Ectoine hyperproduction by engineered Halomonas bluephagenesis.

Ectoine, a crucial osmoprotectant for salt adaptation in halophiles, has gained growing interest in cosmetics and medical industries. However, its production remains challenged by stringent fermentation process in model microorganisms and low production level in its native producers. Here, we systematically engineered the native ectoine producer Halomonas bluephagenesis for ectoine production by overexpressing ectABC operon, increasing precursors availability, enhancing product transport system and optimizing its growth medium. The final engineered H. bluephagenesis produced 85 g/L ectoine in 52 h under open unsterile incubation in a 7 L bioreactor in the absence of plasmid, antibiotic or inducer. Furthermore, it was successfully demonstrated the feasibility of decoupling salt concentration with ectoine synthesis and co-production with bioplastic P(3HB-co-4HB) by the engineered H. bluephagenesis. The unsterile fermentation process and significantly increased ectoine titer indicate that H. bluephagenesis as the chassis of Next-Generation Industrial Biotechnology (NGIB), is promising for the biomanufacturing of not only intracellular bioplastic PHA but also small molecular compound such as ectoine.

Metabolic engineering of Halomonas bluephagenesis for production of five carbon molecular chemicals derived from L-lysine.

5-Aminovaleric acid (5-AVA), 5-hydroxyvalerate (5HV), copolymer P(3HB-co-5HV) of 3-hydroxybutyrate (3HB) and 5HV were produced from L-lysine as a substrate by recombinant Halomonas bluephagenesis constructed based on codon optimization, deletions of competitive pathway and L-lysine export protein, and three copies of davBA genes encoding L-lysine monooxygenase (DavB) and 5-aminovaleramide amidohydrolase (DavA) inserted into its genome to form H. bluephagenesis YF117ΔgabT1+2, which produced 16.4 g L-1 and 67.4 g L-1 5-AVA in flask cultures and in 7 L bioreactor, respectively. It was able to de novo synthesize 5-AVA from glucose by L-lysine-overproducing H. bluephagenesis TD226. Corn steep liquor was used instead of yeast extract for cost reduction during the 5-AVA production. Using promoter engineering based on Pporin mutant library for downstream genes, H. bluephagenesis YF117 harboring pSEVA341-Pporin42-yqhDEC produced 6 g L-1 5HV in shake flask growth, while H. bluephagenesis YF117 harboring pSEVA341-Pporin42-yqhDEC-Pporin278-phaCRE-abfT synthesized 42 wt% P(3HB-co-4.8 mol% 5HV) under the same condition. Thus, H. bluephagenesis was successfully engineered to produce 5-AVA and 5HV in supernatant and intracellular P(3HB-co-5HV) utilizing L-lysine as the substrate.

Flux optimization using multiple promoters in Halomonas bluephagenesis as a model chassis of the next generation industrial biotechnology.

Predictability and robustness are challenges for bioproduction because of the unstable intracellular synthetic activities. With the deeper understanding of the gene expression process, fine-tuning has become a meaningful tool for biosynthesis optimization. This study characterized several gene expression elements and constructed a multiple inducible system that responds to ten different small chemical inducers in halophile bacterium Halomonas bluephagenesis. Genome insertion of regulators was conducted for the purpose of gene cluster stabilization and regulatory plasmid simplification. Additionally, dynamic ranges of the multiple inducible systems were tuned by promoter sequence mutations to achieve diverse scopes for high-resolution gene expression control. The multiple inducible system was successfully employed to precisely control chromoprotein expression, lycopene and poly-3-hydroxybutyrate (PHB) biosynthesis, resulting in colorful bacterial pictures, optimized cell growth, lycopene and PHB accumulation. This study demonstrates a desirable approach for fine-tuning of rational and efficient gene expressions, displaying the significance for metabolic pathway optimization.

Engineering low-salt growth Halomonas Bluephagenesis for cost-effective bioproduction combined with adaptive evolution.

