Accumulation of a Polyhydroxyalkanoate Containing Primarily 3-Hydroxydecanoate from Simple Carbohydrate Substrates by Pseudomonas sp. Strain NCIMB 40135

Constant fed-batch cultivation with glucose and propionate as co-substrate: A strategy to fine-tune polyhydroxyalkanoates monomeric composition in Pseudomonas spp

Pseudomonas sp. LFM693 is a 2-methylisocitrate lyase (prpB) disrupted mutant. This enzyme catalyzes a step in the 2-methylcitrate cycle, the only known and described pathway for propionate oxidation in this organism. The affected mutants can efficiently produce PHA containing even and odd-chain length hydroxyalkanoates (HAeven/odd) in the presence of propionate and glucose. In this study, a constant fed-batch configuration was utilized to control the composition of PHA and decrease the toxicity of propionate. The incorporation of HAodd into the copolymer was linear, ranging from 7 to approximately 30 %, and correlated directly with the propionate/glucose molar ratio in the feeding solution. This allowed for the molecular composition of the mclPHA to be fine-tuned with minimum process monitoring and control. The average PHA content was 52 % cell dry weight with a molar composition that favored 3-hydroxyalkanoates containing C8, C9, and C10. The conversion factor of propionate to HAodd varied between 0.36 and 0.53 mol·mol-1 (YHAodd/prop.), which are significantly lower than the theoretical maximum efficiency (1.0 mol·mol-1). These results along with the lack of 2-methylisocitrate as a byproduct provides further support for the evidence that the mutant prpB- is still capable of oxidizing propionate.

Polyhydroxyalkanoate bio-production and its rise as biomaterial of the future

The first observation of a polyhydroxyalkanoate (PHA) aggregate was in 1888 by Beijenrinck. Despite polyhydroxybutyrate (PHB) being the first type of PHA discovered, it was not extracted and characterized until 1925 by Maurice Lemoigne in France, even before the concept of "macromolecules" was known. After more than 30 years, in 1958, Wilkinson and co-workers rediscovered PHB and its metabolic role in the cells as storage compound. PHB started to be appealing to the industry in the 1980s, when a few companies started to commercialize microbially produced PHAs. During the 1990 s, the focus was on reducing production costs to make PHA production economically feasible, for instance by genetically modified microorganisms and even plants. Since then, many advances have been made: diverse wastes as feedstock, different production processes, and tailored design of biopolymers. This paper summarizes the scientific and technological development of PHAs from their discovery in 1888 until their latest applications and current commercial uses. Future perspectives have been devised too based on the current bottlenecks.

Polyhydroxyalkanoates (PHAs): Biopolymers for Biofuel and Biorefineries

Fossil fuels are energy recourses that fulfill most of the world's energy requirements. However, their production and use cause severe health and environmental problems including global warming and pollution. Consequently, plant and animal-based fuels (also termed as biofuels), such as biogas, biodiesel, and many others, have been introduced as alternatives to fossil fuels. Despite the advantages of biofuels, such as being renewable, environmentally friendly, easy to source, and reducing the dependency on foreign oil, there are several drawbacks of using biofuels including high cost, and other factors discussed in the fuel vs. food debate. Therefore, it is imperative to produce novel biofuels while also developing suitable manufacturing processes that ease the aforementioned problems. Polyhydroxyalkanoates (PHAs) are structurally diverse microbial polyesters synthesized by numerous bacteria. Moreover, this structural diversity allows PHAs to readily undergo methyl esterification and to be used as biofuels, which further extends the application value of PHAs. PHA-based biofuels are similar to biodiesel except for having a high oxygen content and no nitrogen or sulfur. In this article, we review the microbial production of PHAs, biofuel production from PHAs, parameters affecting the production of fuel from PHAs, and PHAs biorefineries. In addition, future work on the production of biofuels from PHAs is also discussed.

Polyhydroxyalkanoates

Human dependence on number of chemicals or chemical derivatives has increased alarmingly. Among the commodity chemicals, plastics are becoming independent for our modern lifestyle, as the usage of plastics is increasing worryingly. However, these synthetic plastics are extremely persistent in nature and accumulate in the environment, thereby leading to serious ecological problems. So, to build our economy sustainably, a need of replacement is necessary. Biomaterials in terms of bioplastics are an anticipated option, being synthesized and catabolized by different organisms with myriad biotechnological applications. Polyhydroxyalkanoates (PHAs) are among such biodegradable bioplastics, which are considered as an effective alternative for conventional plastics due to their similar mechanical properties of plastics. A range of microbes under different nutrient and environmental conditions produce PHAs significantly with the help of enzymes. PHA synthases encoded by phaC genes are the key enzymes that polymerize PHA monomers. Four major classes of PHA synthases can be distinguished based on their primary structures, as well as the number of subunits and substrate specificity. PHAs can also be produced from renewable feedstock under, unlike the petrochemically derived plastics that are produced by fractional distillation of depleting fossil fuels. Polyhydroxybutyrate (PHB) is the simplest yet best known polyester of PHAs, as the PHB derived bioplastics are heat tolerant, thus used to make heat tolerant and clear packaging film. They have several medical applications such as drug delivery, suture, scaffold and heart valves, tissue engineering, targeted drug delivery, and agricultural fields. Genetic modification (GM) may be necessary to achieve adequate yields. The selections of suitable bacterial strains, inexpensive carbon sources, efficient fermentation, and recovery processes are also some aspects important aspects taken into consideration for the commercialization of PHA. PHA producers have been reported to reside at various ecological niches with few among them also produce some byproducts like extracellular polymeric substances, rhamnolipids and biohydrogen gas. So, the metabolic engineering thereafter promises to bring a feasible solution for the production of “green plastic” in order to preserve petroleum reserves and diminish the escalating human and animal health concerns environmental implications.

