Continuous bioconversion of n-octane to octanoic acid by recombinant Escherichia coli (alk(+)) growing in a two-liquid-phase Chemostat

Modulating the import of medium-chain alkanes in E. coli through tuned expression of FadL

Background

In recent years, there have been intensive efforts to develop synthetic microbial platforms for the production, biosensing and bio-remediation of fossil fuel constituents such as alkanes. Building predictable engineered systems for these applications will require the ability to tightly control and modulate the rate of import of alkanes into the host cell. The native components responsible for the import of alkanes within these systems have yet to be elucidated. To shed further insights on this, we used the AlkBGT alkane monooxygenase complex from Pseudomonas putida GPo1 as a reporter system for assessing alkane import in Escherichia coli. Two native E. coli transporters, FadL and OmpW, were evaluated for octane import given their proven functionality in the uptake of fatty acids along with their structural similarity to the P. putida GPo1 alkane importer, AlkL.

Results

Octane import was removed with deletion of fadL, but was restored by complementation with a fadL-encoding plasmid. Furthermore, tuned overexpression of FadL increased the rate of alkane import by up to 4.5- fold. A FadL deletion strain displayed a small but significant degree of tolerance toward hexane and octane relative to the wild type, while the responsiveness of the well-known alkane biosensor, AlkS, toward octane and decane was strongly reduced by 2.7- and 2.9-fold, respectively.

Conclusions

We unequivocally show for the first time that FadL serves as the major route for medium-chain alkane import in E. coli. The experimental approaches used within this study, which include an enzyme-based reporter system and a fluorescent alkane biosensor for quantification and real-time monitoring of alkane import, could be employed as part of an engineering toolkit for optimizing biological systems that depend on the uptake of alkanes. Thus, the findings will be particularly useful for biological applications such as bioremediation and biomanufacturing.

Electronic supplementary material

The online version of this article (doi:10.1186/s13036-016-0026-3) contains supplementary material, which is available to authorized users.

Identification and use of an alkane transporter plug-in for applications in biocatalysis and whole-cell biosensing of alkanes

Effective application of whole-cell devices in synthetic biology and biocatalysis will always require consideration of the uptake of molecules of interest into the cell. Here we demonstrate that the AlkL protein from Pseudomonas putida GPo1 is an alkane import protein capable of industrially relevant rates of uptake of C7-C16 n-alkanes. Without alkL expression, native E.coli n-alkane uptake was the rate-limiting step in both the whole-cell bioconversion of C7-C16 n-alkanes and in the activation of a whole-cell alkane biosensor by C10 and C11 alkanes. By coexpression of alkL as a transporter plug-in, specific yields improved by up to 100-fold for bioxidation of >C12 alkanes to fatty alcohols and acids. The alkL protein was shown to be toxic to the host when overexpressed but when expressed from a vector capable of controlled induction, yields of alkane oxidation were improved a further 10-fold (8 g/L and 1.7 g/g of total oxidized products). Further testing of activity on n-octane with the controlled expression vector revealed the highest reported rates of 120 μmol/min/g and 1 g/L/h total oxidized products. This is the first time AlkL has been shown to directly facilitate enhanced uptake of C10-C16 alkanes and represents the highest reported gain in product yields resulting from its use.

Synthesis of ω-hydroxy dodecanoic acid based on an engineered CYP153A fusion construct

A bacterial P450 monooxygenase-based whole cell biocatalyst using Escherichia coli has been applied in the production of ω-hydroxy dodecanoic acid from dodecanoic acid (C12-FA) or the corresponding methyl ester. We have constructed and purified a chimeric protein where the fusion of the monooxygenase CYP153A from Marinobacter aquaeloei to the reductase domain of P450 BM3 from Bacillus megaterium ensures optimal protein expression and efficient electron coupling. The chimera was demonstrated to be functional and three times more efficient than other sets of redox components evaluated. The established fusion protein (CYP153A_{M. aq.}-CPR) was used for the hydroxylation of C12-FA in in vivo studies. These experiments yielded 1.2 g l^–1 ω-hydroxy dodecanoic from 10 g l^–1 C12-FA with high regioselectivity (> 95%) for the terminal position. As a second strategy, we utilized C12-FA methyl ester as substrate in a two-phase system (5:1 aqueous/organic phase) configuration to overcome low substrate solubility and product toxicity by continuous extraction. The biocatalytic system was further improved with the coexpression of an additional outer membrane transport system (AlkL) to increase the substrate transfer into the cell, resulting in the production of 4 g l^–1 ω-hydroxy dodecanoic acid. We further summarized the most important aspects of the whole-cell process and thereupon discuss the limits of the applied oxygenation reactions referring to hydrogen peroxide, acetate and P450 concentrations that impact the efficiency of the production host negatively.

