenge of distributing carbon flux in GlcNAc production.

Selection for Cell Yield Does Not Reduce Overflow Metabolism in Escherichia coli

Overflow metabolism is ubiquitous in nature, and it is often considered inefficient because it leads to a relatively low biomass yield per consumed carbon. This metabolic strategy has been described as advantageous because it supports high growth rates during nutrient competition. Here, we experimentally evolved bacteria without nutrient competition by repeatedly growing and mixing millions of parallel batch cultures of Escherichia coli. Each culture originated from a water-in-oil emulsion droplet seeded with a single cell. Unexpectedly we found that overflow metabolism (acetate production) did not change. Instead, the numerical cell yield during the consumption of the accumulated acetate increased as a consequence of a reduction in cell size. Our experiments and a mathematical model show that fast growth and overflow metabolism, followed by the consumption of the overflow metabolite, can lead to a higher numerical cell yield and therefore a higher fitness compared with full respiration of the substrate. This provides an evolutionary scenario where overflow metabolism can be favorable even in the absence of nutrient competition.

Purine Auxotrophic Starvation Evokes Phenotype Similar to Stationary Phase Cells in Budding Yeast

Purine auxotrophy is an abundant trait among eukaryotic parasites and a typical marker for many budding yeast strains. Supplementation with an additional purine source (such as adenine) is necessary to cultivate these strains. If not supplied in adequate amounts, purine starvation sets in. We explored purine starvation effects in a model organism, a budding yeast Saccharomyces cerevisiae ade8 knockout, at the level of cellular morphology, central carbon metabolism, and global transcriptome. We observed that purine-starved cells stopped their cycle in G1/G0 state and accumulated trehalose, and the intracellular concentration of AXP decreased, but adenylate charge remained stable. Cells became tolerant to severe environmental stresses. Intracellular RNA concentration decreased, and massive downregulation of ribosomal biosynthesis genes occurred. We proved that the expression of new proteins during purine starvation is critical for cells to attain stress tolerance phenotype Msn2/4p targets are upregulated in purine-starved cells when compared to cells cultivated in purine-rich media. The overall transcriptomic response to purine starvation resembles that of stationary phase cells. Our results demonstrate that the induction of a strong stress resistance phenotype in budding yeast can be caused not only by natural starvation, but also starvation for metabolic intermediates, such as purines.

Adaptive laboratory evolution of Escherichia coli W enhances gamma-aminobutyric acid production using glycerol as the carbon source

The microbial conversion of glycerol into value-added commodity products has emerged as an attractive means to meet the demands of biosustainability. However, glycerol is a non-preferential carbon source for productive fermentation because of its low energy density. We employed evolutionary and metabolic engineering in tandem to construct an Escherichia coli strain with improved GABA production using glycerol as the feedstock carbon. Adaptive evolution of E. coli W under glycerol-limited conditions for 1300 generations harnessed an adapted strain with a metabolic system optimized for glycerol utilization. Mutation profiling, enzyme kinetic assays, and transcriptome analysis of the adapted strain allowed us to decipher the basis of glycerol adaptation at the molecular level. Importantly, increased substrate influx mediated by the mutant glpK and modulation of intracellular cAMP levels were the key drivers of improved fitness in the glycerol-limited condition. Leveraging the enhanced capability of glycerol utilization in the strain, we constructed a GABA-producing E. coli W-derivative with superior GABA production compared to the wild-type. Furthermore, rationally designed inactivation of the non-essential metabolic genes, including ackA, mgsA, and gabT, in the glycerol-adapted strain improved the final GABA titer and specific productivity by 3.9- and 4.3-fold, respectively, compared with the wild-type.

Post-translational Lysine Ac(et)ylation in Bacteria: A Biochemical, Structural, and Synthetic Biological Perspective

Ac(et)ylation is a post-translational modification present in all domains of life. First identified in mammals in histones to regulate RNA synthesis, today it is known that is regulates fundamental cellular processes also in bacteria: transcription, translation, metabolism, cell motility. Ac(et)ylation can occur at the ε-amino group of lysine side chains or at the α-amino group of a protein. Furthermore small molecules such as polyamines and antibiotics can be acetylated and deacetylated enzymatically at amino groups. While much research focused on N-(ε)-ac(et)ylation of lysine side chains, much less is known about the occurrence, the regulation and the physiological roles on N-(α)-ac(et)ylation of protein amino termini in bacteria. Lysine ac(et)ylation was shown to affect protein function by various mechanisms ranging from quenching of the positive charge, increasing the lysine side chains’ size affecting the protein surface complementarity, increasing the hydrophobicity and by interfering with other post-translational modifications. While N-(ε)-lysine ac(et)ylation was shown to be reversible, dynamically regulated by lysine acetyltransferases and lysine deacetylases, for N-(α)-ac(et)ylation only N-terminal acetyltransferases were identified and so far no deacetylases were discovered neither in bacteria nor in mammals. To this end, N-terminal ac(et)ylation is regarded as being irreversible. Besides enzymatic ac(et)ylation, recent data showed that ac(et)ylation of lysine side chains and of the proteins N-termini can also occur non-enzymatically by the high-energy molecules acetyl-coenzyme A and acetyl-phosphate. Acetyl-phosphate is supposed to be the key molecule that drives non-enzymatic ac(et)ylation in bacteria. Non-enzymatic ac(et)ylation can occur site-specifically with both, the protein primary sequence and the three dimensional structure affecting its efficiency. Ac(et)ylation is tightly controlled by the cellular metabolic state as acetyltransferases use ac(et)yl-CoA as donor molecule for the ac(et)ylation and sirtuin deacetylases use NAD⁺ as co-substrate for the deac(et)ylation. Moreover, the accumulation of ac(et)yl-CoA and acetyl-phosphate is dependent on the cellular metabolic state. This constitutes a feedback control mechanism as activities of many metabolic enzymes were shown to be regulated by lysine ac(et)ylation. Our knowledge on lysine ac(et)ylation significantly increased in the last decade predominantly due to the huge methodological advances that were made in fields such as mass-spectrometry, structural biology and synthetic biology. This also includes the identification of additional acylations occurring on lysine side chains with supposedly different regulatory potential. This review highlights recent advances in the research field. Our knowledge on enzymatic regulation of lysine ac(et)ylation will be summarized with a special focus on structural and mechanistic characterization of the enzymes, the mechanisms underlying non-enzymatic/chemical ac(et)ylation are explained, recent technological progress in the field are presented and selected examples highlighting the important physiological roles of lysine ac(et)ylation are summarized.