Biotic and Abiotic Synthesis of Renewable Aliphatic Polyesters from Short Building Blocks Obtained from Biotechnology

Biobased polymers have seen their attractiveness increase in recent decades thanks to the significant development of biorefineries to allow access to a wide variety of biobased building blocks. Polyesters are one of the best examples of the development of biobased polymers because most of them now have their monomers produced from renewable resources and are biodegradable. Currently, these polyesters are mainly produced by using traditional chemical catalysts and harsh conditions, but recently greener pathways with nontoxic enzymes as biocatalysts and mild conditions have shown great potential. Bacterial polyesters, such as poly(hydroxyalkanoate)s (PHA), are the best example of the biotic production of high molar mass polymers. PHAs display a wide variety of macromolecular architectures, which allow a large range of applications. The present contribution aims to provide an overview of recent progress in studies on biobased polyesters, especially those made from short building blocks, synthesized through step-growth polymerization. In addition, some important technical aspects of their syntheses through biotic or abiotic pathways have been detailed.

Bioreactor Operating Strategies for Improved Polyhydroxyalkanoate (PHA) Productivity

Microbial polyhydroxyalkanoates (PHAs) are promising biodegradable polymers that may alleviate some of the environmental burden of petroleum-derived polymers. The requirements for carbon substrates and energy for bioreactor operations are major factors contributing to the high production costs and environmental impact of PHAs. Improving the process productivity is an important aspect of cost reduction, which has been attempted using a variety of fed-batch, continuous, and semi-continuous bioreactor systems, with variable results. The purpose of this review is to summarize the bioreactor operations targeting high PHA productivity using pure cultures. The highest volumetric PHA productivity was reported more than 20 years ago for poly(3-hydroxybutryate) (PHB) production from sucrose (5.1 g L⁻¹ h⁻¹). In the time since, similar results have not been achieved on a scale of more than 100 L. More recently, a number fed-batch and semi-continuous (cyclic) bioreactor operation strategies have reported reasonably high productivities (1 g L⁻¹ h⁻¹ to 2 g L⁻¹ h⁻¹) under more realistic conditions for pilot or industrial-scale production, including the utilization of lower-cost waste carbon substrates and atmospheric air as the aeration medium, as well as cultivation under non-sterile conditions. Little development has occurred in the area of fully continuously fed bioreactor systems over the last eight years.

Carbon Sources for Polyhydroxyalkanoates and an Integrated Biorefinery

Polyhydroxyalkanoates (PHAs) are a group of bioplastics that have a wide range of applications. Extensive progress has been made in our understanding of PHAs’ biosynthesis, and currently, it is possible to engineer bacterial strains to produce PHAs with desired properties. The substrates for the fermentative production of PHAs are primarily derived from food-based carbon sources, raising concerns over the sustainability of their production in terms of their impact on food prices. This paper gives an overview of the current carbon sources used for PHA production and the methods used to transform these sources into fermentable forms. This allows us to identify the opportunities and restraints linked to future sustainable PHA production. Hemicellulose hydrolysates and crude glycerol are identified as two promising carbon sources for a sustainable production of PHAs. Hemicellulose hydrolysates and crude glycerol can be produced on a large scale during various second generation biofuels’ production. An integration of PHA production within a modern biorefinery is therefore proposed to produce biofuels and bioplastics simultaneously. This will create the potential to offset the production cost of biofuels and reduce the overall production cost of PHAs.

Medium chain length polyhydroxyalkanoates consisting primarily of unsaturated 3-hydroxy-5-cis-dodecanoate synthesized by newly isolated bacteria using crude glycerol

Background

Our study aimed to search for novel bacteria capable of producing polyhydroxyalkanoates (PHAs) using crude glycerol residue obtained from biodiesel production in which used cooking oils were the substrates.

Results

Newly isolated bacteria from soils in Thailand were screened for the efficient production of PHAs from crude glycerol. The bacterial strains were cultivated on glucose, refined glycerol, crude glycerol, or various cooking oils (canola oil, palm oil, soybean oil, sunflower oil, corn oil, grape seed oil, olive oil, rice bran oil, camellia seed oil) for growth and PHA production. The effects of the total organic carbon (TOC) concentration and the mole ratio of carbon to nitrogen were investigated in batch cultivation. ¹H NMR, two dimensional-¹H-correlation spectroscopy (2D-¹H-COSY) and ¹³C NMR analyses confirmed four bacterial strains were capable of producing medium-chain-length PHAs (mcl-PHAs), consisting of 3-hydroxyoctanoate (3HO) and 3-hydroxy-5-cis-dodecanoate (3H5DD), from crude glycerol. On the basis of phenotypic features and genotypic investigations, the bacterial strains were assigned as: ASC1, Acinetobacter genus (94.9 % similarity); ASC2, Pseudomonas genus (99.2 % similarity); ASC3, Enterobacter genus (99.2 % similarity); ASC4, Bacillus genus (98.4 % similarity). The highest amount of mcl-PHAs, 17.5 ± 0.8 g/L (content 61.8 ± 3.3 % wt), with 3HO (14.7 ± 2.2 mol %), 3H5DD (85.3 ± 2.2 mol %), and a total biomass of 32.3 ± 0.3 g/L, was obtained from Pseudomonas sp. ASC2 in batch cultivation after 36 h. The mcl-PHAs recovered had a number-average molecular weight (M_N) of 3.6 × 10⁴ Da. Homopolymeric 3H5DD was obtained when the cultivation time was prolonged to 96 h.