Engineered catalytic biofilms for continuous large scale production of n-octanol and (S)-styrene oxide

This study evaluates the technical feasibility of biofilm-based biotransformations at an industrial scale by theoretically designing a process employing membrane fiber modules as being used in the chemical industry and compares the respective process parameters to classical stirred-tank studies. To our knowledge, catalytic biofilm processes for fine chemicals production have so far not been reported on a technical scale. As model reactions, we applied the previously studied asymmetric styrene epoxidation employing Pseudomonas sp. strain VLB120ΔC biofilms and the here-described selective alkane hydroxylation. Using the non-heme iron containing alkane hydroxylase system (AlkBGT) from P. putida Gpo1 in the recombinant P. putida PpS81 pBT10 biofilm, we were able to continuously produce 1-octanol from octane with a maximal productivity of 1.3 g L ⁻¹(aq) day⁻¹ in a single tube micro reactor. For a possible industrial application, a cylindrical membrane fiber module packed with 84,000 polypropylene fibers is proposed. Based on the here presented calculations, 59 membrane fiber modules (of 0.9 m diameter and 2 m length) would be feasible to realize a production process of 1,000 tons/year for styrene oxide. Moreover, the product yield on carbon can at least be doubled and over 400-fold less biomass waste would be generated compared to classical stirred-tank reactor processes. For the octanol process, instead, further intensification in biological activity and/or surface membrane enlargement is required to reach production scale. By taking into consideration challenges such as biomass growth control and maintaining a constant biological activity, this study shows that a biofilm process at an industrial scale for the production of fine chemicals is a sustainable alternative in terms of product yield and biomass waste production.

NADH availability limits asymmetric biocatalytic epoxidation in a growing recombinant Escherichia coli strain

Styrene can efficiently be oxidized to (S)-styrene oxide by recombinant Escherichia coli expressing the styrene monooxygenase genes styAB from Pseudomonas sp. strain VLB120. Targeting microbial physiology during whole-cell redox biocatalysis, we investigated the interdependency of styrene epoxidation, growth, and carbon metabolism on the basis of mass balances obtained from continuous two-liquid-phase cultures. Full induction of styAB expression led to growth inhibition, which could be attenuated by reducing expression levels. Operation at subtoxic substrate and product concentrations and variation of the epoxidation rate via the styrene feed concentration allowed a detailed analysis of carbon metabolism and bioconversion kinetics. Fine-tuned styAB expression and increasing specific epoxidation rates resulted in decreasing biomass yields, increasing specific rates for glucose uptake and the tricarboxylic acid (TCA) cycle, and finally saturation of the TCA cycle and acetate formation. Interestingly, the biocatalysis-related NAD(P)H consumption was 3.2 to 3.7 times higher than expected from the epoxidation stoichiometry. Possible reasons include uncoupling of styrene epoxidation and NADH oxidation and increased maintenance requirements during redox biocatalysis. At epoxidation rates of above 21 micromol per min per g cells (dry weight), the absence of limitations by O(2) and styrene and stagnating NAD(P)H regeneration rates indicated that NADH availability limited styrene epoxidation. During glucose-limited growth, oxygenase catalysis might induce regulatory stress responses, which attenuate excessive glucose catabolism and thus limit NADH regeneration. Optimizing metabolic and/or regulatory networks for efficient redox biocatalysis instead of growth (yield) is likely to be the key for maintaining high oxygenase activities in recombinant E. coli.