Halophilic Halomonas bluephagenesis has been engineered to produce various added-value bio-compounds with reduced costs. However, the salt-stress regulatory mechanism remained unclear. H. bluephagenesis was randomly mutated to obtain low-salt growing mutants via atmospheric and room temperature plasma (ARTP). The resulted H. bluephagenesis TDH4A1B5 was constructed with the chromosomal integration of polyhydroxyalkanoates (PHA) synthesis operon phaCAB and deletion of phaP1 gene encoding PHA synthesis associated protein phasin, forming H. bluephagenesis TDH4A1B5P, which led to increased production of poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-4-hydrobutyrate) (P34HB) by over 1.4-fold. H. bluephagenesis TDH4A1B5P also enhanced production of ectoine and threonine by 50% and 77%, respectively. A total 101 genes related to salinity tolerance was identified and verified via comparative genomic analysis among four ARTP mutated H. bluephagenesis strains. Recombinant H. bluephagenesis TDH4A1B5P was further engineered for PHA production utilizing sodium acetate or gluconate as sole carbon source. Over 33% cost reduction of PHA production could be achieved using recombinant H. bluephagenesis TDH4A1B5P. This study successfully developed a low-salt tolerant chassis H. bluephagenesis TDH4A1B5P and revealed salt-stress related genes of halophilic host strains.

PHB production from food waste hydrolysates by Halomonas bluephagenesis Harboring PHB operon linked with an essential gene.

Food wastes can be hydrolyzed into soluble microbial substrates, contributing to sustainability. Halomonas spp.-based Next Generation Industrial Biotechnology (NGIB) allows open, unsterile fermentation, eliminating the need for sterilization to avoid the Maillard reaction that negatively affects cell growth. This is especially important for food waste hydrolysates, which have a high nutrient content but are unstable due to batch, sources, or storage conditions. These make them unsuitable for polyhydroxyalkanoate (PHA) production, which usually requires limitation on either nitrogen, phosphorous, or sulfur. In this study, H. bluephagenesis was constructed by overexpressing the PHA synthesis operon phaCABCn (cloned from Cupriavidus necator) controlled by the essential gene ompW (encoding outer membrane protein W) promoter and the constitutive porin promoter that are continuously expressed at high levels throughout the cell growth process, allowing poly(3-hydroxybutyrate) (PHB) production to proceed in nutrient-rich (also nitrogen-rich) food waste hydrolysates of various sources. The recombinant H. bluephagenesis termed WZY278 generated 22 g L-1 cell dry weight (CDW) containing 80 wt% PHB when cultured in food waste hydrolysates in shake flasks, and it was grown to 70 g L-1 CDW containing 80 wt% PHB in a 7-L bioreactor via fed-batch cultivation. Thus, unsterilizable food waste hydrolysates can become nutrient-rich substrates for PHB production by H. bluephagenesis able to be grown contamination-free under open conditions.

Biosynthesis of diverse α,ω-diol-derived polyhydroxyalkanoates by engineered Halomonas bluephagenesis.

Polyhydroxyalkanoates (PHA) are a family of biodegradable and biocompatible plastics with potential to replace petroleum based plastics. Diversity of PHA monomer structures provides flexibility in material properties to suit more applications. In this study, 5-hydroxyvalerate (5HV) synthesis pathway was established based on intrinsic alcohol/aldehyde dehydrogenases. The PHA polymerase cloned from Cupriavidus necator functions to polymerize 5HV into its copolymers in ratios ranging from 8% to 32%. Elastic copolymer P(85% 3HB-co-15% 5HV) was generated with an elongation at break and a Young's modulus of 1283% and 73.1 MPa, respectively. The recombinant H. bluephagenesis was able to convert various diols including 1, 3-propanediol, 1, 4-butanediol and 1, 5-pentanediol into PHA, leading to 13 PHA polymers including transparent P(53% 3HB-co-20% 4HB-co-27% 5HV) and sticky P(3HB-co-3HP-co-4HB-co-5HV). The engineered H. bluephagenesis was successfully grown in a 7-L bioreactor to produce the highly elastic P(85% 3HB-co-15% 5HV) and the sticky P(3HB-co-3HP-co-4HB-co-5HV), demonstrating their potential for industrial scale-up.