Conclusions

Novel PHA-producing strains were isolated and identified. These bacterial strains are able to produce mcl-PHAs from crude glycerol. The mcl-PHAs produced contained a high percentage of 3H5DD, which suggests their future application as softeners mixed with other biomaterials. The unsaturated side chain of 3H5DD monomers containing double bounds offers additional potential for improving the properties of the mcl-PHAs or extending their applications to the food industry.

Tight coupling of polymerization and depolymerization of polyhydroxyalkanoates ensures efficient management of carbon resources in Pseudomonas putida

Environmental microbes oscillate between feast and famine and need to carefully manage utilization, storage and conversion of reserve products to exploitable sources of carbon and energy. Polyhydroxyalkanoates (PHAs) are storage polymers that serve bacteria as sources of food materials under physiological conditions of carbon demand. In order to obtain insights into the role of PHA depolymerase (PhaZ) and its relationship to a PHA polymerase (PhaC2) in the carbon management activity of Pseudomonas putida strain U, we created a polymerase hyperexpression strain and a depolymerase knockout mutant of this strain, and examined their synthesis of PHA and expression of their PHA genes. This study revealed that hyperexpression of PhaC2 led to the accumulation of higher amounts of PHA (44%wt) than in the wild-type strain (24%wt) after 24 h of cultivation, which then returned to wild-type levels by 48 h, as a result of elevated depolymerization. The phaZ mutant, however, accumulated higher levels of PHA than the parental strain (62%wt), which were maintained for at least 96 h. Transcriptional analysis of the pha cluster by RT-PCR revealed that PHA operon proteins, including depolymerase, are expressed from the beginning of the growth phase. Hyperexpression of the PhaC2 polymerase was accompanied by an increase in the expression of the PhaZ depolymerase and a decrease in expression of another PHA polymerase, PhaC1. This suggests tight regulatory coupling of PHA polymerase and depolymerase activities that act in synergy, and in concert with other PHA proteins, to provide dynamic PHA granule synthesis and remodelling that rapidly and sensitively respond to changes in availability of carbon and the physiological-metabolic needs of the cell, to ensure optimal carbon resource management.

Microbial production of poly(hydroxybutyrate) from C₁ carbon sources

Polyhydroxybutyrate (PHB) is an attractive substitute for petrochemical plastic due to its similar properties, biocompatibility, and biodegradability. The cost of scaled-up PHB production inhibits its widespread usage. Intensive researches are growing to reduce costs and improve thermomechanical, physical, and processing properties of this green biopolymer. Among cheap substrates which are used for reducing total cost of PHB production, some C₁ carbon sources, e.g., methane, methanol, and CO₂ have received a great deal of attention due to their serious role in greenhouse problem. This article reviews the fundamentals of strategies for reducing PHA production and moves on to the applications of several cheap substrates with a special emphasis on methane, methanol, and CO₂. Also, some explanation for involved microorganisms including the hydrogen-oxidizing bacteria and methanotrophs, their history, culture condition, and nutritional requirements are given. After description of some important strains among the hydrogen-oxidizing and methanotrophic producers of PHB, the article is focused on limitations, threats, and opportunities for application and their future trends.

Genetically modified strains of Ralstonia eutropha H16 with β-ketothiolase gene deletions for production of copolyesters with defined 3-hydroxyvaleric acid contents

β-Ketothiolases catalyze the first step of poly(3-hydroxybutyrate) [poly(3HB)] biosynthesis in bacteria by condensation of two acetyl coenzyme A (acetyl-CoA) molecules to acetoacetyl-CoA and also take part in the degradation of fatty acids. During growth on propionate or valerate, Ralstonia eutropha H16 produces the copolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [poly(3HB-co-3HV)]. In R. eutropha, 15 β-ketothiolase homologues exist. The synthesis of 3-hydroxybutyryl-CoA (3HB-CoA) could be significantly reduced in an 8-fold mutant (Lindenkamp et al., Appl. Environ. Microbiol. 76:5373-5382, 2010). In this study, a 9-fold mutant deficient in nine β-ketothiolase gene homologues (phaA, bktB, H16_A1713, H16_B1771, H16_A1528, H16_B0381, H16_B1369, H16_A0170, and pcaF) was generated. In order to examine the polyhydroxyalkanoate production capacity when short- or long-chain and even- or odd-chain-length fatty acids were provided as carbon sources, the growth and storage behavior of several mutants from the previous study and the newly generated 9-fold mutant were analyzed. Propionate, valerate, octanoate, undecanoic acid, or oleate was chosen as the sole carbon source. On octanoate, no significant differences in growth or storage behavior were observed between wild-type R. eutropha and the mutants. In contrast, during the growth on oleate of a multiple mutant lacking phaA, bktB, and H16_A0170, diminished poly(3HB) accumulation occurred. Surprisingly, the amount of accumulated poly(3HB) in the multiple mutants grown on gluconate differed; it was much lower than that on oleate. The β-ketothiolase activity toward acetoacetyl-CoA in H16ΔphaA and all the multiple mutants remained 10-fold lower than the activity of the wild type, regardless of which carbon source, oleate or gluconate, was employed. During growth on valerate as a sole carbon source, the 9-fold mutant accumulated almost a poly(3-hydroxyvalerate) [poly(3HV)] homopolyester with 99 mol% 3HV constituents.