Solvent-tolerant bacteria for biotransformations in two-phase fermentation systems

Product removal from aqueous media poses a challenge in biotechnological whole-cell biotransformation processes in which substrates and/or products may have toxic effects. The assignment of an additional liquid solvent phase provides a solution, as it facilitates in situ product recovery from aqueous media. In such two-phase systems, toxic substrates and products are present in the aqueous phase in tolerable but still bioavailable amounts. As a matter of course, adequate organic solvents have to possess hydrophobicity properties akin to substrates and products of interest, which in turn involves intrinsic toxicity of the solvents used. The employment of bacteria being able to adapt to otherwise toxic solvents helps to overcome the problem. Adaptive mechanisms enabling such solvent tolerant bacteria to survive and grow in the presence of toxic solvents generally involve either modification of the membrane and cell surface properties, changes in the overall energy status, or the activation and/or induction of active transport systems for extruding solvents from membranes into the environment. It is anticipated that the biotechnological production of a number of important fine chemicals in amounts sufficient to compete economically with chemical syntheses will soon be possible by making use of solvent-tolerant microorganisms.

Production of fuculose-1-phosphate aldolase using operator-repressor titration for plasmid maintenance in high cell density Escherichia coli fermentations

We report a novel application for the operator-repressor titration (ORT) plasmid maintenance system. The ability of ORT to maintain a plasmid during production of DNA has been demonstrated previously. In this study, we have used the ORT system to maintain a plasmid during high cell density cultivation and expression of a recombinant protein. No evidence of plasmid loss was seen during protein expression at high cell densities. In addition, the quantity of protein produced using this system was similar to traditional plasmid maintenance systems.

Process implementation aspects for biocatalytic hydrocarbon oxyfunctionalization

Oxidoreductases catalyze a large variety of regio-, stereo-, and chemoselective hydrocarbon oxyfunctionalizations, reactions, which are important in industrial organic synthesis but difficult to achieve by chemical means. This review summarizes process implementation aspects for the in vivo application of the especially versatile enzyme class of oxygenases, capable of specifically introducing oxygen from molecular oxygen into a large range of organic molecules. Critical issues such as reaching high enzyme activity and specificity, product degradation, cofactor recycling, reactant toxicity, and substrate and oxygen mass transfer can be overcome by biochemical process engineering and biocatalyst engineering. Both strategies provide a growing toolset to facilitate process implementation, optimization, and scale-up. Major advances were achieved via heterologous overexpression of oxygenase genes, directed evolution, metabolic engineering, and in situ product removal. Process examples from industry and academia show that the combined use of different concepts enables efficient oxygenase-based whole-cell catalysis of various commercially interesting reactions such as the biosynthesis of chiral compounds, the specific oxyfunctionalization of complex molecules, and also the synthesis of medium-priced chemicals. Better understanding of the cell metabolism and future developments in both biocatalyst and bioprocess engineering are expected to promote the implementation of many and various industrial biooxidation processes.

Recent advances in petroleum microbiology

Recent advances in molecular biology have extended our understanding of the metabolic processes related to microbial transformation of petroleum hydrocarbons. The physiological responses of microorganisms to the presence of hydrocarbons, including cell surface alterations and adaptive mechanisms for uptake and efflux of these substrates, have been characterized. New molecular techniques have enhanced our ability to investigate the dynamics of microbial communities in petroleum-impacted ecosystems. By establishing conditions which maximize rates and extents of microbial growth, hydrocarbon access, and transformation, highly accelerated and bioreactor-based petroleum waste degradation processes have been implemented. Biofilters capable of removing and biodegrading volatile petroleum contaminants in air streams with short substrate-microbe contact times (<60 s) are being used effectively. Microbes are being injected into partially spent petroleum reservoirs to enhance oil recovery. However, these microbial processes have not exhibited consistent and effective performance, primarily because of our inability to control conditions in the subsurface environment. Microbes may be exploited to break stable oilfield emulsions to produce pipeline quality oil. There is interest in replacing physical oil desulfurization processes with biodesulfurization methods through promotion of selective sulfur removal without degradation of associated carbon moieties. However, since microbes require an environment containing some water, a two-phase oil-water system must be established to optimize contact between the microbes and the hydrocarbon, and such an emulsion is not easily created with viscous crude oil. This challenge may be circumvented by application of the technology to more refined gasoline and diesel substrates, where aqueous-hydrocarbon emulsions are more easily generated. Molecular approaches are being used to broaden the substrate specificity and increase the rates and extents of desulfurization. Bacterial processes are being commercialized for removal of H(2)S and sulfoxides from petrochemical waste streams. Microbes also have potential for use in removal of nitrogen from crude oil leading to reduced nitric oxide emissions provided that technical problems similar to those experienced in biodesulfurization can be solved. Enzymes are being exploited to produce added-value products from petroleum substrates, and bacterial biosensors are being used to analyze petroleum-contaminated environments.

