Impact of hfq and sigE on the tolerance of Zymomonas mobilis ZM4 to furfural and acetic acid stresses

Zymomonas mobilis, as an ethanologenic microorganism with many desirable industrial features, faces crucial obstacles in the lignocellulosic ethanol production process. A significant hindrance occurs during the pretreatment procedure that not only produces fermentable sugars but also releases severe toxic compounds. As diverse parts of regulation networks are involved in different aspects of complicated tolerance to inhibitors, we developed ZM4-hfq and ZM4-sigE strains, in which hfq and sigE genes were overexpressed, respectively. ZM4-hfq is a transcription regulator and ZM4-sigE is a transcription factor that are involved in multiple stress responses. In the present work, by overexpressing these two genes, we evaluated their impact on the Z. mobilis tolerance to furfural, acetic acid, and sugarcane bagasse hydrolysates. Both recombinant strains showed increased growth rates and ethanol production levels compared to the parental strain. Under a high concentration of furfural, the growth rate of ZM4-hfq was more inhibited compared to ZM4-sigE. More precisely, fermentation performance of ZM4-hfq revealed that the yield of ethanol production was less than that of ZM4-sigE, because more unused sugar had remained in the medium. In the case of acetic acid, ZM4-sigE was the superior strain and produced four and two-fold more ethanol compared to the parental strain and ZM4-hfq, respectively. Comparison of inhibitor tolerance between single and multiple toxic inhibitors in the fermentation of sugarcane bagasse hydrolysate by ZM4-sigE strain showed similar results. In addition, ethanol production performance was considerably higher in ZM4-sigE as well. Finally, the results of the qPCR analysis suggested that under both furfural and acetic acid treatment experiments, overproduction of both hfq and sigE improves the Z. mobilis tolerance and its ethanol production capability. Overall, our study showed the vital role of the regulatory elements to overcome the obstacles in lignocellulosic biomass-derived ethanol and provide a platform for further improvement by directed evolution or systems metabolic engineering tools.

Heterologous expression of a glycosyl hydrolase and cellular reprogramming enable Zymomonas mobilis growth on cellobiose

Plant-derived fuels and chemicals from renewable biomass have significant potential to replace reliance on petroleum and improve global carbon balance. However, plant biomass contains significant fractions of oligosaccharides that are not usable natively by many industrial microorganisms, including Escherichia coli, Saccharomyces cerevisiae, and Zymomonas mobilis. Even after chemical or enzymatic hydrolysis, some carbohydrate remains as non-metabolizable oligosaccharides (e.g., cellobiose or longer cellulose-derived oligomers), thus reducing the efficiency of conversion to useful products. To begin to address this problem for Z. mobilis, we engineered a strain (Z. mobilis GH3) that expresses a glycosyl hydrolase (GH) with β-glucosidase activity from a related α-proteobacterial species, Caulobacter crescentus, and subjected it to an adaptation in cellobiose medium. Growth on cellobiose was achieved after a prolonged lag phase in cellobiose medium that induced changes in gene expression and cell composition, including increased expression and extracellular release of GH. These changes were reversible upon growth in glucose-containing medium, meaning they did not result from genetic mutation but could be retained upon transfer of cells to fresh cellobiose medium. After adaptation to cellobiose, our GH-expressing strain was able to convert about 50% of cellobiose to glucose within 24 h and use it for growth and ethanol production. Alternatively, pre-growth of Z. mobilis GH3 in sucrose medium enabled immediate growth on cellobiose. Proteomic analysis of cellobiose- and sucrose-adapted strains revealed upregulation of secretion-, transport-, and outer membrane-related proteins, which may aid release or surface display of GHs, entry of cellobiose into the periplasm, or both. Our two key findings are that Z. mobilis can be reprogrammed to grow on cellobiose as a sole carbon source and that this reprogramming is related to a natural response of Z. mobilis to sucrose that promotes sucrase production.

Increased salt tolerance in Zymomonas mobilis strain generated by adaptative evolution

BACKGROUND

Ethanologenic alphaproteobacterium Zymomonas mobilis has been acknowledged as a promising biofuel producer. There have been numerous efforts to engineer this species applicable for an industrial-scale bioethanol production. Although Z. mobilis is robustly resilient to certain abiotic stress such as ethanol, the species is known to be sensitive to saline stress at a mild concentration, which hampers its industrial use as an efficient biocatalyst. To overcome this issue, we implemented a laboratory adaptive evolution approach to obtain salt tolerant Z. mobilis strain.

RESULTS

During an adaptive evolution, we biased selection by cell morphology to exclude stressed cells. The evolved strains significantly improved growth and ethanol production in the medium supplemented with 0.225 M NaCl. Furthermore, comparative metabolomics revealed that the evolved strains did not accumulate prototypical osmolytes, such as proline, to counter the stress during their growth. The sequenced genomes of the studied strains suggest that the disruption of ZZ6_1149 encoding carboxyl-terminal protease was likely responsible for the improved phenotype.

CONCLUSIONS

The present work successfully generated strains able to grow and ferment glucose under the saline condition that severely perturbs parental strain physiology. Our approach to generate strains, cell shape-based diagnosis and selection, might be applicable to other kinds of strain engineering in Z. mobilis.

Capacity for survival in global warming: Adaptation of mesophiles to the temperature upper limit

The Intergovernmental Panel on Climate Change recommends keeping the increase in temperature to less than a two-degree increase by the end of the century, but the direct impact of global warming on ecosystems including microbes has not been investigated. Here we performed thermal adaptation of two species and three strains of mesophilic microbes for improvement of the survival upper limit of temperature, and the improvement was evaluated by a newly developed method. To understand the limitation and variation of thermal adaptation, experiments with mutators and by multiple cultures were performed. The results of experiments including genome sequencing and analysis of the characteristics of mutants suggest that these microbes bear a genomic potential to endure a 2–3°C rise in temperature but possess a limited variation of strategies for thermal adaptation.

Hyper-production of levansucrase from Zymomonas mobilis KIBGEIB14 using submerged fermentation technique

The industrial utilization of enzymes requires the high yield of enzyme production for the synthesis of polymers by microorganisms. Therefore, it is necessary to optimize different production parameters of levansucrase in order to increase its industrial applications. Zymomonas mobilis KIBGE-IB14 was considered as a promising candidate for the large scale production of levan among wide range of microorganisms. The current investigation is aimed to optimize the production parameters of levansucrase by Z. mobilis KIBGE-IB14 isolated from molasses. The results indicated that bacterial growth as well as enzyme production was greatly influenced by both physical and chemical conditions. It was revealed that high enzyme titers were achieved at 30°C with pH 6.5 after 24 hours of incubation in a modified medium. Moreover, the enzyme exhibited its induction in the presence of sucrose used as a substrate. Thus, the present study demonstrated that newly isolated Z. mobilis KIBGE-IB14 can be used as a plausible producer of levansucrase for industrial applications.

