Proteomic response analysis of a threonine-overproducing mutant of Escherichia coli

Two-stage carbon distribution and cofactor generation for improving l-threonine production of Escherichia coli

L-Threonine, a kind of essential amino acid, has numerous applications in food, pharmaceutical, and aquaculture industries. Fermentative l-threonine production from glucose has been achieved in Escherichia coli. However, there are still several limiting factors hindering further improvement of l-threonine productivity, such as the conflict between cell growth and production, byproduct accumulation, and insufficient availability of cofactors (adenosine triphosphate, NADH, and NADPH). Here, a metabolic modification strategy of two-stage carbon distribution and cofactor generation was proposed to address the above challenges in E. coli THRD, an l-threonine producing strain. The glycolytic fluxes towards tricarboxylic acid cycle were increased in growth stage through heterologous expression of pyruvate carboxylase, phosphoenolpyruvate carboxykinase, and citrate synthase, leading to improved glucose utilization and growth performance. In the production stage, the carbon flux was redirected into l-threonine synthetic pathway via a synthetic genetic circuit. Meanwhile, to sustain the transaminase reaction for l-threonine production, we developed an l-glutamate and NADPH generation system through overexpression of glutamate dehydrogenase, formate dehydrogenase, and pyridine nucleotide transhydrogenase. This strategy not only exhibited 2.02- and 1.21-fold increase in l-threonine production in shake flask and bioreactor fermentation, respectively, but had potential to be applied in the production of many other desired oxaloacetate derivatives, especially those involving cofactor reactions.

High-level production of L-threonine by recombinant Escherichia coli with combined feeding strategies

The process of L-threonine production using Escherichia coli TRFC was investigated, and the result showed that there was a large amount of acetic acid in the broth. The effects of acetic acid, which is a known inhibitory metabolite in E. coli cultivation, on L-threonine production by recombinant E. coli TRFC were evaluated, and the result indicated that the growth of E. coli TRFC and L-threonine formation were significantly inhibited in the presence of acetic acid. Two combined feeding strategies were applied to L-threonine fed-batch fermentation in order to investigate the effects of the feeding strategy on L-threonine fermentation. The results showed that using the combined feeding strategy of pseudo-exponential feeding and glucose-stat feeding resulted in high cell density (36.67 g L⁻¹) and L-threonine production (124.57 g L⁻¹) as well as low accumulation of by-products. This work provides a useful approach for large-scale production of L-threonine.

Proteomic analysis of Bacillus thuringiensis strain 4.0718 at different growth phases

The growth process of Bacillus thuringiensis Bt4.0718 strain was studied using proteomic technologies. The proteins of Bt whole cells at three phases—middle vegetative, early sporulation, and late sporulation—were extracted with lysis buffer, followed with separation by 2-DE and identified by MALDI-TOF/TOF MS. Bioactive factors such as insecticidal crystal proteins (ICPs) including Cry1Ac(3), Cry2Aa, and BTRX28, immune inhibitor (InhA), and InhA precursor were identified. InhA started to express at the middle vegetative phase, suggesting its contribution to the survival of Bt in the host body. At the early sporulation phase, ICPs started their expression. CotJC, OppA, ORF1, and SpoIVA related to the formation of crystals and spores were identified, the expression characteristics of which ensured the stable formation of crystals and spores. This study provides an important foundation for further exploration of the stable expression of ICPs, the smooth formation of crystals, and the construction of recombinant strains.

Microbial metabolic engineering for L-threonine production

L-threonine, one of the three major amino acids produced throughout the world, has a wide application in industry, as an additive or as a precursor for the biosynthesis of other chemicals. It is predominantly produced through microbial fermentation the efficiency of which largely depends on the quality of strains. Metabolic engineering based on a cogent understanding of the metabolic pathways of L-threonine biosynthesis and regulation provides an effective alternative to the traditional breeding for strain development. Continuing efforts have been made in revealing the mechanisms and regulation of L-threonine producing strains, as well as in metabolic engineering of suitable organisms whereby genetically-defined, industrially competitive L-threonine producing strains have been successfully constructed. This review focuses on the global metabolic and regulatory networks responsible for L-threonine biosynthesis, the molecular mechanisms of regulation, and the strategies employed in strain engineering.

Dietary L-homoserine spares threonine in chicks

Four chick bioassays were conducted to evaluate the threonine (Thr) replacement value of l-homoserine (HS). Growth rate was increased (P < 0.05) by dietary addition of 800 mg l-HS/kg diet to a purified diet severely deficient in Thr or by the addition of 800 or 1000 mg of l-HS/kg diet to a corn-peanut meal diet distinctly deficient in Thr. The addition of an isomolar level of alpha-ketobutyrate, a catabolic product of both Thr and HS, did not elicit a response. Standard-curve methodology predicted a Thr replacement value of 38 +/- 9% for HS. Interactions (P < 0.01) were observed in assays 2 and 4 between dietary Thr adequacy and 800 or 1000 mg l-HS/kg supplementation. Thus, HS improved growth performance when added to a Thr-deficient diet (0.46 g Thr/100 g diet), but it decreased growth performance when added to the same diet containing surfeit Thr (0.80 g Thr/100 g diet). The results indicate that low levels of HS elicit a growth response in young chicks fed Thr-deficient diets.