Efficient biosynthetic incorporation of tryptophan and indole analogs in an integral membrane protein

Biosynthetic incorporation of tryptophan (Trp) analogs such as 7-azatryptophan, 5-hydroxytryptophan, and fluorotryptophan into a protein can facilitate its structural analysis by spectroscopic techniques such as fluorescence, phosphorescence, nuclear magnetic resonance, and Fourier transform infrared. Until now, the approach has dealt primarily with soluble proteins. In this article, we demonstrate that four different Trp analogs can be very efficiently incorporated into a membrane protein as demonstrated for the mannitol transporter of Escherichia coli (EII(mtl)). EII(mtl) overexpression was under control of the lambdaP(R) promoter, and the E. coli Trp auxotroph M5219 was used as host. This strain constitutively expresses the heat labile repressor protein of the lambdaP(R) promoter. Together with the presence of the repressor gene on the EII(mtl) plasmid, this resulted in a tightly controlled promoter system, a prerequisite for high Trp analog incorporation. A new method for determining the analog incorporation efficiency is presented that is suitable for membrane proteins. The procedure involves fitting of the phosphorescence spectrum as a linear combination of the Trp and Trp analog contributions, taking into account the influence of the protein environment on the Trp analog spectrum. The data show that the analog content of EII(mtl) samples is very high (>95%). In addition, we report here that biosynthetic incorporation of Trp analogs can also be effected with less expensive indole analogs, which in vivo are converted to L-Trp analogs.

In situ product removal (ISPR) in whole cell biotechnology during the last twenty years

This review sums up the activity in the field of in situ product removal in whole cell bioprocesses over the last 20 years. It gives a complete summary of ISPR operations with microbial cells and cites a series of interesting ISPR applications in plant and animal cell technology. All the ISPR projects with microbial cells are categorized according to their products, their ISPR techniques, and their applied configurations of the ISPR set-up. Research on ISPR application has primarily increased in the field of microbial production of aromas and organic acids such lactic acid over the last ten years. Apart from the field of de novo formation of bioproducts, ISPR is increasingly applied to microbial bioconversion processes. However, despite of the large number of microbial whole cell ISPR projects (approximately 250), very few processes have been transferred to an industrial scale. The proposed processes have mostly been too complex and consequently not cost effective. Therefore, this review emphasizes that the planning of a successful whole cell ISPR process should not only consider the choice of ISPR technique according to the physicochemical properties of the product, but also the potential configuration of the whole process set-up. Furthermore, additional process aspects, biological and legal constraint need to be considered from the very beginning for the design of an ISPR project. Finally, future trends of new, modified or improved ISPR techniques are given.

Pilot-scale production of (S)-styrene oxide from styrene by recombinant Escherichia coli synthesizing styrene monooxygenase

Recombinant Escherichia coli JM101(pSPZ10) cells produce the styrene monooxygenase of Pseudomonas sp. strain VLB120, which catalyzes the oxidation of styrene to (S)-styrene oxide at an enantiomeric excess larger than 99%. This biocatalyst was used to produce 388 g of styrene oxide in a two-liquid phase 30-L fed-batch bioconversion. The average overall volumetric activity was 170 U per liter over a period of more than 10 h, equivalent to mass transfer rates of 10.2 mmoles per liter per hour at a phase ratio of 0.5. At this transfer rate, the biotransformation system appeared to be substrate mass-transfer limited. The reactor had an estimated power input in the order of 5 W. L(-1), which is close to values typically obtained with commercially operating units. The product could be easily purified by fractional distillation to a purity in excess of 97%. The process illustrates the feasibility of recombinant whole cell biotransformations in two-liquid phase systems with toxic substrates and products.