Expression of the ace operon in Escherichia coli is triggered in response to growth rate-dependent flux-signal of ATP

The signal that triggers the expression of the ace operon and, in turn, the transition of central metabolism's architecture from acetogenic to gluconeogenic in Escherichia coli remains elusive despite extensive research both in vivo and in vitro. Here, with the aid of flux analysis together with measurements of the enzymic activity of isocitrate lyase (ICL) and its aceA-messenger ribonucleuc acid (mRNA) transcripts, we provide credible evidence suggesting that the expression of the ace operon in E. coli is triggered in response to growth rate-dependent threshold flux-signal of adenosine triphosphate (ATP). Flux analysis revealed that the shortfall in ATP supply observed as the growth rate ($\mu $) diminishes from µmax to ≤ 0.43h-1 ($ \pm 0.02;n4)\ $is partially redressed by up-regulating flux through succinyl CoA synthetase. Unlike glycerol and glucose, pyruvate cannot feed directly into the two glycolytic ATP-generating reactions catalyzed by phosphoglycerokinase and pyruvate kinase. On the other hand, glycerol, which upon its conversion to D-glyceraldehyde, feeds into the phosphorylation and dephosphorylation parts of glycolysis including the substrate-level phosphorylation-ATP generating reactions, thus preventing ATP flux from dropping to the critical threshold signal required to trigger the acetate-diauxic switch until glycerol is fully consumed. The mRNA transcriptional patterns of key gluconeogenic enzymes, namely, ackA, acetate kinase; pta, phosphotransacetylase; acs, acetyl CoA synthetase and aceA, ICL, suggest that the pyruvate phenotype is better equipped than the glycerol phenotype for the switch from acetogenic to gluconeogenic metabolism.

Physiological Characterization of Sulfolobus acidocaldarius in a Controlled Bioreactor Environment

The crenarchaeal model organism Sulfolobus acidocaldarius is typically cultivated in shake flasks. Although shake flasks represent the state-of-the-art for the cultivation of this microorganism, in these systems crucial process parameters, like pH or substrate availability, are only set initially, but cannot be controlled during the cultivation process. As a result, a thorough characterization of growth parameters under controlled conditions is still missing for S. acidocaldarius. In this study, we conducted chemostat cultivations at 75 °C using a growth medium containing L-glutamate and D-glucose as main carbon sources. Different pH values and dilution rates were applied with the goal to physiologically characterize the organism in a controlled bioreactor environment. Under these controlled conditions a pH optimum of 3.0 was determined. Washout of the cells occurred at a dilution rate of 0.097 h⁻¹ and the optimal productivity of biomass was observed at a dilution rate of 0.062 h⁻¹. While both carbon sources were taken up by S. acidocaldarius concomitantly, a 6.6-fold higher affinity for L-glutamate was shown. When exposed to suboptimal growth conditions, S. acidocaldarius reacted with a change in the respiratory behavior and an increased trehalose production rate in addition to a decreased growth rate.

Bacterial Hsp90 Facilitates the Degradation of Aggregation-Prone Hsp70-Hsp40 Substrates

In eukaryotes, the 90-kDa heat shock proteins (Hsp90s) are profusely studied chaperones that, together with 70-kDa heat shock proteins (Hsp70s), control protein homeostasis. In bacteria, however, the function of Hsp90 (HtpG) and its collaboration with Hsp70 (DnaK) remains poorly characterized. To uncover physiological processes that depend on HtpG and DnaK, we performed comparative quantitative proteomic analyses of insoluble and total protein fractions from unstressed wild-type (WT) Escherichia coli and from knockout mutants ΔdnaKdnaJ (ΔKJ), ΔhtpG (ΔG), and ΔdnaKdnaJΔhtpG (ΔKJG). Whereas the ΔG mutant showed no detectable proteomic differences with wild-type, ΔKJ expressed more chaperones, proteases and ribosomes and expressed dramatically less metabolic and respiratory enzymes. Unexpectedly, we found that the triple mutant ΔKJG showed higher levels of metabolic and respiratory enzymes than ΔKJ, suggesting that bacterial Hsp90 mediates the degradation of aggregation-prone Hsp70-Hsp40 substrates. Further in vivo experiments suggest that such Hsp90-mediated degradation possibly occurs through the HslUV protease.