Engineering an oleic acid-induced system for Halomonas, E. coli and Pseudomonas.

Ligand-induced system plays an important role for microbial engineering due to its tunable gene expression control over timings and levels. An oleic acid (OA)-induced system was recently constructed based on protein FadR, a transcriptional regulator involved in fatty acids metabolism, for metabolic control in Escherichia coli. In this study, we constructed a synthetic FadR-based OA-induced systems in Halomonas bluephagenesis by hybridizing the porin promoter core region and FadR-binding operator (fadO). The dynamic control range was optimized over 150-fold, and expression leakage was significantly reduced by tuning FadR expression and positioning fadO, forming a series of OA-induced systems with various expression strengths, respectively. Additionally, ligand orthogonality and cross-species portability were also studied and showed highly linear correlation among Halomonas spp., Escherichia coli and Pseudomonas spp. Finally, OA-induced systems with medium- and small-dynamic control ranges were employed to dynamically control the expression levels of morphology associated gene minCD, and monomer precursor 4-hydroxybutyrate-CoA (4HB-CoA) synthesis pathway for polyhydroxyalkanoates (PHA), respectively, in the presence of oleic acid as an inducer. As a result, over 10 g/L of poly-3-hydroxybutyrate (PHB) accumulated by elongated cell sizes, and 6 g/L of P(3HB-co-9.57 mol% 4HB) were obtained by controlling the dose and induction time of oleic acid only. This study provides a systematic approach for ligand-induced system engineering, and demonstrates an alternative genetic tool for dynamic control of industrial biotechnology.

Rapid Quantification of Polyhydroxyalkanoates Accumulated in Living Cells Based on Green Fluorescence Protein-Labeled Phasins: The qPHA Method.

Polyhydroxyalkanoates (PHAs) are microbial polyesters that have the potential to replace nonbiodegradable petroplastics. A real-time in situ PHA quantification method has long been awaited to replace the traditional method, which is time- and labor-consuming. Quantification of PHA in living cells was finally developed from fluorescence intensities generated from the green fluorescence protein (GFP) fused with the Halomonas bluephagenesis phasin proteins. Phasins PhaP1 and PhaP2 were used to fuse with GFP, which reflected PHA accumulation with an R-square of over 0.9. Also, a standard correlation was established to calculate PHA contents based on the fluorescence and cell density recorded via a microplate reader with an R-square of over 0.95 when grown on various substrates. The PhaP2-GFP containing H. bluephagenesis was applied successfully to quantify PHA synthesis in a 7.5 L fermenter with high precision. Moreover, the method was found to be feasible in non-natural PHA producers such as Escherichia coli, demonstrating its broad applicability.

Engineering the permeability of Halomonas bluephagenesis enhanced its chassis properties.

Bacterial outer membrane (OM), an asymmetric lipid bilayer functioning as a self-protective barrier with reduced permeability for Gram-negative bacteria, yet wasting nutrients and energy to synthesize, has not been studied for its effect on bioproduction. Here we construct several OM-defected halophile Halomonas bluephagenesis strains to investigate the effects of OM on bioproduction. We achieve enhanced chassis properties of H. bluephagenesis based on positive cellular properties among several OM-defected strains. The OM-defected H. bluephagenesis WZY09 demonstrates better adaptation to lower salinity, increasing 28%, 30% and 12% on dry cell mass (DCM), poly(3-hydroxybutyrate) (PHB) accumulation and glucose to PHB conversion rate, respectively, including enlarged cell sizes and 21-folds reduced endotoxin. Interestingly, a poly(3-hydroxybutyrate-co-21mol%4-hydroxybutyrate) (P(3HB-co-21mol%4HB)) is produced by H. bluephagenesis WZY09 derivate WZY249, increasing 60% and 260% on polyhydroxyalkanoate (PHA) production and 4HB content, respectively. Furthermore, increased electroporation efficiency, more sensitive isopropyl ß-D-1-thio-galactopyranoside (IPTG) induction, better oxygen uptake, enhanced antibiotics sensitivity and ectoine secretion due to better membrane permeability are observed if OM defected, demonstrating significant OM defection impacts for further metabolic engineering, synthetic biology studies and industrial applications.