Bioconversion of hydrophobic compounds in a continuous closed-gas-loop bioreactor: feasibility assessment and epoxide production

Microorganisms can be used as catalysts to produce organic compounds in a highly chemo-, regio- and enantioselective manner, and whole cells do not require the costly addition of cofactors for redox reactions. However, bioconversions are slow compared to alternative chemical reactions, and the biocatalyst works at its best in an aqueous medium, while the transformations of interest frequently involve compounds with a low-aqueous solubility and that are toxic to microorganisms. This results in low-volumetric productivity in classical bioreactors. The Continuous Closed-Gas-Loop Bioreactor is described here-a reactor system with high productivity, but without the problems associated with two-phase systems, such as an emulsified product stream and phase toxicity. Its working principle is to recirculate a gas phase through a bioreaction compartment and a saturator/absorber module where the product accumulates as a clear organic solution. A wide range of bioconversions should be possible in this set-up, and proof of concept was established for the epoxidation of 1,7-octadiene to (R)-1,2-epoxyoct-7-ene by a native strain of Pseudomonas oleovorans. This reaction represents a group of terminal alkene epoxidations where the bioconversion substrate does not support growth of the microorganism. Practical results at a 5l-scale are presented for this bioconversion for both batch and continuous operation with respect to the aqueous phase, showing continuous stable epoxidation at productivities >14 micromol min(-1) L(-1) (U L(-1)). The results confirm that the metabolism does not allow a simple optimization strategy, because growth and biotransformation substrates compete for the same enzyme sites, and conversely growth on a substrate using this very enzyme system is necessary for longterm bioconversion. Integrated removal of the CO(2) formed via the liquid overflow was estimated from theory and verified in experimental work.

Expression, stability and performance of the three-component alkane mono-oxygenase of Pseudomonas oleovorans in Escherichia coli

We tested the synthesis and in vivo function of the inducible alkane hydroxylase of Pseudomonas oleovorans GPo1 in several Escherichia coli recombinants. The enzyme components (AlkB, AlkG and AlkT) were synthesized at various rates in different E. coli hosts, which after induction produced between twofold and tenfold more of the Alk components than did P. oleovorans. The enzyme components were less stable in recombinant E. coli hosts than in P. oleovorans. In addition, the specific activity of the alkane mono-oxygenase component AlkB was five or six times lower in E. coli than in P. oleovorans. Evidently, optimal functioning of the hydroxylase system requires factors or a molecular environment that are available in Pseudomonas but not in E. coli. These factors are likely to include correct interactions of AlkB with the membrane and incorporation of iron into the AlkG and AlkB apoproteins.

Engineering of a stable whole-cell biocatalyst capable of (S)-styrene oxide formation for continuous two-liquid-phase applications

Recombinant strains of Pseudomonas putida KT2440 carrying genetic expression cassettes with xylene oxygenase- and styrene monooxygenase-encoding genes on their chromosomes could be induced in shaking-flask experiments to specific activities that rivaled those of multicopy-plasmid-based Escherichia coli recombinants. Such strains maintained the introduced styrene oxidation activity in continuous two-liquid-phase cultures for at least 100 generations, although at a lower level than in the shaking-flask experiments. The data suggest that placement of target genes on the chromosome might be a suitable route for the construction of segregationally stable and highly active whole-cell biocatalysts.

Integrated two-liquid phase bioconversion and product-recovery processes for the oxidation of alkanes: process design and economic evaluation