The Low Energy-Coupling Respiration in Zymomonas mobilis Accelerates Flux in the Entner-Doudoroff Pathway

Performing oxidative phosphorylation is the primary role of respiratory chain both in bacteria and eukaryotes. Yet, the branched respiratory chains of prokaryotes contain alternative, low energy-coupling electron pathways, which serve for functions other than oxidative ATP generation (like those of respiratory protection, adaptation to low-oxygen media, redox balancing, etc.), some of which are still poorly understood. We here demonstrate that withdrawal of reducing equivalents by the energetically uncoupled respiratory chain of the bacterium Zymomonas mobilis accelerates its fermentative catabolism, increasing the glucose consumption rate. This is in contrast to what has been observed in other respiring bacteria and yeast. This effect takes place after air is introduced to glucose-consuming anaerobic cell suspension, and can be simulated using a kinetic model of the Entner-Doudoroff pathway in combination with a simple net reaction of NADH oxidation that does not involve oxidative phosphorylation. Although aeration hampers batch growth of respiring Z. mobilis culture due to accumulation of toxic byproducts, nevertheless under non-growing conditions respiration is shown to confer an adaptive advantage for the wild type over the non-respiring Ndh knock-out mutant. If cells get occasional access to limited amount of glucose for short periods of time, the elevated glucose uptake rate selectively improves survival of the respiring Z. mobilis phenotype.

Neutral red-mediated microbial electrosynthesis by Escherichia coli, Klebsiella pneumoniae, and Zymomonas mobilis

The aim of this work was to compare the effects of electrosynthesis on different bacterial species. The effects of neutral red-mediated electrosynthesis on the metabolite profiles of three microorganisms: Escherichia coli, Klebsiella pneumoniae, and Zymomonas mobilis, were measured and compared and contrasted. A statistically comprehensive analysis of neutral red-mediated electrosynthesis is presented using the analysis of end-product profiles, current delivered, and changes in cellular protein expression. K. pneumoniae displayed the most dramatic response to electrosynthesis of the three bacteria, producing 93% more ethanol and 76% more lactate vs. control fermentation with no neutral red and no electron delivery. Z. mobilis showed no response to electrosynthesis except elevated acetate titers. Stoichiometric comparison showed that NAD(+) reduction by neutral red could not account for changes in metabolites during electrosynthesis. Neutral red-mediated electrosynthesis was shown to have multifarious effects on the three species.

Pre-treatment step with Leuconostoc mesenteroides or L. pseudomesenteroides strains removes furfural from Zymomonas mobilis ethanolic fermentation broth

Furfural is an inhibitor of growth and ethanol production by Zymomonas mobilis. This study used a naturally occurring (not GMO) biological pre-treatment to reduce that amount of furfural in a model fermentation broth. Pre-treatment involved inoculating and incubating the fermentation broth with strains of Leuconostoc mesenteroides or Leuconostoc pseudomesenteroides. The Leuconostoc strains converted furfural to furfuryl alcohol without consuming large amounts of dextrose in the process. Coupling this pre-treatment to ethanolic fermentation reduced furfural in the broth and improved growth, dextrose uptake and ethanol formation. Pre-treatment permitted ethanol formation in the presence of 5.2 g L(-1) furfural, which was otherwise inhibitive. The pre-treatment and presence of the Leuconostoc strains in the fermentation broth did not interfere with Z. mobilis ethanolic fermentation or the amounts of ethanol produced. The method suggests a possible technique for reducing the effect that furfural has on the production of ethanol for use as a biofuel.

Tolerance of S. cerevisiae and Z. mobilis to inhibitors produced during dilute acid hydrolysis of soybean meal

The objective of this research was to determine the minimum inhibitory concentration of 5-hydroxymethyl furfural, furfural, and acetic acid on Saccharomyces cerevisiae (NRRL Y-2233) and Zymomonas mobilis subspecies mobilis (NRRL B-4286) in both detoxified hydrolyzed soybean meal and synthetic YM broth spiked with the three compounds. Soybean meal was hydrolyzed with dilute sulfuric acid (0.0, 0.5, 1.25, and 2.0% wt v(-1)) at three temperatures (105, 120, and 135°C) and three durations (15, 30, and 45 min) followed by detoxification with activated carbon. Of all the combinations, only the treatments obtained at 135°C, 2.0% H2SO4, and 45 min and the one at 135°C, 1.25% H2SO4, and 45 min showed inhibition in the growth of the tested microorganisms. Spiked YM broths showed inhibition for the highest levels of inhibitors, either applied individually or in combination.

"Fish-in-net", a novel method for cell immobilization of Zymomonas mobilis

BACKGROUND

Inorganic mesoporous materials exhibit good biocompatibility and hydrothermal stability for cell immobilization. However, it is difficult to encapsulate living cells under mild conditions, and new strategies for cell immobilization are needed. We designed a "fish-in-net" approach for encapsulation of enzymes in ordered mesoporous silica under mild conditions. The main objective of this study is to demonstrate the potential of this approach in immobilization of living cells.

METHODOLOGY/PRINCIPAL FINDINGS

Zymomonas mobilis cells were encapsulated in mesoporous silica-based materials under mild conditions by using a "fish-in-net" approach. During the encapsulation process, polyethyleneglycol was used as an additive to improve the immobilization efficiency. After encapsulation, the pore size, morphology and other features were characterized by various methods, including scanning electron microscopy, nitrogen adsorption-desorption analysis, transmission electron microscopy, fourier transform infrared spectroscopy, and elemental analysis. Furthermore, the capacity of ethanol production by immobilized Zymomonas mobilis and free Zymomonas mobilis was compared.

CONCLUSIONS/SIGNIFICANCE

In this study, Zymomonas mobilis cells were successfully encapsulated in mesoporous silica-based materials under mild conditions by the "fish-in-net" approach. Encapsulated cells could perform normal metabolism and exhibited excellent reusability. The results presented here illustrate the enormous potential of the "fish-in-net" approach for immobilization of living cells.

Hydrogen sulfide formation as well as ethanol production in different media by cysND- and/or cysIJ-inactivated mutant strains of Zymomonas mobilis ZM4

Many bacteria reduce inorganic sulfate to sulfide to satisfy their need for sulfur, one of the most important elements for biological life. But little is known about the metabolic pathways involving hydrogen sulfide (H₂S) in mesophilic bacteria. By genomic sequence analysis, a complete set of genes for the assimilatory sulfate reduction pathway has been identified in the ethanologen Zymomonas mobilis. In this study, the first ATP sulfurylase- and final sulfite reductase-encoding genes cysND and cysIJ, respectively, in the putative pathway from sulfate to sulfite in Z. mobilis ZM4 was singly or doubly inactivated by homologous recombination and a site-specific FLP-FRT recombination. The resultant mutants, ∆cysND, ∆cysIJ and ∆cysND-cat∆cysIJ, were unable to produce detectable H₂S in glucose or sucrose-containing rich medium and sweet sorghum juice, in which the wild-type ZM4 produced detectable H₂S. While adding sulfite (SO₃²⁻) into media impaired the growth of the mutants and ZM4 to varying degrees, the sulfite restored the H₂S formation in the ∆cysND in the above media, but not in the ∆cysIJ and ∆cysND-cat∆cysIJ mutants. Although it seemed that the inactivation of cysND and cysIJ did not exert a significant negative effect on the cell growth at least in glucose or sucrose medium, the ethanol production of all mutants was inferior to that of ZM4 in sucrose medium and sweet sorghum juice. In addition, adding L-cysteine to glucose-containing rich media restored H₂S formation of all mutants, indicating the existence of another pathway for producing H₂S in Z. mobilis. All these results would help to further elucidate the metabolic pathways involving H₂S in Z. mobilis and exploit the biotechnological applications of this industrially important bacterium.