Synthetic biology: an emerging research field in China

Synthetic biology is considered as an emerging research field that will bring new opportunities to biotechnology. There is an expectation that synthetic biology will not only enhance knowledge in basic science, but will also have great potential for practical applications. Synthetic biology is still in an early developmental stage in China. We provide here a review of current Chinese research activities in synthetic biology and its different subfields, such as research on genetic circuits, minimal genomes, chemical synthetic biology, protocells and DNA synthesis, using literature reviews and personal communications with Chinese researchers. To meet the increasing demand for a sustainable development, research on genetic circuits to harness biomass is the most pursed research within Chinese researchers. The environmental concerns are driven force of research on the genetic circuits for bioremediation. The research on minimal genomes is carried on identifying the smallest number of genomes needed for engineering minimal cell factories and research on chemical synthetic biology is focused on artificial proteins and expanded genetic code. The research on protocells is more in combination with the research on molecular-scale motors. The research on DNA synthesis and its commercialisation are also reviewed. As for the perspective on potential future Chinese R&D activities, it will be discussed based on the research capacity and governmental policy.

Production and characterization of PHA from recombinant E. coli harbouring phaC1 gene of indigenous Pseudomonas sp. LDC-5 using molasses

Polyhydroxyalkanoates (PHA) are biodegradable and biocompatible green thermoplastics, synthesized by wide variety of bacteria as an intracellular carbon and energy storage intermediate. They are used as an alternative to nonrenewable petroleum derived plastics. The current interest in these biopolyesters is stimulated by the search for cost-effective capitalized production. This paper attempts to achieve maximized production rate from recombinant system using inexpensive substrate. Molasses from agro-industrial waste was used to produce PHA from recombinant E.coli in batch culture. PHA yield in molasses (3.06g/L ± 0.05‒75.5%) was higher than that of sucrose (2.5g/L ± 0.05 - 65.1%). Properties of the polymer produced from molasses and sucrose were analyzed by DSC, TGA, DTA, GC/MS, TLC and optical rotation studies. The findings suggested that molasses enhanced PHA production in recombinant E.coli.

Bacterial polyhydroxyalkanoates

Polyhydroxyalkanoates (PHAs) are polyesters of hydroxyalkanoates (HAs) synthesized by numerous bacteria as intracellular carbon and energy storage compounds and accumulated as granules in the cytoplasm of cells. More than 80 HAs have been detected as constituents of PHAs, which allows these thermoplastic materials to have various mechanical properties resembling hard crystalline polymer or elastic rubber depending on the incorporated monomer units. Even though PHAs have been recognized as good candidates for biodegradable plastics, their high price compared with conventional plastics has limited their use in a wide range of applications. A number of bacteria including Alcaligenes eutrophus, Alcaligenes latus, Azotobacter vinelandii, methylotrophs, pseudomonads, and recombinant Escherichia coli have been employed for the production of PHAs, and the productivity of greater than 2 g PHA/L/h has been achieved. Recent advances in understanding metabolism, molecular biology, and genetics of the PHA-synthesizing bacteria and cloning of more than 20 different PHA biosynthesis genes allowed construction of various recombinant strains that were able to synthesize polyesters having different monomer units and/or to accumulate much more polymers. Also, genetically engineered plants harboring the bacterial PHA biosynthesis genes are being developed for the economical production of PHAs. Improvements in fermentation/separation technology and the development of bacterial strains or plants that more efficiently synthesize PHAs will bring the costs down to make PHAs competitive with the conventional plastics.

Biosynthesis of medium chain length poly(3-hydroxyalkanoates) (mcl PHAs) from cosmetic co-products by Pseudomonas raguenesii sp. nov., isolated from Tetiaroa, French Polynesia

A new bacterium, designated as strain TE9 was isolated from a microbial mat in French Polynesia and was studied for its ability to synthesize medium chain length poly-beta-hydroxyalkanoates (mcl PHAs) during cultivation on cosmetics co-products. The composition of PHAs was analysed by coupled gas chromatography mass spectroscopy (GC/MS), nuclear magnetic resonance (NMR) and Fourier Transform InfraRed (FTIR) spectroscopy. PHAs were composed of C6-C14 3-hydroxyacids monomers, with a predominance of 3-hydroxyoctanoate (3HO), 3-hydroxydecanoate (3HD) and 3-hydroxydodecanoate (3HDD). Differential scanning calorimetry (DSC) experiments allowed the characterization of elastomeric materials with a melting point T(m) near 50 degrees C, enthalpy of fusion DeltaH(m) from 27 to 32 J/g, and glass transition temperature T(g) of -43 degrees C. Molecular weights ranged from 175,000 to 358,000 g/mol. On the basis of the phenotypical features and genotypic investigations, strain TE9 was assigned to the Pseudomonas genus and the name of Pseudomonas raguenesii sp. nov. is proposed.