Genomics and proteomics in process development: opportunities and challenges

Global gene expression profiling by genomic and proteomic analyses has changed the face of drug discovery and biological research in the past few years. The benefit of these technologies in the area of process development for recombinant protein production has been increasingly realized. This review discusses the application of genome-wide expression profiling tools in the design and optimization of bioprocesses, with the emphasis on the effect on process development of mammalian cell culture. Despite the lack of genome sequence information for most of the relevant mammalian cell lines used, these technologies can be applied during various process development steps. Although there are only a few examples in the literature that present a major improvement in productivity based on genomics and proteomics, further advances in analytical tools and genome sequencing technologies will greatly increase our knowledge at the molecular level and will drive the design of future bioprocesses.

Proteome analysis of recombinant Escherichia coli producing human glucagon-like peptide-1

The proteomic response of recombinant Escherichia coli producing human glucagon-like peptide-1 was analyzed by two-dimensional gel electrophoresis. Protein spots in two-dimensional gel could be identified by using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and their expression profiles were compared with those of nonproducing cells. Thirty-five intracellular proteins exhibited differential expression levels between the production and control strains. These changes reflected physiological responses to heterologous peptide production in recombinant E. coli. Specifically, physiological changes included the down-regulation of proteins involved in the central carbon metabolism, biosynthesis of cellular building blocks and peptides, and up-regulation of cell protection proteins and some sugar transport proteins. This comprehensive analysis would provide useful information for understanding physiological alterations to heterologous peptide production and for designing efficient metabolic engineering strategies for the production of recombinant peptides in E. coli.

The response of Escherichia coli to exposure to the biocide polyhexamethylene biguanide

The global response of Escherichia coli to the broad-spectrum biocide polyhexamethylene biguanide (PHMB) was investigated using transcriptional profiling. The transcriptional analyses were validated by direct determination of the PHMB-tolerance phenotypes of derivatives of E. coli MG1655 carrying either insertionally inactivated genes and/or plasmids expressing the cognate open reading frames from a heterologous promoter in the corresponding chromosomally inactivated strains. The results showed that a wide range of genes was altered in transcriptional activity and that all of the corresponding knockout strains subsequently challenged with biocide were altered in tolerance. Of particular interest was the induction of the rhs genes and the implication of enzymes involved in the repair/binding of nucleic acids in the generation of tolerance, suggesting a novel dimension in the mechanism of action of PHMB based on its interaction with nucleic acids.

Effect of rpoS gene knockout on the metabolism of Escherichia coli during exponential growth phase and early stationary phase based on gene expressions, enzyme activities and intracellular metabolite concentrations

The RNA polymerase sigma factor, encoded by rpoS gene, controls the expression of a large number of genes in Escherichia coli under stress conditions. The present study investigated the growth characteristics and metabolic pathways of rpoS gene knockout mutant of E. coli growing in LB media under aerobic condition. The analyses were made based on gene expressions obtained by DNA microarray and RT-PCR, enzyme activities and intracellular metabolite concentrations at the exponential and early stationary phases of growth. Although the glucose utilization pattern of the mutant was similar to the parent strain, the mutant failed to utilize acetate throughout the cultivation period. Microarray data indicated that the expression levels of several important genes of acetate metabolism such as acs, aceAB, cysDEK, fadR, etc. were significantly altered in the absence of rpoS gene. Interestingly, there was an increased activity of TCA cycle during the exponential growth phase, which was gradually diminished at the onset of stationary phase. Moreover, rpoS mutation had profound effect on the expression of several other genes of E. coli metabolic pathways that were not described earlier. The changes in the gene expressions, enzyme activities and intracellular metabolite concentrations of the rpoS mutant are discussed in details with reference to the major metabolic pathways of E. coli.

Proteome response of Escherichia coli fed-batch culture to temperature downshift

During fed-batch cultivation of Escherichia coli K-12, the proteomic response to a temperature downshift from 37 to 20 degrees C was quantitatively monitored and analyzed by using two-dimensional electrophoresis. When the temperature of exponentially growing E. coli K-12 culture was downshifted to 20 degrees C, the synthesis level of 57 intracellular proteins showed significant changes for a prolonged period of time, compared to the fed-batch culture controlled at 37 degrees C. Thus, these proteins are regarded as important stress proteins responsive to cold shock, which were analyzed by using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and identified using the E. coli SWISS-2DPAGE database. Most of the identified proteins were shown to be involved in energy metabolism, several cellular molecule biosynthetic pathways and catabolism, cell processes, flagellar biosynthesis and motility, and protein translation and folding. The systematic approach to the monitoring of proteomic responses and the detailed analysis results reported in this article would be useful in understanding the metabolic adaptation to lowered culture temperature and designing efficient fermentation strategies for the production of recombinant proteins and metabolites using E. coli strains.