A scalable microfluidic chip for bacterial suspension culture

Microfluidic systems could, in principle, enable high-throughput breeding and screening of microbial strains for industrial applications, but parallel and scalable culture and detection chips are needed before complete microbial selection systems can be integrated and tested. Here we demonstrate a scalable multi-channel chip that is capable of bacterial suspension culture. The key invention is a multi-layered chip design, which enables a single set of control channels to function as serial peristaltic pumps to drive parallel culture chamber loops. Such design leads to scalability of the culture chip. We demonstrate that E. coli growth in the chip is equivalent or superior to conventional suspension culture on shaking beds. The chip could also be used for suspension culture of other microbes such as Bacillus subtilis, Pseudomonas stutzeri, and Zymomonas mobilis, indicating its general applicability for bacterial suspension culture.

Kinetic modeling to optimize pentose fermentation inZymomonas mobilis

Zymomonas mobilis engineered to express four heterologous enzymes required for xylose utilization ferments xylose along with glucose. A network of pentose phosphate (PP) pathway enzymatic reactions interacting with the native glycolytic Entner Doudoroff (ED) pathway has been hypothesized. We have investigated this putative reaction network by developing a kinetic model incorporating all of the enzymatic reactions of the PP and ED pathways, including those catalyzed by the heterologous enzymes. Starting with the experimental literature on in vitro characterization of each enzymatic reaction, we have developed a kinetic model to enable dynamic simulation of intracellular metabolite concentrations along the network of interacting PP and ED metabolic pathways. This kinetic model is useful for performing in silico simulations to predict how varying the different enzyme concentrations will affect intracellular metabolite concentrations and ethanol production rate during continuous fermentation of glucose and xylose mixtures. Among the five enzymes whose concentrations were varied as inputs to the model, ethanol production in the continuous fermentor was optimized when xylose isomerase (XI) was present at the highest level, followed by transaldolase (TAL). Predictions of the model that the interconnecting enzyme phosphoglucose isomerase (PGI) does not need to be overexpressed were recently confirmed through experimental investigations. Through such systematic analysis, we can develop efficient strategies for maximizing the fermentation of both glucose and xylose, while minimizing the expression of heterologous enzymes.

Enhancement of Acid Tolerance in Zymomonas mobilis by a Proton-Buffering Peptide

A portion of the cbpA gene from Escherichia coli K-12 encoding a 24 amino acid proton-buffering peptide (Pbp) was cloned via the shuttle vector pJB99 into E. coli JM105 and subsequently into Zymomonas mobilis CP4. Expression of Pbp was confirmed in both JM105 and CP4 by HPLC. Z. mobilis CP4 carrying pJB99-2 (Pbp) exhibited increased acid tolerance (p < 0.05) in acidified TSB (HCl [pH 3.0] or acetic acid [pH 3.5]), glycine-HCl buffer (pH 3.0), and sodium acetate-acetic acid buffer (pH 3.5) in comparison to the parent strain (CP4) and CP4 with pJB99 (control plasmid). Although the expression of Pbp influenced survival at a low pH, the minimum growth pH was unaffected. Growth of Z. mobilis in the presence of ampicillin also significantly increased acid tolerance by an unknown mechanism. Results from this study demonstrate that the production of a peptide with a high proportion of basic amino acids can contribute to protection from low pH and weak organic acids such as acetic acid.

Physiological regulation of the properties of alcohol dehydrogenase II (ADH II) of Zymomonas mobilis: NADH renders ADH II resistant to cyanide and aeration

The variable cyanide-sensitivity of the iron-containing alcohol dehydrogenase isoenzyme (ADH II) of the ethanol-producing bacterium Zymomonas mobilis was studied. In aerobically grown permeabilized cells, cyanide caused gradual inhibition of ADH II, which was largely prevented by externally added NADH. Cyanide-sensitivity of ADH II was highest in cells grown under conditions of vigorous aeration, in which intracellular NADH concentration was low. Anaerobically grown bacteria, as well as those cultivated aerobically in the presence of cyanide, maintained higher intracellular NADH levels along with a more cyanide-resistant ADH II. It was demonstrated that cyanide acted as a competitive inhibitor of ADH II, competing with nicotinamide nucleotides. NADH increased both cyanide-resistance and oxygen-resistance of ADH II.

Transcriptional analysis of a gene cluster involved in glucose tolerance in Zymomonas mobilis: evidence for an osmoregulated promoter

Exponentially growing cells of Zymomonas mobilis normally exhibit a lag period of up to 3 h when they are transferred from a liquid medium containing 2% glucose to a liquid medium containing 10% glucose. A mutant of Z. mobilis (CU1) exhibited a lag period of more than 20 h when it was grown under the same conditions, whereas it failed to grow on a solid medium containing 10% glucose. The glucose-defective phenotype of mutant CU1 was due to a spontaneous insertion in a putative gene (ORF4) identified as part of an operon (glc) which includes three additional putative genes (ORF1, ORF2, and ORF3) with no obvious involvement in the glucose tolerance mechanism. The common promoter controlling glc operon transcription, designated P(glc), was found to be osmoregulated and stimulated by the putative product of ORF4 in an autoregulated fashion, as indicated by expression of the gfp reporter gene. Additionally, reverse transcriptase PCR analysis showed that the gene cluster produces a single mRNA, which verified the operon organization of this transcription unit. Further transcriptional analysis demonstrated that glc operon expression is regulated by the concentration of glucose, which supported the hypothesis that this operon is directly involved in the uncharacterized glucose tolerance mechanism of Z. mobilis.

Over-expression of xylulokinase in a xylose-metabolising recombinant strain of Zymomonas mobilis

The broad host range vector pBBR1MCS-2 has been evaluated as an expression vector for Zymomonas mobilis. The transformation efficiency of this vector was 2 x 10(3) CFU per mug of DNA in a recombinant strain of Z. mobilis ZM4/AcR containing the plasmid pZB5. Stable replication for this expression vector was demonstrated for 50 generations. This vector was used to study xylose metabolism in acetate resistant Z. mobilis ZM4/AcR (pZB5) by over-expression of xylulokinase (XK), as previous studies had suggested that XK could be the rate-limiting enzyme for such strains. Based on the above vector, a recombinant plasmid pJX1 harboring xylB (expressing XK) under control of a native Z. mobilis promotor Ppdc was constructed. When this plasmid was introduced into ZM4/AcR (pZB5) a 3-fold higher XK expression was found compared to the control strain. However, fermentation studies with ZM4/AcR (pZB5, pJX1) on xylose medium did not result in any increase in rate of growth or xylose metabolism, suggesting that XK expression was not rate-limiting for ZM4/AcR (pZB5) and related strains.

Cellulosic fuel ethanol: alternative fermentation process designs with wild-type and recombinant Zymomonas mobilis

Iogen (Canada) is a major manufacturer of industrial cellulase and hemicellulase enzymes for the textile, pulp and paper, and poultry feed industries. Iogen has recently constructed a 40 t/d biomass-to-ethanol demonstration plant adjacent to its enzyme production facility. The integration of enzyme and ethanol plants results in significant reduction in production costs and offers an alternative use for the sugars generated during biomass conversion. Iogen has partnered with the University of Toronto to test the fermentation performance characteristics of metabolically engineered Zymomonas mobilis created at the National Renewable Energy Laboratory. This study focused on strain AX101, a xylose- and arabinose-fermenting stable genomic integrant that lacks the selection marker gene for antibiotic resistance. The "Iogen Process" for biomass depolymerization consists of a dilute-sulpfuric acid-catalyzed steam explosion, followed by enzymatic hydrolysis. This work examined two process design options for fermentation, first, continuous cofermentation of C5 and C6 sugars by Zm AX101, and second, separate continuous fermentations of prehydrolysate by Zm AX101 and cellulose hydrolysate by either wildtype Z. mobilis ZM4 or an industrial yeast commonly used in the production of fuel ethanol from corn. Iogen uses a proprietary process for conditioning the prehydrolysate to reduce the level of inhibitory acetic acid to at least 2.5 g/L. The pH was controlled at 5.5 and 5.0 for Zymomonas and yeast fermentations, respectively. Neither 2.5 g/L of acetic acid nor the presence of pentose sugars (C6:C5 = 2:1) appreciably affected the high-performance glucose fermentation of wild-type Z. mobilis ZM4. By contrast, 2.5 g/L of acetic acid significantly reduced the rate of pentose fermentation by strain AX101. For single-stage continuous fermentation of pure sugar synthetic cellulose hydrolysate (60 g/L of glucose), wild-type Zymomonas exhibited a four-fold higher volumetric productivity compared with industrial yeast. Low levels of acetic acid stimulated yeast ethanol productivity. The glucose-to-ethanol conversion efficiency for Zm and yeast was 96 and 84%, respectively.

