The Feasibility of Thermophilic Caldimonas manganoxidans as a Platform for Efficient PHB Production

Application of extremophile cell factories in industrial biotechnology

Due to the extreme living conditions, extremophiles have unique characteristics in morphology, structure, physiology, biochemistry, molecular evolution mechanism and so on. Extremophiles have superior growth and synthesis capabilities under harsh conditions compared to conventional microorganisms, allowing for unsterilized fermentation processes and thus better performance in low-cost production. In recent years, due to the development and optimization of molecular biology, synthetic biology and fermentation technology, the identification and screening technology of extremophiles has been greatly improved. In this review, we summarize techniques for the identification and screening of extremophiles and review their applications in industrial biotechnology in recent years. In addition, the facts and perspectives gathered in this review suggest that next-generation industrial biotechnology (NGIBs) based on engineered extremophiles holds the promise of simplifying biofuturing processes, establishing open, non-sterilized continuous fermentation production systems, and utilizing low-cost substrates to make NGIBs attractive and cost-effective bioprocessing technologies for sustainable manufacturing.

Production of polyhydroxyalkanoates by the thermophile Cupriavidus cauae PHS1

Thermophilic production of polyhydroxyalkanoate is considered a very promising way to overcome the problems that may arise when using mesophilic strains. This study reports the first thermophilic polyhydroxybutyrate-producing Cupriavidus species, which are known as the best polyhydroxybutyrate-producing microorganisms. Cupriavidus cauae PHS1 harbors a phbCABR cluster with high similarity to the corresponding proteins of C. necator H16 (80, 93, 96, and 97 %). This strain can produce polyhydroxybutyrate from a range of substrates, including acetate (5 g/L) and phenol (1 g/L), yielding 7.6 % and 18.9 % polyhydroxybutyrate, respectively. Moreover, the strain produced polyhydroxybutyrate at temperatures ranging from 25 to 50 °C, with the highest polyhydroxybutyrate content (47 °C) observed at 45 °C from gluconate. Additionally, the strain could incorporate 3-hydroxyvalerate (12.5 mol. %) into the polyhydroxybutyrate polymer using levulinic acid as a precursor. Thus, Cupriavidus cauae PHS1 may be a promising polyhydroxybutyrate producer as alternative for mesophilic polyhydroxybutyrate-producing Cupriavidus species.

Producing medium-chain-length polyhydroxyalkanoate from diverse feedstocks by deregulating unsaturated fatty acid biosynthesis in Escherichia coli

The fatty acid metabolism in Escherichia coli has served as a basic metabolic chassis for medium-chain-length polyhydroxyalkanoate (mcl-PHA) production. In this study, the phaG and phaC1 genes from Pseudomonas entomophila L48 were first cloned as pGRN08. E. coli BL21P (E. coli BL21(DE3) ΔptsG) containing pGRN08 was able to produce 23 ± 3 and 7 ± 0 mg/L homopolymer poly(3-hydroxydecanoate)(P(3HD)) from glucose and xylose, respectively. Next, a gene, PSEEN0908 (encoding a putative 3-hydroxyacyl-CoA ligase), from P. entomophila L48 was found to increase the performance of mcl-PHA production. The induction of the fatty acid biosynthesis repressor (FabR), a transcription regulator that represses UFA biosynthesis, in E. coli substantially increased the mcl-PHA production by an order of magnitude from both unrelated and related carbon source conversion. A mcl-PHA concentration of 179 ± 1 mg/L and a content of 5.79 ± 0.16 % were obtained, where 31 mol% was 3-hydroxyoctanoate (3HO) and 69 mol% was 3HD.

The Transcription Activator AtxA from Bacillus Anthracis was Employed for Developing a Tight-Control, High-Level, Modulable, and Stationary-Phase Specific Transcription Activity in Escherichia Coli

The strong transcriptional activity of the virulent gene pagA in Bacillus anthracis has been proven to be anthrax toxin activator (AtxA)-regulated. However, the obscure pagA transcription mechanism hinders practical applications of this strong promoter. In this study, a 509-bp DNA fragment [termed 509sequence, (−508)-(+1) relative to the P2 transcription start site] was cloned upstream of rbs-GFPuv as pTOL02B to elucidate the AtxA-regulated transcription. The 509sequence was dissected into the −10 sequence, −35 sequence, AT_rich tract, SLI/SLII and upstream site. In conjunction with the heterologous co-expression of AtxA (under the control of the T7 promoter), the −10 sequence (TATACT) was sufficient for the AtxA-regulated transcription. Integration of pTOL02F + pTOLAtxA as pTOL03F showed that the AtxA-regulated transcription exhibited a strong specific fluorescence intensity/common analytical chemistry term (OD₆₀₀) of 40 597 ± 446 and an induction/repression ratio of 122. An improved induction/repression ratio of 276 was achieved by cultivating Escherichia coli/pTOL03F in M9 minimal medium. The newly developed promoter system termed P_AtxA consists of AtxA, the −10 sequence and Escherichia RNA polymerase. These three elements synergistically and cooperatively formed a previously undiscovered transcription system, which exhibited a tight-control, high-level, modulable and stationary-phase-specific transcription. The P_AtxA was used for phaCAB expression for the stationary-phase polyhydroxybutyrate production, and the results showed that a PHB yield, content and titer of 0.20 ± 0.27 g/g-glucose, 68 ± 11% and 1.5 ± 0.4 g/l can be obtained. The positive inducible P_AtxA, in contrast to negative inducible, should be a useful tool to diversify the gene information flow in synthetic biology.