Methyl-branched poly(hydroxyalkanoate) biosynthesis from 13-methyltetradecanoic acid and mixed isostearic acid isomer substrates

Pseudomonas resinovorans, a known medium-chain-length (mcl-) poly(hydroxyalkanoate) (PHA) producer, was grown on 13-methyltetradecanoic acid (13-MTDA) and a mixture of isostearic acid (IA) isomers to produce methyl-branched mcl-PHA polymers. Shake-flask experiments revealed polymer productivities (the percent of the cell mass that is polymer) of 31 +/- 1% (n = 3) and 23 +/- 3% (n = 3) when grown in 13-MTDA and IA, respectively. Monomer content was determined by a combination of gas chromatography/mass spectrometry (GC/MS) of the acid hydrolyzed, silylated methyl esters, and nuclear magnetic resonance spectroscopy. Results showed that the mcl-PHA polymer derived from 13-MTDA was primarily composed of 3-hydroxy-7-methyloctanoic acid and 3-hydroxy-9-methyldecanoic acid (67 and 16 mol% by GC/MS, respectively). In contrast, the mcl-polymers synthesized from the IA isomeric mixture were more complex, containing both even and odd chain-length monomers as well as varying distributions of methyl-branched derivatives. The PHA distributions among the C8, C10, C12, and C14 carbon chain-length monomers included three isomers of C8, five isomers of C10, seven isomers of C12, and nine isomers of C14 each containing one linear-chain derivative and n-6 methyl-branched derivatives where n equals the total number of carbon atoms in each monomer unit (C8-C14).

A genome-scale metabolic reconstruction of Pseudomonas putida KT2440: iJN746 as a cell factory

Background

Pseudomonas putida is the best studied pollutant degradative bacteria and is harnessed by industrial biotechnology to synthesize fine chemicals. Since the publication of P. putida KT2440's genome, some in silico analyses of its metabolic and biotechnology capacities have been published. However, global understanding of the capabilities of P. putida KT2440 requires the construction of a metabolic model that enables the integration of classical experimental data along with genomic and high-throughput data. The constraint-based reconstruction and analysis (COBRA) approach has been successfully used to build and analyze in silico genome-scale metabolic reconstructions.

Results

We present a genome-scale reconstruction of P. putida KT2440's metabolism, iJN746, which was constructed based on genomic, biochemical, and physiological information. This manually-curated reconstruction accounts for 746 genes, 950 reactions, and 911 metabolites. iJN746 captures biotechnologically relevant pathways, including polyhydroxyalkanoate synthesis and catabolic pathways of aromatic compounds (e.g., toluene, benzoate, phenylacetate, nicotinate), not described in other metabolic reconstructions or biochemical databases. The predictive potential of iJN746 was validated using experimental data including growth performance and gene deletion studies. Furthermore, in silico growth on toluene was found to be oxygen-limited, suggesting the existence of oxygen-efficient pathways not yet annotated in P. putida's genome. Moreover, we evaluated the production efficiency of polyhydroxyalkanoates from various carbon sources and found fatty acids as the most prominent candidates, as expected.

Conclusion

Here we presented the first genome-scale reconstruction of P. putida, a biotechnologically interesting all-surrounder. Taken together, this work illustrates the utility of iJN746 as i) a knowledge-base, ii) a discovery tool, and iii) an engineering platform to explore P. putida's potential in bioremediation and bioplastic production.

Production and characterization of polyhydroxyalkanoates in Pseudomonas aeruginosa ATCC 9027 from glucose, an unrelated carbon source

The production and characterization of polyhydroxyalkanoic acids (PHAs) from glucose in Pseudomonas aeruginosa ATCC 9027 is described. We determined that the synthesis of PHAs was not due to a complete lack of nitrogen source, as previously reported for other microorganisms. The synthesis of PHAs was observed during exponential growth and it depended on the carbon/nitrogen ratio in the culture. More significantly, the specific PHA accumulation rate in this phase was higher than that observed in the storage phase. This phenomenon was a consequence of higher extracellular production rates of gluconate and 2-ketogluconate detected during the storage phase. Therefore, the production of those acids decreased the synthesis of PHAs in P. aeruginosa. The maximum percentage of PHA accumulation obtained was 10.8% of the cell dry matter when all the glucose was consumed. The monomer composition of this PHA consisted only of saturated 3-hydroxy fatty acids (octanoic, decanoic, and dodecanoic acids) as shown by gas chromatography - mass spectroscopy and nuclear magnetic resonance analyses, where 3-hydroxydecanoic acid was the main component because of the high affinity of its PhaC synthase for this monomer. The physical properties of this PHA were determined by differential scanning calorimetry and gel permeation chromatography.