The paradoxical cyanide-stimulated respiration of Zymomonas mobilis: cyanide sensitivity of alcohol dehydrogenase (ADH II)

The respiratory inhibitor cyanide stimulates growth of the ethanologenic bacterium Zymomonas mobilis, perhaps by diverting reducing equivalents from respiration to ethanol synthesis, thereby minimizing accumulation of toxic acetaldehyde. This study sought to identify cyanide-sensitive components of respiration. In aerobically grown, permeabilized Z. mobilis cells, addition of 200 microM cyanide caused gradual inhibition of ADH II, the iron-containing alcohol dehydrogenase isoenzyme, which, in aerobic cultures, might be oxidizing ethanol and supplying NADH to the respiratory chain. In membrane preparations, NADH oxidase was inhibited more rapidly, but to a lesser extent, than ADH II. The time-course of inhibition of whole-cell respiration resembled that of NADH oxidase, yet the inhibition was almost complete, and was accompanied by an increase of intracellular NADH concentration. Cyanide did not significantly affect the activity of ADH I, the zinc-containing alcohol dehydrogenase isoenzyme. When an aerobic batch culture was grown in the presence of 200 microM cyanide, cyanide-resistant ADH II activity was observed, its appearance correlating with the onset of respiration. It is concluded that the membrane-associated respiratory chain, but not ADH II, is responsible for the whole-cell cyanide sensitivity, while the cyanide-resistant ADH II is needed for respiration in the presence of cyanide, and represents an adaptive response of Z. mobilis to cyanide, analogous to the induction of alternative terminal oxidases in other bacteria.

Catalytic activity of zymomonas mobilis extracellular "levan-levansucrase" complex in sucrose medium

The fructan biosynthesis by ethanol sedimented "levan-levansucrase" complex from Zymomonas mobilis fermentation broth as well as purified levansucrase was investigated. The fructooligosaccharide (FOS) producing activity of "levan-levamsucrase" sediment was investigated in 55% sucrose syrup at 45 degrees C. It was shown that FOS in the syrup were presented by 1-kestose, 6-kestose, neokestose and nystose. The increase of gluconic acid concentration was observed in the reaction mixture during the incubation suggesting about presence of glucose/fructose oxidoreductase in "levan-levansucrase" sediment. The influence of ethanol, glycerol and NaCl on levan and fructooligosaccharide formation by "levan-levansucrase" complex and purified levansucrase was studied and the changes in the ratio between different activities of levansucrase (sucrose hydrolysis, levan biosynthesis and FOS formation) were observed. Ethanol increases the FOS biosynthesis part in total activity of purified levansucrase. The technology of the production of prebiotics containing food product--fructan syrup by "levan-levansucrase " sediment as biocatalyst was developed.

Steady-state measurements of lactic acid production in a wild-type and a putative D-lactic acid dehydrogenase-negative mutant of Zymomonas mobilis: influence of glycolytic flux

This work represents a continuation of our investigation into environmental conditions that promote lactic acid synthesis by Zymomonas mobilis. The characteristic near theoretical yield of ethanol from glucose by Z. mobilis can be compromised by the synthesis of D- and L-lactic acid. The production of lactic acid is exacerbated by the following conditions: pH 6.0, yeast extract, and reduced growth rate. At a specific growth rate of 0.048/h, the average yield of DL-lactate from glucose in a yeast extract-based medium at pH 6.0 was 0.15 g/g. This represents a reduction in ethanol yield of about 10% relative to the yield at a growth rate of 0.15/h. Very little lactic acid was produced at pH 5.0 or using a defined salts medium (without yeast extract) Under permissive and comparable culture conditions, a tetracycline-resistant, D-ldh negative mutant produced about 50% less lactic acid than its parent strain Zm ATCC 39676. D-lactic acid was detected in the cell-free spent fermentation medium of the mutant, but this could be owing to the presence of a racemase enzyme. Under the steady-state growth conditions provided by the chemostat, the specific rate of glucose consumption was altered at a constant growth rate of 0.075/h. Shifting from glucose-limited to nitrogen-limited growth, or increasing the temperature, caused an increase in the specific rate of glucose catabolism. There was good correlation between an increase in glycolytic flux and a decrease in lactic acid yield from glucose. This study points to a mechanistic link between the glycolytic flux and the control of end-product glucose metabolism. Implications of reduced glycolytic flux in pentose-fermenting recombinant Z. mobilis strains, relative to increased byproduct synthesis, is discussed.

Characterization of heterologous and native enzyme activity profiles in metabolically engineered Zymomonas mobilis strains during batch fermentation of glucose and xylose mixtures

Zymomonas mobilis has been metabolically engineered to broaden its substrate utilization range to include D-xylose and L-arabinose. Both genomically integrated and plasmid-bearing Z. mobilis strains that are capable of fermenting the pentose D-xylose have been created by incorporating four genes: two genes encoding xylose utilization metabolic enzymes (xylA/xylB) and two genes encoding pentose phosphate pathway enzymes (talB/tktA). We have characterized the activities of the four newly introduced enzymes for xylose metabolism, along with those of three native glycolytic enzymes, in two different xylose-fermenting Z. mobilis strains. These strains were grown on glucose-xylose mixtures in computer-controlled fermentors. Samples were collected and analyzed to determine extracellular metabolite concentrations as well as the activities of several intracellular enzymes in the xylose and glucose uptake and catabolism pathways. These measurements provide new insights on the possible bottlenecks in the engineered metabolic pathways and suggest methods for further improving the efficiency of xylose fermentation.

Respective effects of sodium and chloride ions on filament formation and growth and ethanol production in Zymomonas mobilis fermentations

AIMS

To elucidate the separate effects of the cation and anion in Zymomonas mobilis cultures inhibited by NaCl.

METHODS AND RESULTS

In batch cultures containing 0.175 mol l-1 NaCl, Z. mobilis developed filaments and growth and ethanol production were inhibited. Chloride ions (added as 0.175 mol l-1 NH4Cl) produced similar filamentous growth, while sodium ions at the same concentration (as Na2SO4) did not. Growth and ethanol production were more strongly inhibited by Na2SO4 than by NH4Cl. Ammonium and sulphate ions had no inhibitory effects at these concentrations.

CONCLUSIONS

Filament formation in NaCl-inhibited Z. mobilis is due entirely to the chloride ion, while the sodium ion has the major effect on growth and ethanol production.