Global coordination of metabolic pathways in Escherichia coli by active and passive regulation

Microorganisms adjust metabolic activity to cope with diverse environments. While many studies have provided insights into how individual pathways are regulated, the mechanisms that give rise to coordinated metabolic responses are poorly understood. Here, we identify the regulatory mechanisms that coordinate catabolism and anabolism in Escherichia coli. Integrating protein, metabolite, and flux changes in genetically implemented catabolic or anabolic limitations, we show that combined global and local mechanisms coordinate the response to metabolic limitations. To allocate proteomic resources between catabolism and anabolism, E. coli uses a simple global gene regulatory program. Surprisingly, this program is largely implemented by a single transcription factor, Crp, which directly activates the expression of catabolic enzymes and indirectly reduces the expression of anabolic enzymes by passively sequestering cellular resources needed for their synthesis. However, metabolic fluxes are not controlled by this regulatory program alone; instead, fluxes are adjusted mostly through passive changes in the local metabolite concentrations. These mechanisms constitute a simple but effective global regulatory program that coarsely partitions resources between different parts of metabolism while ensuring robust coordination of individual metabolic reactions.

Control and regulation of acetate overflow in Escherichia coli

Overflow metabolism refers to the production of seemingly wasteful by-products by cells during growth on glucose even when oxygen is abundant. Two theories have been proposed to explain acetate overflow in Escherichia coli – global control of the central metabolism and local control of the acetate pathway – but neither accounts for all observations. Here, we develop a kinetic model of E. coli metabolism that quantitatively accounts for observed behaviours and successfully predicts the response of E. coli to new perturbations. We reconcile these theories and clarify the origin, control, and regulation of the acetate flux. We also find that, in turns, acetate regulates glucose metabolism by coordinating the expression of glycolytic and TCA genes. Acetate should not be considered a wasteful end-product since it is also a co-substrate and a global regulator of glucose metabolism in E. coli. This has broad implications for our understanding of overflow metabolism.

Proteome Regulation Patterns Determine Escherichia coli Wild-Type and Mutant Phenotypes

It is generally recognized that proteins constitute the key cellular component in shaping microbial phenotypes. Due to limited cellular resources and space, optimal allocation of proteins is crucial for microbes to facilitate maximum proliferation rates while allowing a flexible response to environmental changes. To account for the growth condition-dependent proteome in the constraint-based metabolic modeling of Escherichia coli, we consolidated a coarse-grained protein allocation approach with the explicit consideration of enzymatic constraints on reaction fluxes. Besides representing physiologically relevant wild-type phenotypes and flux distributions, the resulting protein allocation model (PAM) advances the predictability of the metabolic responses to genetic perturbations. A main driver of mutant phenotypes was ascribed to inherited regulation patterns in protein distribution among metabolic enzymes. Moreover, the PAM correctly reflected metabolic responses to an augmented protein burden imposed by the heterologous expression of green fluorescent protein. In summary, we were able to model the effects of important and frequently applied metabolic engineering approaches on microbial metabolism. Therefore, we want to promote the integration of protein allocation constraints into classical constraint-based models to foster their predictive capabilities and application for strain analysis and engineering purposes.

IMPORTANCE Predictive metabolic models are important, e.g., for generating biological knowledge and designing microbes with superior performance for target compound production. Yet today’s whole-cell models either show insufficient predictive capabilities or are computationally too expensive to be applied to metabolic engineering purposes. By linking the inherent genotype-phenotype relationship to a complete representation of the proteome, the PAM advances the accuracy of simulated phenotypes and intracellular flux distributions of E. coli. Being equally computationally lightweight as classical stoichiometric models and allowing for the application of established in silico tools, the PAM and related simulation approaches will foster the use of a model-driven metabolic research. Applications range from the investigation of mechanisms of microbial evolution to the determination of optimal strain design strategies in metabolic engineering, thus supporting basic scientists and engineers alike.

Evolutionary dynamics and structural consequences of de novo beneficial mutations and mutant lineages arising in a constant environment

BACKGROUND

Microbial evolution experiments can be used to study the tempo and dynamics of evolutionary change in asexual populations, founded from single clones and growing into large populations with multiple clonal lineages. High-throughput sequencing can be used to catalog de novo mutations as potential targets of selection, determine in which lineages they arise, and track the fates of those lineages. Here, we describe a long-term experimental evolution study to identify targets of selection and to determine when, where, and how often those targets are hit.