Molecular characterization of Pseudomonas sp. LDC-5 involved in accumulation of poly 3-hydroxybutyrate and medium-chain-length poly 3-hydroxyalkanoates

Polyhydroxyalkanoates (PHAs) are biological polyesters, of which, Short-Chain-Length-Medium-Chain-Length (SCL-MCL) PHA copolymers are important because of their wide range of applications. The present study focused on molecular characterization of Pseudomonas sp. LDC-5 that is identified as SCL-MCL producer. Phase contrast, fluorescent and electron microscopic observation confirmed the presence of PHA granules in Pseudomonas sp. LDC-5. PCR analysis indicated the presence of expected amplicon for SCL phaC gene ( approximately 500 bp), MCL phaC1 with phaZ ( approximately 1.3), and phaC2 with phaZ ( approximately 1.5 kb). Sequence analysis of the PHA synthase gene of Pseudomonas sp. LDC-5 revealed significant differences in phaC1 and phaC2 which were further confirmed by recombinant studies. Recombinant Escherichia coli harboring the partial phaC1 gene was able to accumulate PHA, whereas E. coli with phaC2 did not accumulate PHA as verified by fold analysis, immunoblotting, Gas Chromatography (GC), Differential scanning calorimetry (DSC), and FTIR studies. The predicted theoretical three-dimensional structure revealed that PhaC1 is consistent with alpha/beta hydrolase fold. Monomer composition showed the presence of monomer ranging from C4 to C12: 1 when glucose and sodium octanoate fed as the carbon source. DSC revealed melting temperature peak at 153.12 degrees C and glass transition (T(g)) peaks at -0.37 degrees C. Thermogravimetric analysis revealed that the polymer was stable up to 276 degrees C. Fourier Transform Infrared Spectroscopy (FT-IR) spectral analysis showed the PHA specific wave number at 1,739.67 and 1,161.07 cm(-1). The potential of Pseudomonas sp. LDC-5 and its properties are discussed.

Raw materials

Industrial fermentations need raw materials that fulfill the requirements of the organism (suitable carbon and nitrogen source, minerals and specific nutrients) and that are available in a high quantity and quality. This contribution gives a comprehensive overview, including the new trends and progress of recent years. The use of feedstock based on several raw materials such as sugar, starch, inulin and lignocellulose is discussed. Biomass-based raw materials are by far the most applied feedstocks for fermentation. However, there are also raw materials for fermentations derived from the petrochemical industry. These substrates are especially hydrocarbons, alcohols and carboxylic acids. Some applications are given in this chapter.

Ecophysiology of different filamentous Alphaproteobacteria in industrial wastewater treatment plants

The ecophysiology of five filamentous species affiliated to the Alphaproteobacteria was investigated in industrial activated sludge systems. The five species, 'Candidatus Alysiosphaera europaea', 'Candidatus Monilibacter batavus', 'Candidatus Alysiomicrobium bavaricum', 'Candidatus Sphaeronema italicum' and Meganema perideroedes, are very abundant in industrial wastewater treatment plants and are often involved in bulking incidents. The morphology of these filamentous bacterial species resembled Eikelboom's Nostocoida limicola, or Type 021N, and could only be correctly identified by using fluorescence in situ hybridization (FISH), applying species-specific gene probes. Two physiological groupings of the five species were found using microautoradiography combined with FISH. Group 1 ('Ca. Monilibacter batavus' and 'Ca. Sphaeronema italicum') utilized many short-chained fatty acids (acetate, pyruvate and propionate), whereas Group 2 ('Ca. Alysiosphaera europaea', 'Ca. Alysiomicrobium bavaricum' and Meganema perideroedes) could also exploit several sugars, amino acids and ethanol. All species had polyhydroxyalkanoate granules present and several of the species had a very large storage capacity. No activity was found under strict anaerobic conditions, while uptake of substrate was observed in the presence of nitrate or nitrite as potential electron acceptor. However, for all species a reduced number of substrates could be consumed under these conditions compared to aerobic conditions. Only a little exo-enzymic activity was found and nearly all species had a hydrophobic cell surface. Based on knowledge of the ecophysiological potential, control strategies are suggested.

Comparative study of promoters for the production of polyhydroxyalkanoates in recombinant strains of Wautersia eutropha

Recombinant strains of Wautersia eutropha expressing an artificial polyhydroxyalkanoate (PHA) biosynthesis operon under the control of different native promoters linked to polyhydroxybutyrate (PHB) (P(phb)), acetoin (P(acoE), P(acoD), and P(acoX)) or pyruvate (P(pdhE)) metabolism were constructed and tested. The promoters were representative either of the enterobacterial sigma70 (P(phb), P(acoE), and P(pdhE))- or sigma54 (P(acoD) and P(acoX))-dependent promoters. To obtain polymers consisting of C4-C12 monomer units, an artificial operon consisting of the PHA synthase gene from Pseudomonas sp. 61-3 (phaC1 (Ps)) tandemly linked to the W. eutropha genes encoding beta-ketothiolase (phbA (We)) and nicotinamide adenine dinucleotide phosphate dependent acetoacetyl-coenzyme A (CoA) reductase (phbB (We)) was constructed. All recombinant strains produced PHA, indicating that the PHA biosynthesis genes were expressed under the control of the different promoters. Cell growth and PHA synthesis on MS medium complemented with gluconate or octanoate, and different concentrations of acetoin (0, 0.15, and 0.3%) clearly differed among the recombinant strains. While the P(acoD) and P(acoX) promoters mediated only low PHA yields (<1%) in the presence of the inducer acetoin, the remaining promoters-independent of the addition of acetoin-resulted in the production of PHA polymers with high 3HB fractions (90-100 mol%) and with high 3HO contents (70-86 mol%) from gluconate and octanoate, respectively. Interestingly, on octanoate-MS medium with 0.15% acetoin, the P(acoE) promoter mediated the synthesis of PHA with a relatively high 3HB fraction (48 mol%). While PHAs with high 3HB contents were obtained, the overall PHA product yields were low (<10%); thus, their potential application for further commercial exploitation appears limited.