SIGNIFICANCE AND IMPACT OF THE STUDY

To avoid filamentous growth, chloride ions should be excluded from ethanol fermentations employing Z. mobilis, e.g. sulphuric is preferable to hydrochloric acid when hydrolysing lignocellulosic substrates prior to fermentation.

Evaluation of a recombinant Klebsiella oxytoca strain for ethanol production from cellulose by simultaneous saccharification and fermentation: comparison with native cellobiose-utilising yeast strains and performance in co-culture with thermotolerant yeast and Zymomonas mobilis

In the simultaneous saccharification and fermentation to ethanol of 100 g l(-1) microcrystalline cellulose, the cellobiose-fermenting recombinant Klebsiella oxytoca P2 outperformed a range of cellobiose-fermenting yeasts used in earlier work, despite producing less ethanol than reported earlier for this organism under similar conditions. The time taken by K. oxytoca P2 to produce up to about 33 g l(-1) ethanol was much less than for any other organism investigated, including ethanol-tolerant strains of Saccharomyces pastorianus, Kluyveromyces marxianus and Zymomonas mobilis. Ultimately, it produced slightly less ethanol (maximum 36 g l(-1)) than these organisms, reflecting its lower ethanol tolerance. Significant advantages were obtained by co-culturing K. oxytoca P2 with S. pastorianus, K. marxianus or Z. mobilis, either isothermally, or in conjunction with temperature-profiling to raise the cellulase activity. Co-cultures produced significantly more ethanol, more rapidly, than either of the constituent strains in pure culture at the same inoculum density. K. oxytoca P2 dominated the early stages of the co-cultures, with ethanol production in the later stages due principally to the more ethanol tolerant strain. The usefulness of K. oxytoca P2 in cellulose simultaneous saccharification and fermentation should be improved by mutation of the strain to increase its ethanol tolerance.

Comparative ethanol productivities of different Zymomonas recombinants fermenting oat hull hydrolysate

Iogen Corporation of Ottawa, Canada, has recently built a 50 t/d biomass-to-ethanol demonstration plant adjacent to its enzyme production facility. Iogen has partnered with the University of Toronto to test the C6/C5 cofermentation performance characteristics of National Renewable Energy Laboratory's metabolically engineered Zymomonas mobilis using its biomass hydrolysates. In this study, the biomass feedstock was an agricultural waste, namely oat hulls, which was hydrolyzed in a proprietary two-stage process involving pretreatment with dilute sulfuric acid at 200-250 degrees C, followed by cellulase hydrolysis. The oat hull hydrolysate (OHH) contained glucose, xylose, and arabinose in a mass ratio of about 8:3:0.5. Fermentation media, prepared from diluted hydrolysate, were nutritionally amended with 2.5 mL/L of corn steep liquor (50% solids) and 1.2 g/L of diammonium phosphate. The estimated cost for large-scale ethanol production using this minimal level of nutrient supplementation was 4.4cents/gal of ethanol. This work examined the growth and fermentation performance of xylose-utilizing, tetracycline-resistant, plasmid-bearing, patented, recombinant Z. mobilis cultures: CP4:pZB5, ZM4:pZB5, 39676:pZB4L, and a hardwood prehydrolysate-adapted variant of 39676:pZB4L (designated as the "adapted" strain). In pH-stat batch fermentations with unconditioned OHH containing 6% (w/v) glucose, 3% xylose, and 0.75% acetic acid, rec Zm ZM4:pZB5 gave the best performance with a fermentation time of 30 h, followed by CP4:pZB5 at 48 h, with corresponding volumetric productivities of 1.4 and 0.89 g/ (L x h), respectively. Based on the available glucose and xylose, the process ethanol yield for both strains was 0.47 g/g (92% conversion efficiency). At 48 h, the process yield for rec Zm 39676:pZB4L and the adapted strain was 0.32 and 0.34 g/g, respectively. None of the test strains was able to ferment arabinose. Acetic acid tolerance appeared to be a major determining factor in cofermentation performance.

Enzymes of the Entner-Doudoroff and pyruvate decarboxylation pathways in Zymomonas mobilis wild-type CP4 and mutant strains grown in continuous culture

The osmotolerant Zymomonas mobilis strain suc40, (containing plasmid pDS3154-inaZ), which is capable of producing simultaneously ethanol and ice nuclei protein, was cultivated in a chemically defined complete sucrose medium, as well as in a sugar beet molasses medium in continuous culture. The strain exhibited the normal Monod's relationship between biomass and dilution rate, and between growth substrate concentration and dilution rate. Specific activities of a number of enzymes that appear to control important steps in the metabolic flux of the Entner-Doudoroff and pyruvate decarboxylation pathways were investigated over a range of growth rates in steady state cultures. With the exception of glucose-6-phosphate dehydrogenase and gluconate kinase, all of the enzymes exhibited a very similar pattern for the wild type Z. mobilis CP4 and for the osmotolerant mutants, independent of the media used; the enzyme patterns remained relatively constant over the studied growth range. The specific activity of glucose-6-phosphate dehydrogenase was increased 2-fold by decreasing dilution rate in sugar beet molasses. The specific activity of gluconate kinase was 2-fold lower at medium growth rates compared with that at either low or high growth rates. Pyruvate kinase, pyruvate decarboxylase and alcohol dehydrogenase activities were significantly higher compared with those of the enzymes governing the early steps of the Entner-Doudoroff pathway. The present study, which was designed to determine the behaviour of important enzymes in sucrose metabolism of Z. mobilis suc40/pDS3154-inaZ grown in continuous culture showed that the microorganism required regulation of specific enzyme activities at the transcriptional level when sugar beet molasses were used as the growth medium.

Production of glucose-fructose oxidoreductase and ethanol by Zymomonas mobilis ATCC 29191 in medium containing corn steep liquor as a source of vitamins

Different concentrations of corn steep liquor (CSL) were tested in the cultivation of Zymomonas mobilis. Cell growth, ethanol production, and the formation of glucose-fructose oxidoreductase (GFOR) and glucono-delta-lactonase (GL), the enzymes responsible for the bio-production of gluconic acid and sorbitol, were examined. The cell yields using 25 g CSL l(-1) and 40 g CSL l(-1) (Y(X,S) approximately 0.031 g g(-1)) were close to that obtained with 5 g yeast extract (YE) l(-1). With 5 g CSL l(-1) and 15 g CSL l(-1), the nutritional limitation led to smaller Y(X/S). Using 100 g CSL l(-1) produced an inhibitory effect on cell growth. Similar ethanol yields (92-95%) were calculated for each concentration of CSL and also for YE medium. The highest specific GFOR/GL activities (13.2-13.5 U g(-1) dry cell) were reached with 25 g CSL l(-1) and 40 g CSL l(-1), values comparable to that achieved with 5 g YE l(-1). The results confirm that CSL is an effective and cheap supplement for Z. mobilis medium, increasing the economic potential of a large-scale bio-production of sorbitol and gluconic acid by untreated Z. mobilis cells. The economic feasibility of the process is discussed.

Simultaneous ethanol and bacterial ice nuclei production from sugar beet molasses by a Zymomonas mobilis CP4 mutant expressing the inaZ gene of Pseudomonas syringae in continuous culture

AIM

The aim of this work was to construct a Zymomonas mobilis mutant capable of simultaneous ethanol and ice nuclei production from agricultural by-product such as sugar beet molasses, in steady-state continuous culture.