RESULTS

We experimentally evolved replicate Escherichia coli populations that originated from a mutator/nonsense suppressor ancestor under glucose limitation for between 300 and 500 generations. Whole-genome, whole-population sequencing enabled us to catalog 3346 de novo mutations that reached > 1% frequency. We sequenced the genomes of 96 clones from each population when allelic diversity was greatest in order to establish whether mutations were in the same or different lineages and to depict lineage dynamics. Operon-specific mutations that enhance glucose uptake were the first to rise to high frequency, followed by global regulatory mutations. Mutations related to energy conservation, membrane biogenesis, and mitigating the impact of nonsense mutations, both ancestral and derived, arose later. New alleles were confined to relatively few loci, with many instances of identical mutations arising independently in multiple lineages, among and within replicate populations. However, most never exceeded 10% in frequency and were at a lower frequency at the end of the experiment than at their maxima, indicating clonal interference. Many alleles mapped to key structures within the proteins that they mutated, providing insight into their functional consequences.

CONCLUSIONS

Overall, we find that when mutational input is increased by an ancestral defect in DNA repair, the spectrum of high-frequency beneficial mutations in a simple, constant resource-limited environment is narrow, resulting in extreme parallelism where many adaptive mutations arise but few ever go to fixation.

Treadmilling FtsZ polymers drive the directional movement of sPG-synthesis enzymes via a Brownian ratchet mechanism

The FtsZ protein is a central component of the bacterial cell division machinery. It polymerizes at mid-cell and recruits more than 30 proteins to assemble into a macromolecular complex to direct cell wall constriction. FtsZ polymers exhibit treadmilling dynamics, driving the processive movement of enzymes that synthesize septal peptidoglycan (sPG). Here, we combine theoretical modelling with single-molecule imaging of live bacterial cells to show that FtsZ's treadmilling drives the directional movement of sPG enzymes via a Brownian ratchet mechanism. The processivity of the directional movement depends on the binding potential between FtsZ and the sPG enzyme, and on a balance between the enzyme's diffusion and FtsZ's treadmilling speed. We propose that this interplay may provide a mechanism to control the spatiotemporal distribution of active sPG enzymes, explaining the distinct roles of FtsZ treadmilling in modulating cell wall constriction rate observed in different bacteria.

Bacterial fitness landscapes stratify based on proteome allocation associated with discrete aero-types

The fitness landscape is a concept commonly used to describe evolution towards optimal phenotypes. It can be reduced to mechanistic detail using genome-scale models (GEMs) from systems biology. We use recently developed GEMs of Metabolism and protein Expression (ME-models) to study the distribution of Escherichia coli phenotypes on the rate-yield plane. We found that the measured phenotypes distribute non-uniformly to form a highly stratified fitness landscape. Systems analysis of the ME-model simulations suggest that this stratification results from discrete ATP generation strategies. Accordingly, we define "aero-types", a phenotypic trait that characterizes how a balanced proteome can achieve a given growth rate by modulating 1) the relative utilization of oxidative phosphorylation, glycolysis, and fermentation pathways; and 2) the differential employment of electron-transport-chain enzymes. This global, quantitative, and mechanistic systems biology interpretation of fitness landscape formed upon proteome allocation offers a fundamental understanding of bacterial physiology and evolution dynamics.

Dynamic Co-Cultivation Process of Corynebacterium glutamicum Strains for the Fermentative Production of Riboflavin

Residual streams from lignocellulosic processes contain sugar mixtures of glucose, xylose, and mannose. Here, the industrial workhorse Corynebacterium glutamicum was explored as a research platform for the rational utilization of a multiple sugar substrate. The endogenous manA gene was overexpressed to enhance mannose utilization. The overexpression of the xylA gene from Xanthomonas campestris in combination with the endogenous xylB gene enabled xylose consumption by C. glutamicum. Furthermore, riboflavin production was triggered by overexpressing the sigH gene from C. glutamicum. The resulting strains were studied during batch fermentations in flasks and 2 L lab-scale bioreactors separately using glucose, mannose, xylose, and a mixture of these three sugars as a carbon source. The production of riboflavin and consumption of sugars were improved during fed-batch fermentation thanks to a dynamic inoculation strategy of manA overexpressing strain and xylAB overexpressing strain. The final riboflavin titer, yield, and volumetric productivity from the sugar mixture were 27 mg L−1, 0.52 mg g−1, and 0.25 mg L−1 h−1, respectively. It reached a 56% higher volumetric productivity with 45% less by-product formation compared with an equivalent process inoculated with a single strain overexpressing the genes xylAB and manA combined. The results indicate the advantages of dynamic multi strains processes for the conversion of sugar mixtures.

Bacterial Hsp90 Facilitates the Degradation of Aggregation-Prone Hsp70-Hsp40 Substrates

In eukaryotes, the 90-kDa heat shock proteins (Hsp90s) are profusely studied chaperones that, together with 70-kDa heat shock proteins (Hsp70s), control protein homeostasis. In bacteria, however, the function of Hsp90 (HtpG) and its collaboration with Hsp70 (DnaK) remains poorly characterized. To uncover physiological processes that depend on HtpG and DnaK, we performed comparative quantitative proteomic analyses of insoluble and total protein fractions from unstressed wild-type (WT) Escherichia coli and from knockout mutants ΔdnaKdnaJ (ΔKJ), ΔhtpG (ΔG), and ΔdnaKdnaJΔhtpG (ΔKJG). Whereas the ΔG mutant showed no detectable proteomic differences with wild-type, ΔKJ expressed more chaperones, proteases and ribosomes and expressed dramatically less metabolic and respiratory enzymes. Unexpectedly, we found that the triple mutant ΔKJG showed higher levels of metabolic and respiratory enzymes than ΔKJ, suggesting that bacterial Hsp90 mediates the degradation of aggregation-prone Hsp70-Hsp40 substrates. Further in vivo experiments suggest that such Hsp90-mediated degradation possibly occurs through the HslUV protease.