Engineering Pseudomonas entomophila for synthesis of copolymers with defined fractions of 3-hydroxybutyrate and medium-chain-length 3-hydroxyalkanoates.

Polyhydroxyalkanoates (PHA) composed of both short-chain-length (SCL) and medium-chain-length (MCL) monomers (SCL-co-MCL PHA) combine the advantages of high strength and elasticity provided by SCL PHA and MCL PHA, respectively. Synthesis of SCL-co-MCL PHA, namely, copolymers of 3-hydroxybutyrate (3HB) and MCL 3-hydroxyalkanoates (3HA) such as 3-hydroxydecanoate (3HD) and longer chain 3HA, has been a challenge for a long time. This study aims to engineer Pseudomonas entomophila for synthesizing P(3HB-co-MCL 3HA) via weakening its ß-oxidation pathway combined with insertion of 3HB synthesis pathway consisting of ß-ketothiolase (phaA) and acetoacetyl-CoA reductase (phaB). 3HB and MCL 3HA polymerization is catalyzed by a low specificity PHA synthase (phaC), namely, mutated PhaC61-3. The link between the fatty acid de novo synthesis and PHA synthesis was further blocked to increase the supply for SCL and MCL monomers in P. entomophila. The so-constructed P. entomophila was successfully used to synthesize novel PHA copolymers of P(3HB-co-3HD), P(3HB-co-3HDD) and P(3HB-co-3H9D) consisting of 3HB and 3-hydroxydecanoate (3HD), 3-hydroxydodecanoate (3HDD) and 3-hydroxy-9-decanent (3H9D), respectively. MCL 3HA compositions of P(3HB-co-3HD) and P(3HB-co-3HDD) can be adjusted from 0 to approximate 100 mol%. Results demonstrated that the engineered P. entomophila could be a platform for tailor-made P(3HB-co-MCL 3HA).

Chromosome engineering of the TCA cycle in Halomonas bluephagenesis for production of copolymers of 3-hydroxybutyrate and 3-hydroxyvalerate (PHBV).

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) is a promising biopolyester with good mechanical properties and biodegradability. Large-scale production of PHBV is still hindered by the high production cost. CRISPR/Cas9 method was used to engineer the TCA cycle in Halomonas bluephagenesis on its chromosome for production of PHBV from glucose as a sole carbon source. Two TCA cycle related genes sdhE and icl encoding succinate dehydrogenase assembly factor 2 and isocitrate lysase were deleted, respectively, in H. bluephagenesis TD08AB containing PHBV synthesis genes on the chromosome, to channel more flux to increase the 3-hydroxyvalerate (3HV) ratio of PHBV. Due to a poor growth behavior of the mutant strains, H. bluephagenesis TY194 equipped with a medium strength Pporin-194 promoter was selected for further studies. The sdhE and/or icl mutant strains of H. bluephagenesis TY194 were constructed to show enhanced cell growth, PHBV synthesis and 3HV molar ratio. Gluconate was used to activate ED pathway and thus TCA cycle to increase 3HV content. H. bluephagenesis TY194 (ΔsdhEΔicl) was found to synthesize 17mol% 3HV in PHBV. Supported by the synergetic function of phosphoenolpyruvate carboxylase and Vitreoscilla hemoglobin encoded by genes ppc and vgb inserted into the chromosome of H. bluephagenesis TY194 (ΔsdhE) serving to enhance TCA cycle activity, a series of strains were generated that could produce PHBV containing 3-18mol% 3HV using glucose as a sole carbon source. Shake flask studies showed that H. bluephagenesis TY194 (ΔsdhE, G7::Pporin-ppc) produced 6.3 g/L cell dry weight (CDW), 65% PHBV in CDW and 25mol% 3HV in PHBV when grown in glucose and gluconate. 25mol% 3HV was the highest reported via chromosomal expression system. PHBV copolymers with different 3HV molar ratios were extracted and characterized. Next-generation industrial biotechnology (NGIB) based on recombinant H. bluephagenesis grown under unsterile and continuous conditions, allows production of P(3HB-0∼25mol% 3HV) in a convenient way with reduced production complexity and cost.