Pseudomonas oleovorans and recombinant strains containing the alkane oxidation genes can produce alkane oxidation products in two-liquid phase bioreactor systems. In these bioprocesses the cells, which grow in the aqueous phase, oxidize apolar, non-water soluble substrates. The apolar products typically accumulate in the emulsified apolar phase. We have studied both the bioconversion systems and several downstream processing systems to separate and purify alkanols from these two-liquid phase media. Based on the information generated in these studies, we have now designed bioconversion and downstream processing systems for the production of 1-alkanols from n-alkanes on a 10 kiloton/yr scale, taking the conversion of n-octane to 1-octanol as a model system. Here, we describe overall designs of fed-batch and continuous-fermentation processes for the oxidation of octane to 1-octanol by Pseudomonas oleovorans, and we discuss the economics of these processes. In both systems the two-liquid phase system consists of an apolar phase with hexadecene as the apolar carrier solvent into which n-octane is dissolved, while the cells are present in the aqueous phase. In one system, multiple-batch fermentations are followed by continuous processing of the product from the separated apolar phase. The second system is based on alkane oxidation by continuously growing cultures, again followed by continuous processing of the product. Fewer fermentors were required and a higher space-time-yield was possible for production of 1-octanol in a continuous process. The overall performance of each of these two systems has been modeled with Aspen software. Investment and operating costs were estimated with input from equipment manufacturers and bulk-material suppliers. Based on this study, the production cost of 1-octanol is about 7 US$kg-1 when produced in the fed-batch process, and 8 US$kg-1 when produced continuously. The comparison of upstream and downstream capital costs and production costs showed significantly higher upstream costs for the fed-batch process and slightly higher upstream costs for continuous fermentation. The largest cost contribution was due to variable production costs, mainly resulting from media costs. The organisms used in these systems are P. putida alk+ recombinants which oxidize alkanes, but cannot oxidize the resulting alkanols further. Hence, such cells need a second carbon source, which in these systems is glucose. Although the continuous process is about 10% more expensive than the fed-batch process, improvements to reduce overall cost can be achieved more easily for continuous than for fed-batch fermentation by decreasing the dilution rate while maintaining near constant productivity. Improvements relevant to both processes can be achieved by increasing the biocatalyst performance, which results in improved overall efficiency, decreased capital investment, and hence, decreased production cost. Copyright 1999 John Wiley & Sons, Inc.

Epoxidation of 1,7-octadiene by pseudomonas oleovorans in a membrane bioreactor

A growing cell culture of Pseudomonas oleovorans was used to biotransform 1,7-octadiene to 1,2-epoxy-7,8-octene in a continuous-flow bioreactor with an external membrane module. A dense silicone rubber membrane was used to contact an organic phase, containing both the reactant (1,7-octadiene) and the growth substrate (heptane), with an aqueous biomedium phase containing the biocatalyst. Heptane and octadiene delivery to the aqueous phase, and epoxide extraction into the solvent, occurred by diffusion across the dense membrane under a concentration-driving force. In addition, a liquid feed of heptane and octadiene was pumped directly into the bioreactor to increase the rate of delivery of these compounds to the aqueous phase. In this system 1,2-epoxy-7,8-octene accumulated in a pure solvent phase, thus, product recovery problems associated with emulsion formation were avoided. Furthermore, no phase breakthrough of either liquid across the membrane was observed. In this system, the highest volumetric productivity obtained was 30 U.L-1, and this was achieved at a dilution rate of 0.07 h-1, 70 m2. m-3 of membrane area, and a steady-state biomass concentration of 2. 5 g.L-1. The system was stable for over 1250 h. Decreasing the dilution rate led to an increased biomass concentration, however, the specific activity was significantly reduced, and therefore, an optimal dilution rate was determined at 0.055 h-1. Copyright 1999 John Wiley & Sons, Inc.

Medium chain length alkane solvent-cell transfer rates in two-liquid phase, pseudomonas oleovorans cultures

The oxidation of medium chain length alkanes and alkenes (C6 to C12) by Pseudomonas oleovorans and related, biocatalytically active recombinant organisms, in two-liquid phase cultures can be used for the biochemical production of several interesting fine chemicals. The volumetric productivities that can be attained in two-liquid phase systems can be, in contrast to aqueous fermentations, limited by the transport of substrates from an apolar phase to the cells residing in the aqueous phase and by toxic effects of apolar solvents on microbial cells. We have assessed the impact of these possible limitations on attainable productivities in two-liquid phase fermentations operated with mcl-alkanes. Pseudomonas oleovorans grows well in two-liquid phase media containing a bulk n-octane phase as the sole carbon source. However, cells are also damaged, typically resulting in a cell lysis rate of about 0.08 to 0. 10 h-1. These rates could be lowered by 50 to 70% to 0.03 h-1 and substrate yields increased from 0.55 to 0.85 g g-1 by diluting octane in non-metabolizable long-chain hydrocarbon solvents. Transfer rates of medium chain length (mcl) alkanes from the apolar phase to the cells were determined by following growth and the rate at which carbon-containing metabolites accumulated in the different phases of the cultures. mcl-Alkane solvent-cell transfer rates of at least 79, 64, and 18 mmol per liter of aqueous medium per hour were determined for n-heptane, n-octane, and n-decane, respectively. Rates of up to 30 mmol L-1 h-1 were observed under octane-limiting conditions in systems where the apolar substrate was dissolved to concentrations below 3% (v/v) in hexadecene. Based on low power input experiments, we estimated the maximum obtainable mass transfer rates in large scale processes to be in the range of 13 mmol L-1 h-1 for decane and higher than 45 mmol L-1 h-1 for octane and heptane. The results indicate that high solvent to cell mass transfer rates and minimized cell damage will enable high production rates in two-liquid phase bioprocesses, justifying ongoing efforts to attain high densities of catalytically, highly active cells in such systems. Copyright 1998 John Wiley & Sons, Inc.