METHODS AND RESULTS

A sucrose-hypertolerant mutant of Z. mobilis strain CP4, named suc40, capable of growing on 40% (w/v) sucrose medium was isolated following N-methyl-N'-nitro-N-nitrosoguanidine treatment. Plasmid pDS3154 carrying the inaZ gene of Pseudomonas syringae was conjugally transferred and expressed in suc40. The potential for simultaneous ethanol and bacterial ice nuclei production was assessed in steady-state continuous cultures over a range of dilution rates from 0.04 to 0.13 h(-1). In addition, the fatty acid and phospholipid profile of the three strains was also investigated. Ethanol production up to 43 g l(-1) was achieved at dilution rates below 0.10 h(-1) in sugar beet molasses. Ice nucleation activity gradually increased with increasing dilution rate and the greatest activity, -3.4 log (ice nuclei per cell), was observed at the highest dilution rate (0.13 h(-1)). Both mutant strains displayed a different fatty acid and phospholipid profile compared with the wild-type strain.

CONCLUSIONS

The ability of the mutant and recombinant plasmid-containing strains to grow on high sugar concentrations and in high osmotic pressure environments (molasses) can be attributed to their phospholipid and fatty acid contents.

SIGNIFICANCE AND IMPACT OF THE STUDY

Taking into account that sugar beet molasses is a low cost agricultural by-product, the simultaneous ethanol and bacterial ice nucleation production achieved under the studied conditions is considered very promising for industrial applications.

Isolation of the dxr gene of Zymomonas mobilis and characterization of the 1-deoxy-D-xylulose 5-phosphate reductoisomerase

The gene encoding the second enzyme of the 2C-methyl-D-erythritol 4-phosphate (MEP) pathway for isopentenyl diphosphate biosynthesis, 1-deoxy-D-xylulose 5-phosphate (DXP) reductoisomerase, was cloned and sequenced from Zymomonas mobilis. The deduced amino acid sequence showed the highest identity (48.2%) to the DXP reductoisomerase of Escherichia coli. Biochemical characterization of the purified DXP reductoisomerase showed a strict dependence of the enzyme on NADPH and divalent cations (Mn(2+), Co(2+) or Mg(2+)). The enzyme is a dimer with a molecular mass of 39 kDa per subunit and has a specific activity of 19.5 U mg protein(-1). Catalysis of the intramolecular rearrangement and reduction of DXP to MEP is competitively inhibited by the antibiotic fosmidomycin with a K(i) of 0.6 microM.

Continuous fermentation studies with xylos-utilizing recombinant Zymomonas mobilis

This study examined the continuous cofermentation performance characteristics of a dilute-acid "prehydrolysate-adapted" recombinant Zymomonas 39676:pZB4L and builds on the pH-stat batch fermentations with this recombinant that we reported on last year. Substitution of yeast extract by 1% (w/v) corn steep liquor (CSL) (50% solids) and Mg (2 mM) did not alter the cofermentation performance. Using declared assumptions, the cost of using CSL and Mg was estimated to be 12.5 cents/gal of ethanol with a possibility of 50% cost reduction using fourfold less CSL with 0.1% diammonium phosphate. Because of competition for a common sugar transporter that exhibits a higher affinity for glucose, utilization of glucose was complete whereas xylose was always present in the chemostat effluent. The ethanol yield, based on sugar used, was 94% of theoretical maximum. Altering the sugar ratio of the synthetic dilute acid hardwood prehydrolysate did not appear to significantly change the pattern of xylose utilization. Using a criterion of 80% sugar utilization for determining the maximum dilution rate (Dmax), changing the composition of the feed from 4% xylose to 3%, and simultaneously increasing the glucose from 0.8 to 1.8% shifted Dmax from 0.07 to 0.08/h. With equal amounts of both sugars (2.5%), Dmax was 0.07/h. By comparison to a similar investigation with rec Zm CP4:pZB5 with a 4% equal mixture of xylose and glucose, we observed that at pH 5.0, the Dmax was 0.064/h and shifted to 0.084/h at pH 5.75. At a level of 0.4% (w/v) acetic acid in the CSL-based medium with 3% xylose and 1.8% glucose at pH 5.75, the Dmax for the adapted recombinant shifted from 0.08 to 0.048/h, and the corresponding maximum volumetric ethanol productivity decreased 45%, from 1.52 to 0.84 g/(L.h). Under these conditions of continuous culture, linear regression of a Pirt plot of the specific rate of sugar utilization vs D showed that 4 g/L of acetic acid did not affect the maximum growth yield (0.030 g dry cell mass/g sugar), but did increase the maintenance coefficient twofold, from 0.46 to 1.0 g of sugar/(g of cell.h).

Comparative energetics of glucose and xylose metabolism in recombinant Zymomonas mobilis

Recombinant Zymomonas mobilis CP4:pZB5 was grown with pH control in batch and continuous modes with either glucose or xylose as the sole carbon and energy source. In batch cultures in which the ratio of the final cell mass concentration to the amount of sugar in the medium was constant (i.e., under conditions that promote "coupled growth"), maximum specific rates of glucose and xylose consumption were 8.5 and 2.1 g/(g of cell.h), respectively; maximum specific rates of ethanol production for glucose and xylose were 4.1 and 1.0 g/(g of cell.h), respectively; and average growth yields from glucose and xylose were 0.055 and 0.034 g of dry cell mass (DCM)/g of sugar, respectively. The corresponding value of YATP for glucose and xylose was 9.9 and 5.1 g of DCM/mol of ATP, respectively. YATP for the wild-type culture CP4 with glucose was 10.4 g of DCM/mol of ATP. For single substrate chemostat cultures in which the growth rate was varied as the dilution rate (D), the maximum or "true" growth yield (max Yx/s) was calculated from Pirt plots as the inverse of the slope of the best-fit linear regression for the specific sugar utilization rate as a function of D, and the "maintenance coefficient" (m) was determined as the y-axis intercept. For xylose, values of max Yx/s and m were 0.0417 g of DCM/g of xylose (YATP = 6.25) and 0.04 g of xylose/(g of cell.h), respectively. However, with glucose there was an observed deviation from linearity, and the data in the Pirt plot was best fit with a second-order polynomial in D. At D > 0.1/h, YATP = 8.71 and m = 2.05 g of glu/(g of cell.h) whereas at D < 0.1/h, YATP = 4.9 g of DCM/mol of ATP and m = 0.04 g of glu/(g of cell.h). This observation provides evidence to question the validity of the unstructured growth model and the assumption that Pirt's maintenance coefficient is a constant that is independent of the growth rate. Collectively, these observations with individual sugars and the values assigned to various growth and fermentation parameters will be useful in the development of models to predict the behavior of rec Zm in mixed substrate fermentations of the type associated with biomass-to-ethanol processes.

Cyanide inhibits respiration yet stimulates aerobic growth of Zymomonas mobilis

Potassium cyanide at submillimolar concentrations (20-500 microM) inhibited the high respiration rates of aerobic cultures of Zymomonas mobilis but, remarkably, stimulated culture growth. In batch culture, after an extended lag phase, exponential growth persisted longer, resulting in higher biomass densities. In aerobic chemostat cultures, elevated biomass concentration was observed in the presence of cyanide. This growth stimulation effect is attributed to decreased production of the inhibitory metabolite acetaldehyde at lowered respiration rates, when more reducing equivalents are channelled to alcohol dehydrogenase. Growth in the presence of cyanide did not alter the membrane cytochrome content. In non-growing cyanide-preincubated cells, with ethanol as the respiratory substrate, cyanide increased ATP levels; in such cells, a large part of the cyanide-sensitive respiration was inhibited within a few seconds after ethanol addition, while inhibition of the rest of respiration took several minutes. The more cyanide-sensitive respiration was apparently energy-nongenerating, and was absent in membrane preparations. Pelleting of membranes from cell-free extracts produced 'soluble' fractions in which a b-type haem was detectable by reduced minus oxidized difference spectroscopy. The function of the Z. mobilis respiratory chain in cell growth and respiratory protection, and the possible physiological role of aerobic generation of inhibitory metabolites, are discussed.