Microbial Upgrading of Acetate into Value-Added Products-Examining Microbial Diversity, Bioenergetic Constraints and Metabolic Engineering Approaches

Ecological concerns have recently led to the increasing trend to upgrade carbon contained in waste streams into valuable chemicals. One of these components is acetate. Its microbial upgrading is possible in various species, with Escherichia coli being the best-studied. Several chemicals derived from acetate have already been successfully produced in E. coli on a laboratory scale, including acetone, itaconic acid, mevalonate, and tyrosine. As acetate is a carbon source with a low energy content compared to glucose or glycerol, energy- and redox-balancing plays an important role in acetate-based growth and production. In addition to the energetic challenges, acetate has an inhibitory effect on microorganisms, reducing growth rates, and limiting product concentrations. Moreover, extensive metabolic engineering is necessary to obtain a broad range of acetate-based products. In this review, we illustrate some of the necessary energetic considerations to establish robust production processes by presenting calculations of maximum theoretical product and carbon yields. Moreover, different strategies to deal with energetic and metabolic challenges are presented. Finally, we summarize ways to alleviate acetate toxicity and give an overview of process engineering measures that enable sustainable acetate-based production of value-added chemicals.

A General Process-Based Model for Describing the Metabolic Shift in Microbial Cell Cultures

The metabolic shift between respiration and fermentation at high glucose concentration is a widespread phenomenon in microbial world, and it is relevant for the biotechnological exploitation of microbial cell factories, affecting the achievement of high-cell-densities in bioreactors. Starting from a model already developed for the yeast Saccharomyces cerevisiae, based on the System Dynamics approach, a general process-based model for two prokaryotic species of biotechnological interest, such as Escherichia coli and Bacillus subtilis, is proposed. The model is based on the main assumption that glycolytic intermediates act as central catabolic hub regulating the shift between respiratory and fermentative pathways. Furthermore, the description of a mixed fermentation with secondary by-products, characteristic of bacterial metabolism, is explicitly considered. The model also represents the inhibitory effect on growth and metabolism of self-produced toxic compounds relevant in assessing the late phases of high-cell density culture. Model simulations reproduced data from experiments reported in the literature with different strains of non-recombinant and recombinant E. coli and B. subtilis cultured in both batch and fed-batch reactors. The proposed model, based on simple biological assumptions, is able to describe the main dynamics of two microbial species of relevant biotechnological interest. It demonstrates that a reductionist System Dynamics approach to formulate simplified macro-kinetic models can provide a robust representation of cell growth and accumulation in the medium of fermentation by-products.

Microbial upgrading of acetate into 2,3-butanediol and acetoin by E. coli W

BACKGROUND

Acetate is an abundant carbon source and its use as an alternative feedstock has great potential for the production of fuel and platform chemicals. Acetoin and 2,3-butanediol represent two of these potential platform chemicals.

RESULTS

The aim of this study was to produce 2,3-butanediol and acetoin from acetate in Escherichia coli W. The key strategies to achieve this goal were: strain engineering, in detail the deletion of mixed-acid fermentation pathways E. coli W ΔldhA ΔadhE Δpta ΔfrdA 445_Ediss and the development of a new defined medium containing five amino acids and seven vitamins. Stepwise reduction of the media additives further revealed that diol production from acetate is mediated by the availability of aspartate. Other amino acids or TCA cycle intermediates did not enable growth on acetate. Cultivation under controlled conditions in batch and pulsed fed-batch experiments showed that aspartate was consumed before acetate, indicating that co-utilization is not a prerequisite for diol production. The addition of aspartate gave cultures a start-kick and was not required for feeding. Pulsed fed-batches resulted in the production of 1.43 g l-1 from aspartate and acetate and 1.16 g l-1 diols (2,3-butanediol and acetoin) from acetate alone. The yield reached 0.09 g diols per g acetate, which accounts for 26% of the theoretical maximum.

CONCLUSION

This study for the first time showed acetoin and 2,3-butanediol production from acetate as well as the use of chemically defined medium for product formation from acetate in E. coli. Hereby, we provide a solid base for process intensification and the investigation of other potential products.