Engineering self-flocculating Halomonas campaniensis for wastewaterless open and continuous fermentation.

Halomonas has been developed as a platform for the next generation industrial biotechnology allowing open and nonsterile growth without microbial contamination under a high-salt concentration and alkali pH. To reduce downstream cost associated with continuous centrifugation and salt containing wastewater treatment, Halomonas campaniensis strain LS21 was engineered to become self-flocculating by knocking out an etf operon encoding two subunits of an electron transferring flavoprotein in the predicted electron transfer chain. Self-flocculation could be attributed to the decrease of the surface charge and increase of the cellular hydrophobicity resulted from deleted etf. A wastewaterless fermentation strategy based on the self-flocculating H. campaniensis was developed for growth and the production of poly-3-hydroxybutyrate (PHB) as an example. Most microbial cells flocculated and precipitated to the bottom of the bioreactor within 1 min after stopping the aeration and agitation. The supernatant can be used again without sterilization or inoculation for the growth of the next batch after collecting the precipitated cell mass. The wastewaterless process was conducted for four runs without generating wastewater. PHB accumulation by the self-flocculent strain was enhanced via promoter and ribosome binding site optimizations, the productivities of cell dry weight and PHB were increased from 0.45 and 0.18 g·L -1 ·hr -1 for the batch process compared to 0.82 and 0.33 g·L -1 ·hr -1 for the wastewaterless continuous process, respectively. This has clearly demonstrated the advantages of the wastewaterless process in that it not only reduces wastewater but also increases cell growth and product formation efficiency in a given period of time.

Superhydrophobic Polyhydroxyalkanoates: Preparation and Applications.

Poly( R-3-hydroxybutyrate- co- R-3-hydroxyhexanoate) (PHBHHx), a family member of microbial polyhydroxyalkanoates (PHA), is a biodegradable and biocompatible material with some hydrophobicity and reasonable strength for packaging and tissue engineering applications. In this study, superhydrophobic PHBHHx is fabricated via a simple nonsolvent-assisted process. The material can absorb all tested hydrophobic solvents and oil up to 6-fold of the material weights from water, permitting applications for cleaning environmental oil or solvent pollutions with convenience of disposal after the usage due to its biodegradability. With an excellent combination of biodegradability and biocompatibility, superhydrophobic PHBHHx films are evaluated for antibioadhesion properities to exploit possible implant usages. Up to 100% reductions for platelet adhesions on the superhydrophobic PHBHHx surfaces are observed compared with that on the control material surfaces. Superhydrophobic biodegradable and biocompatible PHBHHx films demonstrate promising low value and high volume or high value and low volume applications.

Highly Efficient Fluorescent Material Based on Rare-Earth-Modified Polyhydroxyalkanoates.

Fluorescent materials play an important role in biomedical fields. However, the main types of fluorescent materials suffer from several disadvantages especially the biotoxicity, which largely restrict its wider applications in biological fields. In this study, a highly efficient rare-earth-modified fluorescent material was successfully designed and fabricated based on polyhydroxyalkanoates, which are known as biodegradable and biocompatible materials. A new Functional-PHA polymer was microbially synthesized by engineered Halomonas bluephagenesis and was used as a basal matrix to generate the rare-earth-modified PHA. N-Acetyl-l-cysteine-grafted PHA (NAL-grafted-PHA) was first produced via a UV-initiated thiol-ene click reaction and the rare earth metal ions (Eu3+ and Tb3+) were subsequently chelated onto the NAL-grafted-PHA through the coordination effect. The composite material exhibited intense photoluminescence properties under UV laser excitation, indicating the excellent features as fluorescent material. The enhanced hydrophilicity and superior biocompatibility of rare-earth-chelated PHA were confirmed, suggesting its great potential application value in biomedical fields.