Kinetic and nuclear magnetic resonance studies of xylose metabolism by recombinant Zymomonas mobilis ZM4(pZB5)

The specific rates of growth, substrate utilization, and ethanol production as well as yields of biomass and ethanol production on xylose for the recombinant Zymomonas mobilis ZM4(pZB5) were shown to be much less than those on glucose or glucose-xylose mixtures. Typical fermentations with ZM4(pZB5) growing on glucose-xylose mixtures followed two-phase growth kinetics with the initial uptakes of glucose and xylose being followed by slower growth and metabolic uncoupling on xylose after glucose depletion. The reductions in rates and yields from xylose metabolism were considered in the present investigation and may be due to a number of factors, including the following: (i) the increased metabolic burden from maintenance of plasmid-related functions, (ii) the production of by-products identified as xylitol, acetate, lactate, acetoin, and dihydroxyacetone by (13)C-nuclear magnetic resonance (NMR) spectroscopy and high-performance liquid chromatography, (iii) growth inhibition due to xylitol by the putative inhibitory compound xylitol phosphate, and (iv) the less energized state of ZM4(pZB5). In vivo (31)P-NMR studies have established that the levels of NTP and UDP sugars on xylose were less than those on glucose, and this energy limitation is likely to restrict the growth of the recombinant strain on xylose media.

Bioconversion of glucose and fructose to sorbitol and gluconic acid by untreated cells of Zymomonas mobilis

The bioconversion of glucose and fructose to gluconic acid and sorbitol, respectively, by the enzymes glucose-fructose oxidoreductase (GFOR) and glucono-delta-lactonase (GL), contained in untreated cells of Zymomonas mobilis ATCC 29191, was investigated in batch runs with glucose plus fructose concentrations (S0) varying from 100 to 750 g l-1 in equimolar ratio. When S0 was increased to 650 g l-1, the yields were improved, reaching a maximum of 91% for both products, with productivities of 1.6 and 1.5 g g-1 cell h-1 for gluconic acid and sorbitol, respectively. Above this level (S0 = 750 g l-1), no further improvement in yields was observed and productivities decreased due to the longer process time. The high yields of bioconversion runs with S0 > or = 650 g l-1 are a consequence of the sequential inhibition of the normal metabolism of Z. mobilis by substrates and products, resulting in preferential utilization of substrates via the GFOR/GL system.

A Zymomonas mobilis mutant with delayed growth on high glucose concentrations

Exponentially growing cells of Zymomonas mobilis normally exhibit a lag period of up to 3 h when transferred from 0.11 M (2%) to 0.55 M (10%) glucose liquid medium. A mutant of Z. mobilis (CU1Rif2), fortuitously isolated, showed more than a 20-h lag period when grown under the same conditions, whereas on 0.55 M glucose solid medium, it failed to grow. The growth of CU1Rif2 on elevated concentrations of other fermentable (0.55 M sucrose or fructose) or nonfermentable (0.11 M glucose plus 0.44 M maltose or xylose) sugars appeared to be normal. Surprisingly, CU1Rif2 cells grew without any delay on 0.55 M glucose on which wild-type cells had been incubated for 3 h and removed at the beginning of their exponential phase. This apparent preconditioning was not observed with medium obtained from wild-type cells grown on 0.11 M glucose and supplemented to 0.55 M after removal of the wild-type cells. Undelayed growth of CU1Rif2 on 0.55 M glucose previously conditioned by the wild type was impaired by heating or protease treatment. It is suggested that in Z. mobilis, a diffusible proteinaceous heat-labile factor, transitionally not present in 0.55 M glucose CU1Rif2 cultures, triggers growth on 0.55 M glucose. Biochemical analysis of glucose uptake and glycolytic enzymes implied that glucose assimilation was not directly involved in the phenomenon. By use of a wild-type Z. mobilis genomic library, a 4.5-kb DNA fragment which complemented in low copy number the glucose-defective phenotype as well as glucokinase and glucose uptake of CU1Rif2 was isolated. This fragment carries a gene cluster consisting of four putative coding regions, encoding 167, 167, 145, and 220 amino acids with typical Z. mobilis codon usage, -35 and -10 promoter elements, and individual Shine-Dalgarno consensus sites. However, strong homologies were not detected in a BLAST2 (EMBL-Heidelberg) computer search with known protein sequences.

Metabolic state of Zymomonas mobilis in glucose-, fructose-, and xylose-fed continuous cultures as analysed by 13C- and 31P-NMR spectroscopy

The reasons for the well-known significantly different behaviour of the anaerobic, gram-negative, ethanologenic bacterium Zymomonas mobilis during growth on fructose (i.e. decreased growth and ethanol yields, increased by-product formation) as compared to that on its second natural substrate, glucose, have remained unexplained. A xylose-fermenting recombinant strain of Z. mobilis that was recently constructed in our laboratory also unexpectedly displayed an increased formation of by-products and a strongly reduced growth rate as compared to the parent strain. Therefore, a comprehensive study employing recently developed NMR-based methods for the in vivo analysis of intracellular phosphorylated pool sizes and metabolic fluxes was undertaken to enable a global characterization of the intracellular metabolic state of Z. mobilis during growth on 13C-labelled glucose, fructose and xylose in defined continuous cultures. The 13C-NMR flux analysis indicated that ribose 5-phosphate is synthesized via the nonoxidative pentose phosphate pathway in Z. mobilis, and it identified a metabolic bottleneck in the recombinant xylose-fermenting Z. mobilis strain at the level of heterologous xylulokinase. The 31P-NMR analyses revealed a global alteration of the levels of intracellular phosphorylated metabolites during growth on fructose as compared to that on glucose. The results suggest that this is primarily caused by an elevated concentration of intracellular fructose 6-phosphate.

Membrane D-lactate oxidase in Zymomonas mobilis: evidence for a branched respiratory chain

Respiratory chain composition of the ethanol-producing bacterium Zymomonas mobilis was studied. Its membrane D-lactate oxidase was characterised. With NADH, but not D-lactate as substrate, a cytochrome o-like component was seen in CO difference spectra. Chlorpromazine specifically inhibited reduction of cytochrome d, while myxothiazol eliminated the cytochrome o-like features in CO difference spectra. It is suggested that electrons from NADH are distributed between branches terminated by the cytochrome o-like component, cytochrome a, and cytochrome d. With D-lactate, electrons are transported to cytochrome a, or an unidentified CN(-)-sensitive oxidase, and cytochrome d.

The effect of ethanol and oxygen on the growth of Zymomonas mobilis and the levels of hopanoids and other membrane lipids

Zymomonas mobilis (ATCC 29191) was grown either aerobically or anaerobically in the presence of 2% (wt/vol) glucose and 0, 3, or 6% (vol/vol) ethanol. The rates of growth and the composition of hopanoids, cellular fatty acids, and other lipids in the bacterial membranes were quantitatively analyzed. The bacterium grew in the presence of 3% and 6% ethanol and was more ethanol tolerant when grown anaerobically. In the absence of ethanol, hopanoids comprised about 30% (by mass) of the total cellular lipids. Addition of ethanol to the media caused complex changes in the levels of hopanoids and other lipids. However, there was not a significant increase in any of the hopanoid lipid classes as ethanol concentration was increased. As previously reported, vaccenic acid was the most abundant fatty acid in the lipids of Z. mobilis, and its high constitutive levels were unaffected by the variations in ethanol and oxygen concentrations. A cyclopropane fatty acid accounted for 2.6-6.4 wt % of the total fatty acids in all treatments.