Short-Chain Fatty Acids Modulate Metabolic Pathways and Membrane Lipids in Prevotella bryantii B14

Short-chain fatty acids (SCFAs) are bacterial products that are known to be used as energy sources in eukaryotic hosts, whereas their role in the metabolism of intestinal microbes is rarely explored. In the present study, acetic, propionic, butyric, isobutyric, valeric, and isovaleric acid, respectively, were added to a newly defined medium containing Prevotella bryantii B₁4 cells. After 8 h and 24 h, optical density, pH and SCFA concentrations were measured. Long-chain fatty acid (LCFA) profiles of the bacterial cells were analyzed via gas chromatography-time of flight-mass spectrometry (GC-ToF MS) and proteins were quantified using a mass spectrometry-based, label-free approach. Cultures supplemented with single SCFAs revealed different growth behavior. Structural features of the respective SCFAs were identified in the LCFA profiles, which suggests incorporation into the bacterial membranes. The proteomes of cultures supplemented with acetic and valeric acid differed by an increased abundance of outer membrane proteins. The proteome of the isovaleric acid supplementation showed an increase of proteins in the amino acid metabolism. Our findings indicate a possible interaction between SCFAs, the lipid membrane composition, the abundance of outer membrane proteins, and a modulation of branched chain amino acid biosynthesis by isovaleric acid.

Phosphate starvation controls lactose metabolism to produce recombinant protein in Escherichia coli

Phosphate is one of the major constituents in growth media. It closely regulates central carbon and energy metabolism. Biochemical reactions in central carbon metabolism are known to be regulated by phosphorylation and dephosphorylation of enzymes. Phosphate scarcity can limit microbial productivity. However, microorganisms are evolved to grow in phosphate starvation environments. This study investigates the effect of phosphate-starved response (PSR) stimuli in wild-type and recombinant Escherichia coli cells cultivated in two different substrates, lactose, and glycerol. Phosphate-starved E. coli culture sustained bacterial growth despite the metabolic burden that emanated from recombinant protein expression albeit with altered dynamics of substrate utilisation. Induction of lactose in phosphate-starved culture led to a 2-fold improvement in product titre of rSymlin and a 2.3-fold improvement in product titre of rLTNF as compared with phosphate-unlimited culture. The results obtained in the study are in agreement with the literature to infer that phosphate starvation or limitation can slow down the microbial growth rate in order to produce recombinant proteins. Further, under PSR conditions, gene expression analysis demonstrated that while selected genes (gapdh, pykF, ppsA, icdA) in glycolysis and pentose phosphate pathway (zwf, gnd, talB, tktA) were up-regulated, other genes in lactose (lacY, lacA) and acetate (ackA, pta) pathway were down-regulated. We have demonstrated that cra, crp, phoB, and phoR are involved in the regulation of central carbon metabolism. We propose a novel cross-regulation between lactose metabolism and phosphate starvation. UDP-galactose, a toxic metabolite that is known to cause cell lysis, has been shown to be significantly reduced due to slow uptake of lactose under PSR conditions. Therefore, E. coli employs a decoupling strategy by limiting growth and redirecting metabolic resources to survive and produce recombinant protein under phosphate starvation conditions. KEY POINTS: • Phosphate starvation controls lactose metabolism, which results in less galactose accumulation. • Phosphate starvation modulates metabolic flow of central carbon metabolism. • Product titre improves by 2-fold due to phosphate starvation. • The approach has been successfully applied to production of two different proteins.

Enhancing UDP-Rhamnose Supply for Rhamnosylation of Flavonoids in Escherichia coli by Regulating the Modular Pathway and Improving NADPH Availability

UDP-rhamnose is the main type of sugar donor and endows flavonoids with special activity, selectivity, and pharmacological properties by glycosylation. In this study, several UDP-glucose synthesis pathways and UDP-rhamnose synthases were screened to develop an efficient UDP-rhamnose biosynthesis pathway in Escherichia coli. Maximal UDP-rhamnose production reached 82.2 mg/L in the recombinant strain by introducing the cellobiose phosphorolysis pathway and Arabidopsis thaliana UDP-rhamnose synthase (AtRHM). Quercitrin production of 3522 mg/L was achieved in the recombinant strain by coupling the UDP-rhamnose generation system with A. thaliana rhamnosyltransferase (AtUGT78D1) to recycle UDP-rhamnose. To further increase UDP-rhamnose supply, an NADPH-independent fusion enzyme was constructed, the UTP supply was improved, and NADPH regenerators were overexpressed in vivo. Finally, by optimizing the bioconversion conditions, the highest quercitrin production reached 7627 mg/L with the average productivity of 141 mg/(L h), which is the highest yield of quercitrin and efficiency of UDP-rhamnose supply reported to date in E. coli. Therefore, the method described herein for the regeneration of UDP-rhamnose from cellobiose may be widely used for the rhamnosylation of flavonoids and other bioactive substances.

Escherichia coli metabolism under short-term repetitive substrate dynamics: adaptation and trade-offs

Background

Microbial metabolism is highly dependent on the environmental conditions. Especially, the substrate concentration, as well as oxygen availability, determine the metabolic rates. In large-scale bioreactors, microorganisms encounter dynamic conditions in substrate and oxygen availability (mixing limitations), which influence their metabolism and subsequently their physiology. Earlier, single substrate pulse experiments were not able to explain the observed physiological changes generated under large-scale industrial fermentation conditions.