Identification and characterization of two polyhydroxyalkanoate biosynthesis loci in Pseudomonas sp. strain 3Y2

A Pseudomonas strain, 3Y2, that produced polyhydroxyalkanoate (PHA) polymers consisting of 3-hydroxybutyric acid (3HB) and medium-chain-length 3-hydroxyalkanoate (mcl-HA) units, with up to 30% 3HB, was isolated. Two PHA biosynthesis loci (pha ( Ps-1) and pha ( Ps-2)) from 3Y2 were cloned by polymerase chain reaction amplification techniques. The pha ( Ps-2) locus was similar to the PHA biosynthesis loci of other PHA-producing Pseudomonas strains, with five tandem open reading frames (ORFs) located in the order ORF1( Ps-2)-phaC1 ( Ps-2)-phaZ ( Ps-2)-phaC2 ( Ps-2)-phaD ( Ps-2). The pha ( Ps-1) locus that contains phaC1 ( Ps-1)-phaZ ( Ps-1) appears to have arisen by a duplication event that placed it downstream of a gene (ORF1( Ps-1)), encoding a putative glucose-methanol-choline flavoprotein oxidoreductase. The PHA synthases 1 encoded by phaC1 ( Ps-1) and phaC1 ( Ps-2) were investigated by heterologous expression in Wautersia eutropha PHB(-)4. Both synthases displayed similar substrate specificities for incorporating 3HB and mcl-HA units into PHA. The ability of PhaC1( Ps-1) to confer PHA synthesis, however, appeared reduced compared to that of PhaC1( Ps-2), since cells harboring PhaC1( Ps-1) accumulated 2.5 to 4.6 times less PHA than cells expressing PhaC1( Ps-2). Primary sequence analysis revealed that PhaC1( Ps-1) had markedly diverged from the other PHA synthases with a relatively high substitution rate (14.9 vs 2% within PhaC1( Ps-2)). The mutations affected a highly conserved C-terminal region and the surroundings of the essential active site cysteine (Cys296) with a loss of hydrophobicity. This led us to predict that if phaC1 ( Ps-1) produces a protein product in the native strain, it is likely that PhaC1( Ps-1) may be destined for elimination by the accumulation of inactivating mutations, although its specialization to accommodate different substrates cannot be eliminated.

A mathematical model for regulating monomer composition of the microbially synthesized polyhydroxyalkanoate copolymers

A mathematical model is proposed for predicting the copolymer composition of the microbially synthesized polyhydroxyalkanoate (PHA) copolymers. Based on the biochemical reactions involved in the precursor formation and polymerization pathways, the model correlates the copolymer composition with the cultivation conditions, the enzyme levels and selectivity, and the metabolic pathways. It suggests the following points: (1) in the case of a sole carbon source, the copolymer composition depends mainly on the topology of the metabolic pathways and the selectivity of both the enzymes involved in the precursor formation and the polymerization route; (2) the copolymer composition can be varied in a wide range via alteration of the flux ratio of different types of monomers channeled from two or more independent and simultaneous pathways; (3) the enzymes which should be over-expressed or inhibited to obtain the desired copolymer composition can be predicted. For example, inhibition of the beta-oxidation pathway will increase the content of the monomer units with longer chain length. To test the model, various experiments were envisaged by varying cultivation time, concentration and chain length of the sole carbon source, and molar ratio of the cosubstrates. The predictions from the model agree well with the experimental results. Therefore, the proposed model will be useful in predicting the PHA copolymer composition under different biochemical reaction conditions. In other words, it can provide a guide for the synthesis of desired PHA copolymers.

Polyhydroxyalkanoates: an overview

Polyhydroxyalkanoates have gained major importance due to their structural diversity and close analogy to plastics. These are gaining more and more importance world over. Different sources (natural isolates, recombinant bacteria, plants) and other methods are being investigated to exert more control over the quality, quantity and economics of poly(3-hydroxybutyrate) (PHB) production. Their biodegradability makes them extremely desirable substitutes for synthetic plastics. The PHB biosynthetic genes phbA, phbB and phbC are clustered and organized in one phbCAB operon. The PHB pathway is highly divergent in the bacterial genera with regard to orientation and clustering of genes involved. Inspite of this the enzymes display a high degree of sequence conservation. But how similar are the mechanisms of regulation of these divergent operons is as yet unknown. Structural studies will further improve our understanding of the mechanism of action of these enzymes and aid us in improving and selecting better candidates for increased production. Metabolic engineering thereafter promises to bring a feasible solution for the production of "green plastic".