Growth characteristics of Escherichia coli HB101[pGEc47] on defined medium

This paper shows that differences in growth behavior of Escherichia coli strain HB101 and strain HB101[pGEc47] can be related to yeast extract-enriched medium rather than plasmid properties. An optimal medium for growth of E. coli HB101[pGEc47] was designed based on the individual yield coefficients for specific medium components (NH4+ 6 g g-1, PO43- 14 g g-1, SO42- 50 g g-1). The yield coefficient for L-leucine depends on the glucose content of the medium (20 g g-1 for 3% glucose, 40 g g-1 for 1% glucose) and the yield coefficient for L-proline depends on the cultivation mode (20 g g-1 for batch cultivation, 44 g g-1 for continuous cultivation). Growth on defined medium after medium optimization is as rapid as on complex medium (0. 42-0.45 h-1). The critical dilution rate (DR) in the defined medium above which undesired production of acetic acid occurs is in the range of 0.23-0.26 h-1.

The AlkB monooxygenase of Pseudomonas oleovorans--synthesis, stability and level in recombinant Escherichia coli and the native host

We have studied the synthesis and stability of the monooxygenase AlkB of Pseudomonas oleovorans in its natural host and in recombinant Escherichia coli. Three strains were investigated: the prototype strain P. oleovorans and the E. coli alk+ recombinants HB101 (pGEc47) and W3110 (pGEc47). Plasmid pGEc47 allows regulated expression of alkB and synthesis of active AlkB in E. coli. The E. coli strains were selected because E. coli HB101 (pGEc47) produces similar amounts of AlkB as P. oleovorans (1.5-2% of total cell protein), whereas E. coli W3110 (pGEc47) is able to make substantially (about fivefold) more AlkB. The AlkB synthesis and degradation rates in batch cultures of the three strains were determined by means of isotopic-labeling and immunological techniques. The mean specific AlkB synthesis rates in P. oleovorans, E. coli HB101 (pGEc47) and E. coli W3110 (pGEc47) were approximately 7, 12.5 and 45 microg x mg protein(-1) x h(-1), respectively. The half-lives of AlkB were estimated to be 80, 3 and 15 for P. oleovorans, E. coli HB101 (pGEc47) and E. coli W3110 (pGEc47), respectively. Thus, the intracellular AlkB level in each of the three strains was the result of their AlkB synthesis and degradation rates. The AlkB level during batch growth was modelled by means of experimentally derived parameters for AlkB synthesis and degradation, and showed good agreement with AlkB levels determined by means of immunoblotting in all strains investigated.

Physiological changes and alk gene instability in Pseudomonas oleovorans during induction and expression of alk genes

The alk genes of Pseudomonas oleovorans, which is able to metabolize alkanes and alkenes, are organized in alkST and alkBFGHJKL clusters, in which the expression of alkBFGHJKL is positively regulated by AlkS. Growth of the wild-type strain GPo1 and P. oleovorans GPo12 alk recombinants on octane resulted in changes of cellular physiology and morphology. These changes, which included lower growth rates and a reduction of the number of CFU due to filamentation, were also seen when the cells were grown on aqueous medium, and the alk genes were induced with dicyclopropylketone, a gratuitous inducer of the alk genes. These effects were seen only for recombinants carrying both alkST and alkBFGHJKL operons. Deletion of parts of either alkB or alkJ, which encode two major Alk proteins located in the cytoplasmic membrane, modified but did not eliminate the effects described above, suggesting that they were due to induction and expression of several alk genes. Continuous growth of the cells in the presence of dicyclopropylketone for about 10 generations led to inactivation, but not elimination, of the alk genes. This resulted in a return of the recombinants to normal physiology and growth.