Results

In this study we applied a repetitive feast–famine regime in an aerobic Escherichia coli culture in a time-scale of seconds. The regime was applied for several generations, allowing cells to adapt to the (repetitive) dynamic environment. The observed response was highly reproducible over the cycles, indicating that cells were indeed fully adapted to the regime. We observed an increase of the specific substrate and oxygen consumption (average) rates during the feast–famine regime, compared to a steady-state (chemostat) reference environment. The increased rates at same (average) growth rate led to a reduced biomass yield (30% lower). Interestingly, this drop was not followed by increased by-product formation, pointing to the existence of energy-spilling reactions. During the feast–famine cycle, the cells rapidly increased their uptake rate. Within 10 s after the beginning of the feeding, the substrate uptake rate was higher (4.68 μmol/g_CDW/s) than reported during batch growth (3.3 μmol/g_CDW/s). The high uptake led to an accumulation of several intracellular metabolites, during the feast phase, accounting for up to 34% of the carbon supplied. Although the metabolite concentrations changed rapidly, the cellular energy charge remained unaffected, suggesting well-controlled balance between ATP producing and ATP consuming reactions.

Conclusions

The adaptation of the physiology and metabolism of E. coli under substrate dynamics, representative for large-scale fermenters, revealed the existence of several cellular mechanisms coping with stress. Changes in the substrate uptake system, storage potential and energy-spilling processes resulted to be of great importance. These metabolic strategies consist a meaningful step to further tackle reduced microbial performance, observed under large-scale cultivations.

The Impact of ackA, pta, and ackA-pta Mutations on Growth, Gene Expression and Protein Acetylation in Escherichia coli K-12

Acetate is a characteristic by-product of Escherichia coli K-12 growing in batch cultures with glucose, both under aerobic as well as anaerobic conditions. While the reason underlying aerobic acetate production is still under discussion, during anaerobic growth acetate production is important for ATP generation by substrate level phosphorylation. Under both conditions, acetate is produced by a pathway consisting of the enzyme phosphate acetyltransferase (Pta) producing acetyl-phosphate from acetyl-coenzyme A, and of the enzyme acetate kinase (AckA) producing acetate from acetyl-phosphate, a reaction that is coupled to the production of ATP. Mutants in the AckA-Pta pathway differ from each other in the potential to produce and accumulate acetyl-phosphate. In the publication at hand, we investigated different mutants in the acetate pathway, both under aerobic as well as anaerobic conditions. While under aerobic conditions only small changes in growth rate were observed, all acetate mutants showed severe reduction in growth rate and changes in the by-product pattern during anaerobic growth. The AckA^– mutant showed the most severe growth defect. The glucose uptake rate and the ATP concentration were strongly reduced in this strain. This mutant exhibited also changes in gene expression. In this strain, the atoDAEB operon was significantly upregulated under anaerobic conditions hinting to the production of acetoacetate. During anaerobic growth, protein acetylation increased significantly in the ackA mutant. Acetylation of several enzymes of glycolysis and central metabolism, of aspartate carbamoyl transferase, methionine synthase, catalase and of proteins involved in translation was increased. Supplementation of methionine and uracil eliminated the additional growth defect of the ackA mutant. The data show that anaerobic, fermentative growth of mutants in the AckA-Pta pathway is reduced but still possible. Growth reduction can be explained by the lack of an important ATP generating pathway of mixed acid fermentation. An ackA deletion mutant is more severely impaired than pta or ackA-pta deletion mutants. This is most probably due to the production of acetyl-P in the ackA mutant, leading to increased protein acetylation.

Amino acids are key substrates to Escherichia coli BW25113 for achieving high specific growth rate

Studying substrate consumption in nutrient-rich conditions is challenging because often the growth medium includes undefined components like yeast extract or peptone. For clear and consistent results, it is necessary to use defined medium, where substrate utilization can be followed. In the present work, Escherichia coli BW25113 batch growth in a medium supplemented with 20 proteinogenic amino acids and glucose was studied. Focus was on the quantitative differences in substrate consumption and proteome composition between minimal and nutrient-rich medium. In the latter, 72% of carbon used for biomass growth came from amino acids and 28% from glucose. Serine was identified as the most consumed substrate with 41% of total carbon consumption. Proteome comparison between nutrient-rich and minimal medium revealed changes in TCA cycle and acetate producing enzymes that together with extracellular metabolite data pointed to serine being consumed mainly for energy generation purposes. Serine removal from the growth medium decreased specific growth rate by 22%. In addition, proteome comparison between media revealed a large shift in amino acid synthesis and translation related proteins. Overall, this work describes in quantitative terms the batch growth carbon uptake profile and proteome allocation of E. coli BW25113 in minimal and nutrient-rich medium.

Genome-Scale Metabolic Reconstruction of Acetobacter pasteurianus 386B, a Candidate Functional Starter Culture for Cocoa Bean Fermentation

Acetobacter pasteurianus 386B is a candidate functional starter culture for the cocoa bean fermentation process. To allow in silico simulations of its related metabolism in response to different environmental conditions, a genome-scale metabolic model for A. pasteurianus 386B was reconstructed. This is the first genome-scale metabolic model reconstruction for a member of the genus Acetobacter. The metabolic network reconstruction process was based on extensive genome re-annotation and comparative genomics analyses. The information content related to the functional annotation of metabolic enzymes and transporters was placed in a metabolic context by exploring and curating a Pathway/Genome Database of A. pasteurianus 386B using the Pathway Tools software. Metabolic reactions and curated gene-protein-reaction associations were bundled into a genome-scale metabolic model of A. pasteurianus 386B, named iAp386B454, containing 454 genes, 322 reactions, and 296 metabolites embedded in two cellular compartments. The reconstructed model was validated by performing growth experiments in a defined medium, which revealed that lactic acid as the sole carbon source could sustain growth of this strain. Further, the reconstruction of the A. pasteurianus 386B genome-scale metabolic model revealed knowledge gaps concerning the metabolism of this strain, especially related to the biosynthesis of its cell envelope and the presence or absence of metabolite transporters.