Polyhydroxyalknoate synthesis in plants as a tool for biotechnology and basic studies of lipid metabolism

Polyhydroxyalkanoates (PHAs) are polyesters of hydroxyacids naturally synthesized in bacteria as a carbon reserve. PHAs have properties of biodegradable thermoplastics and elastomers and their synthesis in crop plants is seen as an attractive system for the sustained production of large amounts of polymers at low cost. A variety of PHAs having different physical properties have now been synthesized in a number of transgenic plants, including Arabidopsis thaliana, rape and corn. This has been accomplished through the creation of novel metabolic pathways either in the cytoplasm, plastid or peroxisome of plant cells. Beyond its impact in biotechnology, PHA production in plants can also be used to study some fundamental aspects of plant metabolism. Synthesis of PHA can be used both as an indicator and a modulator of the carbon flux to pathways competing for common substrates, such as acetyl-coenzyme A in fatty acid biosynthesis or 3-hydroxyacyl-coenzyme A in fatty acid degradation. Synthesis of PHAs in plant peroxisome has been used to demonstrate changes in the flux of fatty acids to the beta-oxidation cycle in transgenic plants and mutants affected in lipid biosynthesis, as well as to study the pathway of degradation of unusual fatty acids.

Accumulation of polyhydroxyalkanoic acid containing large amounts of unsaturated monomers in Pseudomonas fluorescens BM07 utilizing saccharides and its inhibition by 2-bromooctanoic acid

A psychrotrophic bacterium, Pseudomonas fluorescens BM07, which is able to accumulate polyhydroxyalkanoic acid (PHA) containing large amounts of 3-hydroxy-cis-5-dodecenoate unit up to 35 mol% in the cell from unrelated substrates such as fructose, succinate, etc., was isolated from an activated sludge in a municipal wastewater treatment plant. When it was grown on heptanoic acid (C(7)) to hexadecanoic acid (C(16)) as the sole carbon source, the monomer compositional characteristics of the synthesized PHA were similar to those observed in other fluorescent pseudomonads belonging to rRNA homology group I. However, growth on stearic acid (C(18)) led to no PHA accumulation, but instead free stearic acid was stored in the cell. The existence of the linkage between fatty acid de novo synthesis and PHA synthesis was confirmed by using inhibitors such as acrylic acid and two other compounds, 2-bromooctanoic acid and 4-pentenoic acid, which are known to inhibit beta-oxidation enzymes in animal cells. Acrylic acid completely inhibited PHA synthesis at a concentration of 4 mM in 40 mM octanoate-grown cells, but no inhibition of PHA synthesis occurred in 70 mM fructose-grown cells in the presence of 1 to 5 mM acrylic acid. 2-Bromooctanoic acid and 4-pentenoic acid were found to much inhibit PHA synthesis much more strongly in fructose-grown cells than in octanoate-grown cells over concentrations ranging from 1 to 5 mM. However, 2-bromooctanoic acid and 4-pentenoic acid did not inhibit cell growth at all in the fructose media. Especially, with the cells grown on fructose, 2-bromooctanoic acid exhibited a steep rise in the percent PHA synthesis inhibition over a small range of concentrations below 100 microM, a finding indicative of a very specific inhibition, whereas 4-pentenoic acid showed a broad, featureless concentration dependence, suggesting a rather nonspecific inhibition. The apparent inhibition constant K(i) (the concentration for 50% inhibition of PHA synthesis) for 2-bromooctanoic acid was determined to be 60 microM, assuming a single-site binding of the inhibitor at a specific inhibition site. Thus, it seems likely that a coenzyme A thioester derivative of 2-bromooctanoic acid specifically inhibits an enzyme linking the two pathways, fatty acid de novo synthesis and PHA synthesis. We suggest that 2-bromooctanoic acid can substitute for the far more expensive (2,000 times) and cell-growth-inhibiting PHA synthesis inhibitor, cerulenin.

Synthesis of polyhydroxyalkanoate in the peroxisome of Saccharomyces cerevisiae by using intermediates of fatty acid beta-oxidation

Medium-chain-length polyhydroxyalkanoates (PHAs) are polyesters having properties of biodegradable thermoplastics and elastomers that are naturally produced by a variety of pseudomonads. Saccharomyces cerevisiae was transformed with the Pseudomonas aeruginosa PHAC1 synthase modified for peroxisome targeting by the addition of the carboxyl 34 amino acids from the Brassica napus isocitrate lyase. The PHAC1 gene was put under the control of the promoter of the catalase A gene. PHA synthase expression and PHA accumulation were found in recombinant S. cerevisiae growing in media containing fatty acids. PHA containing even-chain monomers from 6 to 14 carbons was found in recombinant yeast grown on oleic acid, while odd-chain monomers from 5 to 15 carbons were found in PHA from yeast grown on heptadecenoic acid. The maximum amount of PHA accumulated was 0.45% of the dry weight. Transmission electron microscopy of recombinant yeast grown on oleic acid revealed the presence of numerous PHA inclusions found within membrane-bound organelles. Together, these data show that S. cerevisiae expressing a peroxisomal PHA synthase produces PHA in the peroxisome using the 3-hydroxyacyl coenzyme A intermediates of the beta-oxidation of fatty acids present in the media. S. cerevisiae can thus be used as a powerful model system to learn how fatty acid metabolism can be modified in order to synthesize high amounts of PHA in eukaryotes, including plants.