Synthesis and Characterization of Polyhydroxyalkanoate Organo/Hydrogels.

Synthetic organogels/hydrogels are attracting growing interests due to their potential applications in biomedical fields, organic electronics, and photovoltaics. Photogelation methods for synthesis of organogels/hydrogels have been shown particularly promising because of the high efficiency and simple synthetic procedures. This study synthesized new biodegradable polyhydroxyalkanoates (PHA)-based organogels/hydrogels via UV photo-cross-linking using unsaturated PHA copolymer poly[(R)-3-hydroxyundecanoate-co-(R)-3-hydroxy-10-undecenoate] (PHU10U) with polyethylene glycol dithiol (PDT) as a photo-cross-linker. The PHU10U was synthesized by an engineered Pseudomonas entomophila and characterized via Fourier transform infrared spectroscopy, 1H nuclear magnetic resonance (NMR), and 13C NMR. With decreasing the molar ratio of PHU10U to PDT, both the swelling ratio and pore size were decreased. Meanwhile, increasing densities of the gel networks resulted in a higher compressive modulus. Cell cytotoxicity studies based on the CCK-8 assay on both the PHU10U precursor and PHU10U/PDT hydrogels showed that the novel PHA-based biodegradables acting as hydrogels possess good biocompatibility.

Increasing oxygen availability for improving poly(3-hydroxybutyrate) production by Halomonas.

Technologies enabling high-cell-density growth are required for economical industrial production of most biotechnological products. However, the key factor limiting cell density in bioreactors is the availability of oxygen during the late phases of fermentation. Although the expression of bacterial Vitreoscilla hemoglobin (VHb) is useful for enhanced oxygen availability, bacterial cell membrane makes efficient hemoglobin-oxygen contact a challenge. On the other hand, periplasmic spaces of Gram-negative microorganisms offer an excellent compartment for the intermittent storage of hemoglobin-bound oxygen. In this study, the cell growth was increased by a remarkable 100% using the twin-arginine translocase (Tat) pathway to export active VHb into the periplasm of Escherichia coli, Halomonas bluephagenesis TD01 and H. campaniensis LS21. Furthermore, eight low-oxygen-inducible vgb promoters were constructed in tandem to become a strong promoter cassette termed P8vgb, which better induces expression of both gene vgb encoding VHb and the PHB synthesis operon microaerobically. Both the P8vgb and VHb performed excellently in E. coli and two Halomonas spp., demonstrating their universal applicability for various organisms.

Engineering cell wall synthesis mechanism for enhanced PHB accumulation in E. coli.

The rigidity of bacterial cell walls synthesized by a complicated pathway limit the cell shapes as coccus, bar or ellipse or even fibers. A less rigid bacterium could be beneficial for intracellular accumulation of poly-3-hydroxybutyrate (PHB) as granular inclusion bodies. To understand how cell rigidity affects PHB accumulation, E. coli cell wall synthesis pathway was reinforced and weakened, respectively. Cell rigidity was achieved by thickening the cell walls via insertion of a constitutive gltA (encoding citrate synthase) promoter in front of a series of cell wall synthesis genes on the chromosome of several E. coli derivatives, resulting in 1.32-1.60 folds increase of Young's modulus in mechanical strength for longer E. coli cells over-expressing fission ring FtsZ protein inhibiting gene sulA. Cell rigidity was weakened by down regulating expressions of ten genes in the cell wall synthesis pathway using CRISPRi, leading to elastic cells with more spaces for PHB accumulation. The regulation on cell wall synthesis changes the cell rigidity: E. coli with thickened cell walls accumulated only 25% PHB while cell wall weakened E. coli produced 93% PHB. Manipulation on cell wall synthesis mechanism adds another possibility to morphology engineering of microorganisms.