The Rocky Road From Fed-Batch to Continuous Processing With E. coli

Escherichia coli still serves as a beloved workhorse for the production of many biopharmaceuticals as it fulfills essential criteria, such as having fast doubling times, exhibiting a low risk of contamination, and being easy to upscale. Most industrial processes in E. coli are carried out in fed-batch mode. However, recent trends show that the biotech industry is moving toward time-independent processing, trying to improve the space-time yield, and especially targeting constant quality attributes. In the 1950s, the term "chemostat" was introduced for the first time by Novick and Szilard, who followed up on the previous work performed by Monod. Chemostat processing resulted in a major hype 10 years after its official introduction. However, enthusiasm decreased as experiments suffered from genetic instabilities and physiology issues. Major improvements in strain engineering and the usage of tunable promotor systems facilitated chemostat processes. In addition, critical process parameters have been identified, and the effects they have on diverse quality attributes are understood in much more depth, thereby easing process control. By pooling the knowledge gained throughout the recent years, new applications, such as parallelization, cascade processing, and population controls, are applied nowadays. However, to control the highly heterogeneous cultivation broth to achieve stable productivity throughout long-term cultivations is still tricky. Within this review, we discuss the current state of E. coli fed-batch process understanding and its tech transfer potential within continuous processing. Furthermore, the achievements in the continuous upstream applications of E. coli and the continuous downstream processing of intracellular proteins will be discussed.

Acetate formation during recombinant protein production in Escherichia coli K-12 with an elevated NAD(H) pool

Acetate formation is a disadvantage in the use of Escherichia coli for recombinant protein production, and many studies have focused on optimizing fermentation processes or altering metabolism to eliminate acetate accumulation. In this study, E. coli MEC697 (MG1655 nadR nudC mazG) maintained a larger pool of NAD(H) compared to the wild-type control, and also accumulated lower concentrations of acetate when grown in batch culture on glucose. In steady-state cultures, the elevated total NAD(H) found in MEC697 delayed the threshold dilution rate for acetate formation to a growth rate of 0.27 h^-1. Batch and fed-batch processes using MEC697 were examined for the production of β-galactosidase as a model recombinant protein. Fed-batch culture of MEC697/pTrc99A-lacZ compared to MG1655/pTrc99A-lacZ at a growth rate of 0.22 h^-1 showed only a modest increase of protein formation. However, 1 L batch growth of MEC697/pTrc99A-lacZ resulted in 50% lower acetate formation compared to MG1655/pTrc99A-lacZ and a two-fold increase in recombinant protein production.

Engineering protein production by rationally choosing a carbon and nitrogen source using E. coli BL21 acetate metabolism knockout strains

BACKGROUND

Escherichia coli (E. coli) is a bacteria that is widely employed in many industries for the production of high interest bio-products such as recombinant proteins. Nevertheless, the use of E. coli for recombinant protein production may entail some disadvantages such as acetate overflow. Acetate is accumulated under some culture conditions, involves a decrease in biomass and recombinant protein production, and its metabolism is related to protein lysine acetylation. Thereby, the carbon and nitrogen sources employed are relevant factors in cell host metabolism, and the study of the central metabolism of E. coli and its regulation is essential for optimizing the production of biomass and recombinant proteins. In this study, our aim was to find the most favourable conditions for carrying out recombinant protein production in E. coli BL21 using two different approaches, namely, manipulation of the culture media composition and the deletion of genes involved in acetate metabolism and Nε-lysine acetylation.

RESULTS

We evaluated protein overexpression in E. coli BL21 wt and five mutant strains involved in acetate metabolism (Δacs, ΔackA and Δpta) and lysine acetylation (ΔpatZ and ΔcobB) grown in minimal medium M9 (inorganic ammonium nitrogen source) and in complex TB7 medium (peptide-based nitrogen source) supplemented with glucose (PTS carbon source) or glycerol (non-PTS carbon source). We observed a dependence of recombinant protein production on acetate metabolism and the carbon and nitrogen source employed. The use of complex medium supplemented with glycerol as a carbon source entails an increase in protein production and an efficient use of resources, since is a sub-product of biodiesel synthesis. Furthermore, the deletion of the ackA gene results in a fivefold increase in protein production with respect to the wt strain and a reduction in acetate accumulation.

CONCLUSION

The results showed that the use of diverse carbon and nitrogen sources and acetate metabolism knockout strains can redirect E. coli carbon fluxes to different pathways and affect the final yield of the recombinant protein bioprocess. Thereby, we obtained a fivefold increase in protein production and an efficient use of the resources employing the most suitable strain and culture conditions.