Solvent selection for whole cell biotransformations in organic media

Although they were used historically as antimicrobial agents, there is a modern requirement to devise organic solvent systems for exploitation in the biotransformation by intact cells of substrates that are poorly soluble in water. Water-immiscible solvents are normally less cytotoxic than are water-miscible ones. While a unitary mechanism is excluded, damage to the membrane remains the likeliest major mechanism of cytotoxicity, and may be conveniently assessed using an electronic biomass probe. Studies designed to account for the mechanisms of action of general anesthetics and of uncouplers parallel those designed to account for the cytotoxicity of organic solvents. Although there are hundreds of potential physical descriptors of solvent properties, many are broadly similar to each other, such that the intrinsic dimensionality of solvent space is relatively small (< 10). This opens up the possibility of providing a rational biophysical basis for the optimization of the solvents used for biotransformations. The widely used descriptor of solvent behavior, log P (the octanol:water partition coefficient), is a composite of more fundamental molecular descriptors; this explains why there are rarely good correlations between cytotoxicity and log P when a wide variety of solvents is studied. Although the intrinsic dimensionality of solvent space is relatively small, pure solvents still populate it rather sparsely. Thus, mixtures of solvents can and do provide the opportunity of obtaining a solvent optimal for a biotransformation of interest.

Genetics of alkane oxidation by Pseudomonas oleovorans

Many Pseudomonads are able to use linear alkanes as sole carbon and energy source. The genetics and enzymology of alkane metabolism have been investigated in depth for Pseudomonas oleovorans, which is able to oxidize C5-C12 n-alkanes by virtue of two gene regions, localized on the OCT-plasmid. The so-called alk-genes have been cloned in pLAFR1, and were subsequent analyzed using minicell expression experiments, DNA sequencing and deletion analysis. This has led to the identification and characterization of of the alkBFGHJKL and alkST genes which encode all proteins necessary to convert alkanes to the corresponding acyl-CoA derivatives. These then enter the beta-oxidation-cycle, and can be utilized as carbon- and energy sources. Medium (C6-C12)- or long-chain (C13-C20) n-alkanes can be utilized by many strains, some of which have been partially characterized. The alkane-oxidizing enzymes used by some of these strains (e.g. two P. aeruginosa strains, a P. denitrificans strain and a marine Pseudomonas sp.) appear to be closely related to those encoded by the OCT-plasmid.

The alkane oxidation system of Pseudomonas oleovorans: induction of the alk genes in Escherichia coli W3110 (pGEc47) affects membrane biogenesis and results in overexpression of alkane hydroxylase in a distinct cytoplasmic membrane subfraction

The alkane hydroxylase system of Pseudomonas oleovorans, which catalyses the initial oxidation of aliphatic substrates, is encoded by three genes. One of the gene products, the alkane hydroxylase AlkB, is an integral cytoplasmic membrane protein. Induction leads to the synthesis of 1.5-2% AlkB relative to the total cell protein, both in P. oleovorans and in recombinant Escherichia coli DH1. We present a study on the induction and localization of the alkane hydroxylase in E. coli W3110, which appears to be an interesting host strain because it permits expression levels of AlkB of up to 10-15% of the total cell protein. This expression level had negative effects on cell growth. The phospholipid content of such cells was about threefold higher than that of wild-type W3110. Freeze-fracture electron microscopy showed that induction of the alk genes led to the appearance of membrane vesicles in the cytoplasm; these occurred much more frequently in cells expressing alkB than in the negative control, which contained all of the alk genes except for alkB. Isolation and separation of the membranes of cells expressing alkB by density gradient centrifugation showed the customary cytoplasmic and outer membranes, as well as a low-density membrane fraction. This additional fraction was highly enriched in AlkB, as shown both by SDS-PAGE and enzyme activity measurements. A typical cytoplasmic membrane protein, NADH oxidase, was absent from the low-density membrane fraction. alkB expression in W3110 changed the composition of the phospholipid headgroup in the membrane, as well as the fatty acid composition of the membrane. The major changes occurred in the unsaturated fatty acids: C16:1 and C18:1 increased at the expense of C17:0cyc and C19:0cyc.