Engineering of Halomonas bluephagenesis for low cost production of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) from glucose.

Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-co-4HB)] is one of the most promising biomaterials expected to be used in a wide range of scenarios. However, its large-scale production is still hindered by the high cost. Here we report the engineering of Halomonas bluephagenesis as a low-cost platform for non-sterile and continuous fermentative production of P(3HB-co-4HB) from glucose. Two interrelated 4-hydroxybutyrate (4HB) biosynthesis pathways were constructed to guarantee 4HB monomer supply for P(3HB-co-4HB) synthesis by working in concert with 3-hydroxybutyrate (3HB) pathway. Interestingly, only 0.17 mol% 4HB in the copolymer was obtained during shake flask studies. Pathway debugging using structurally related carbon source located the failure as insufficient 4HB accumulation. Further whole genome sequencing and comparative genomic analysis identified multiple orthologs of succinate semialdehyde dehydrogenase (gabD) that may compete with 4HB synthesis flux in H. bluephagenesis. Accordingly, combinatory gene-knockout strains were constructed and characterized, through which the molar fraction of 4HB was increased by 24-fold in shake flask studies. The best-performing strain was grown on glucose as the single carbon source for 60 h under non-sterile conditions in a 7-L bioreactor, reaching 26.3 g/L of dry cell mass containing 60.5% P(3HB-co-17.04 mol%4HB). Besides, 4HB molar fraction in the copolymer can be tuned from 13 mol% to 25 mol% by controlling the residual glucose concentration in the cultures. This is the first study to achieve the production of P(3HB-co-4HB) from only glucose using Halomonas.

Engineering NADH/NAD+ ratio in Halomonas bluephagenesis for enhanced production of polyhydroxyalkanoates (PHA).

Halomonas bluephagenesis has been developed as a platform strain for the next generation industrial biotechnology (NGIB) with advantages of resistances to microbial contamination and high cell density growth (HCD), especially for production of polyhydroxyalkanoates (PHA) including poly(3-hydroxybutyrate) (PHB), poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P34HB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). However, little is known about the mechanism behind PHA accumulation under oxygen limitation. This study for the first time found that H. bluephagenesis utilizes NADH instead of NADPH as a cofactor for PHB production, thus revealing the rare situation of enhanced PHA accumulation under oxygen limitation. To increase NADH/NAD+ ratio for enhanced PHA accumulation under oxygen limitation, an electron transport pathway containing electron transfer flavoprotein subunits α and ß encoded by etf operon was blocked to increase NADH supply, leading to 90% PHB accumulation in the cell dry weight (CDW) of H. bluephagenesis compared with 84% by the wild type. Acetic acid, a cost-effective carbon source, was used together with glucose to balance the redox state and reduce inhibition on pyruvate metabolism, resulting in 22% more CDW and 94% PHB accumulation. The cellular redox state changes induced by the addition of acetic acid increased 3HV ratio in its copolymer PHBV from 4% to 8%, 4HB in its copolymer P34HB from 8% to 12%, respectively, by engineered H. bluephagenesis. The strategy of systematically modulation on the redox potential of H. bluephagenesis led to enhanced PHA accumulation and controllable monomer ratios in PHA copolymers under oxygen limitation, reducing energy consumption and scale-up complexity.

Biosynthesis of Natural Rubber: Current State and Perspectives.

Natural rubber is a kind of indispensable biopolymers with great use and strategic importance in human society. However, its production relies almost exclusively on rubber-producing plants Hevea brasiliensis, which have high requirements for growth conditions, and the mechanism of natural rubber biosynthesis remains largely unknown. In the past two decades, details of the rubber chain polymerization and proteins involved in natural rubber biosynthesis have been investigated intensively. Meanwhile, omics and other advanced biotechnologies bring new insight into rubber production and development of new rubber-producing plants. This review summarizes the achievements of the past two decades in understanding the biosynthesis of natural rubber, especially the massive information obtained from the omics analyses. Possibilities of natural rubber biosynthesis in vitro or in genetically engineered microorganisms are also discussed.