Graphical Abstract

Construction of an Antibiotic-Free Vector and its Application in the Metabolic Engineering of Escherichia Coli for Polyhydroxybutyrate Production

An antibiotic- and inducer-free culture condition was proposed for polyhydroxybutyrate (PHB) production in recombinant Escherichia coli. First, antibiotic-free vectors were constructed by installing the plasmid maintenance system, alp7, hok/sok, and the hok/sok and alp7 combination into the pUC19 vector. The plasmid stability test showed that pVEC02, the pUC19 vector containing the hok/sok system, was the most effective in achieving antibiotic-free cultivation in the E. coli B strain but not in the K strain. Second, the putative phaCAB operon derived from Caldimonas manganoxidans was inserted into pVEC02 to yield pPHB01 for PHB production in E. coli BL21 (DE3). The putative phaCAB operon was first shown function properly for PHB production and thus, inducer-free conditions were achieved. However, the maintenance of pPHB01 in E. coli requires antibiotics supplementation. Finally, an efficient E. coli ρ factor-independent terminator, thrLABC (ECK120033737), was inserted between the phaCAB operon and the hok/sok system to avoid possible transcriptional carry-over. The newly constructed plasmid pPHB01-1 facilitates an antibiotic- and inducer-free culture condition and induces the production of PHB with a concentration of 3.0 on0.2 g/L, yield of 0.26 /L0.07 g/g-glucose, and content of 44 /g3%. The PHB production using E. coli BL21 (DE3)/pPHB01-1 has been shown to last 84 and 96 h in the liquid and solid cultures.

Electrotransformation of thermophilic bacterium Caldimonas manganoxidans

Caldimonas manganoxidans is a Gram-negative, thermophilic, bioplastic-producing bacterium that is a promising strain to overcome the drawbacks of existing bioplastic manufacturing methods. However, genetic manipulation of this species has not previously been studied. Here, we developed an optimized electrotransformation protocol for C. manganoxidans by screening conditions, including the bacterial growth phase, electroporation buffer, pulse strength, and recovery time. The optimized transformation protocol obtained (3.1 ± 0.78) × 10⁸ colony-forming units/μg DNA of plasmid pBBR1MCS-2. High transformation efficiency was observed when using plasmid DNA isolated from C. manganoxidans. The DNA methylases of Escherichia coli did not affect the transformation efficiency of C. manganoxidans. The electrotransformation technique proposed here will be beneficial for the genetic manipulation of thermophilic Caldimonas species.

Poly(3-Hydroxybutyrate-co-3-Hydroxyvalerate): Enhancement Strategies for Advanced Applications

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate), PHBV, is a microbial biopolymer with excellent biocompatible and biodegradable properties that make it a potential candidate for substituting petroleum-derived polymers. However, it lacks mechanical strength, water sorption and diffusion, electrical and/or thermal properties, antimicrobial activity, wettability, biological properties, and porosity, among others, limiting its application. For this reason, many researchers around the world are currently working on how to overcome the drawbacks of this promising material. This review summarises the main advances achieved in this field so far, addressing most of the chemical and physical strategies to modify PHBV and placing particular emphasis on the combination of PHBV with other materials from a variety of different structures and properties, such as other polymers, natural fibres, carbon nanomaterials, nanocellulose, nanoclays, and nanometals, producing a wide range of composite biomaterials with increased potential applications. Finally, the most important methods to fabricate porous PHBV scaffolds for tissue engineering applications are presented. Even though great advances have been achieved so far, much research needs to be conducted still, in order to find new alternative enhancement strategies able to produce advanced PHBV-based materials able to overcome many of these challenges.

Co-Expression of ORFCma with PHB Depolymerase (PhaZCma ) in Escherichia coli Induces Efficient Whole-Cell Biodegradation of Polyesters

Whole-cell degradation of polyesters not only avoids the tedious process of enzyme separation, but also allows the degraded product to be reused as a carbon source. In this study, Escherichia coli BL21(DE3) harboring phaZ_Cma , a gene encoding poly(3-hydroxybutyrate) (PHB) depolymerase from Caldimonas manganoxidans, is constructed. The extra-cellular fraction of E. coli/pPHAZ exhibits a fast PHB degradation rate where it only took 35 h to completely degrade PHB films, while C. manganoxidans takes 81 h to do the same. The co-expression of ORF_Cma (a putative periplasmic substrate binding protein that is within the same operon of phaZ_Cma ) further improves the PHB degradation. While 28 h is needed for E. coli/pPHAZ to cause an 80% weight loss in PHB films, E. coli/pORFPHAZ needs only 21 h. Furthermore, it is able to degrade at-least four different polyesters, PHB, poly(lactic acid) (PLA), polycaprolactone (PCL), and poly(butylene succinate-co-adipate) (PBSA). Testing of the time course of 3-hydroxybutyrate concentration and the turbidity of the degradation solutions over time shows that PhaZ_Cma has both exo- and endo-enzymatic activity. The whole-cell E. coli/pORFPHAZ can be used for recycling various polyesters while ORF_Cma can potentially be a universal element for enhancing the secretion of recombinant protein.