Changes in the growth and enzyme level of Zymomonas mobilis under oxygen-limited conditions at low glucose concentration

Zymomonas mobilis growing aerobically with 20 g glucose-1 (carbon-limited) in a chemostat exhibited an increase in both the molar growth yield (Yx/s) and the maximum molar growth yield (Yx/smax) and a decrease in both the specific substrate consumption rate (qs) and the maintenance energy consumption rate (me). Stepwise increase in the input oxygen partial pressure showed that anaerobic-to-aerobic transitional adaptation occurred in four stages: anaerobic (0 mm HgO2), oxygen-limited (7.6- 230 mm HgO2), intermediate (273 mm HgO2), and oxygen excess (290 mm HgO2). The steady-state biomass concentration, Yx/s, and intracellular ATP content increased between oxygen partial pressures of 7.6 and 120 mm HgO2, accompanied by a decrease in the qs and the specific acid production rate. The membrane ATPase activity decreased with increasing oxygen partial pressure and reached its lowest levels at 273 mm HgO2, which was the highest input oxygen partial pressure where steady-state conditions were possible. Glucokinase, glucose-6-phosphate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, and alcohol dehydrogenase activities also decreased when the oxygen partial pressure was increased above 15 mm Hg, whereas pyruvate decarboxylase was unaffected by aeration. Growth inhibition at 290 mm HgO2 was characterised by a drastic reduction in the pyruvate kinase activity and a collapse in the intracellular ATP pool. The growth and enzyme data suggest that at low glucose concentrations and oxygen-limited conditions, the increase in biomass yields is a reflection of a redirection of ATP usage rather than a net increase in energy production.

Transposon mutagenesis and strain construction in Zymomonas mobilis

Conjugative or mobilizable plasmids carrying the transposable elements Tn5, Tn501 or mini Mu were readily transferred from Escherichia coli donors into Zymomonas mobilis recipients with frequencies depending both on donor and recipient strain used. With the exception of pULB113 (RP4::mini Mu), all foreign plasmids exhibited high instability in Z. mobilis transconjugants under both selective and non-selective conditions. Transposition events and consequent mutagenesis occurred readily in Z. mobilis transconjugant strains, with Tn5 and Tn501 being far less successful than mini Mu. Transposon mutagenesis with the help of mini Mu resulted in the isolation of a large number of independent auxotrophs with polyauxotrophs, cysteine, methionine and isoleucine requiring-isolates being the most frequent. When chromosomal DNA from all these mutants was digested with various restriction enzymes and the resulting restriction patterns were hybridized with a mini Mu probe, the majority of these mutants appeared to have insertions at different sites of the chromosome. Thus, transposon mutagenesis by mini Mu is proven to be a simple and efficient tool for mutant production and the genetic analysis of Z. mobilis.

An elevation of the molar growth yield of Zymomonas mobilis during aerobic exponential growth

Elevated values of molar growth yield (Yx/s = 14-26 g mol-1) were obtained during exponential growth (mu > 0.4 h-1) of Zymomonas mobilis ATCC 29191 by using reduced concentrations of glucose (6. 25-100 mM) and increased oxygen supply (Eh > 300 mV) in the growth medium, as compared to the Yx/s of anaerobic exponential growth (8-10 g mol-1). Aerobically grown cells showed an increased maximum growth rate (mumax), and a reduced specific glucose consumption rate (qs), and specific ethanol formation rate (qp), thus demonstrating a more pronounced energy-coupling growth under oxic conditions. These results can be neither explained by the concept of a solely operating Entner-Doudoroff pathway as an ATP source in aerobically growing cultures of Z. mobilis nor considered to be consistent with existing data on the lack of the Pasteur effect in this bacterium. Therefore, the results rather give evidence for the essential contribution of aerobic ATP generation under the reported conditions.

Expression of the Escherichia coli pmi gene, encoding phosphomannose-isomerase in Zymomonas mobilis, leads to utilization of mannose as a novel growth substrate, which can be used as a selective marker

Wild-type Zymomonas mobilis can utilize only three substrates (sucrose, glucose, and fructose) as sole carbon sources, which are largely converted into ethanol and carbon dioxide. Here, we show that although D-mannose is not used as a growth substrate, it is taken up via the glucose uniport system (glucose facilitator protein) with a Vmax similar to that of glucose. Moreover, D-mannose was phosphorylated by a side activity of the resident fructokinase to mannose-6-phosphate. Fructokinase was purified to homogeneity from an frk-recombinant Z. mobilis strain showing a specific activity of 205 +/- 25 U of protein mg-1 with fructose (K(m), 0.75 +/- 0.06 mM) and 17 +/- 2 U mg-1 (relative activity, 8.5%) with mannose (K(m), 0.65 +/- 0.08 mM). However, no phosphomannoseisomerase activity could be detected for Z. mobilis, and this appeared to be the reason for the lack of growth on mannose. Therefore, we introduced the Escherichia coli gene pmi (manA) in Z. mobilis under the control of a lacIq-Ptac system on a broad-host-range plasmid (pZY507; Cmr). Subsequently, in pmi-recombinant cells of Z. mobilis, phosphomannoseisomerase was expressed in a range of from 3 U (without isopropyl-beta-D-thiogalactopyranoside [IPTG]) to 20 U mg-1 of protein in crude extracts (after IPTG induction). Recombinant cells of different Z. mobilis strains utilized mannose (4%) as the sole carbon source with a growth rate of 0.07 h-1, provided that they contained fructokinase activity. When the frk gene was additionally expressed from the same vector, fructokinase activities of as much as 9.7 U mg-1 and growth rates of as much as 0.25 h-1 were detected, compared with 0.34 h-1 on fructose for wild-type Z. mobilis. Selection for growth on mannose was used to monitor plasmid transfer of pZY507pmi from E. coli to Z. mobilis strains and could replace the previous selection for antibiotic resistance.

Sorbitol promotes growth of Zymomonas mobilis in environments with high concentrations of sugar: evidence for a physiological function of glucose-fructose oxidoreductase in osmoprotection

The gram-negative ethanologenic bacterium Zymomonas mobilis is able to grow in media containing high concentrations of glucose or other sugars. A novel compatible solute for bacteria, sorbitol, which enhances growth of Z. mobilis at glucose concentrations exceeding 0.83 M (15%), is described. Added sorbitol was accumulated intracellularly up to 1 M to counteract high external glucose concentrations (up to 1.66 M or 30%). Accumulation of sorbitol was triggered by a glucose upshift (e.g., from 0.33 to 1.27 M or 6 to 23%) and was prevented by the uncoupler CCCP (carbonyl cyanide m-chlorophenylhydrazone; 100 microM). The sorbitol transport system followed Michaelis-Menten kinetics, with an apparent Km of 34 mM and a Vmax of 11.2 nmol.min-1.mg-1 (dry mass). Sorbitol was produced by the cells themselves and was accumulated when growing on sucrose (1 M or 36%) by the action of the periplasmic enzyme glucose-fructose oxidoreductase, which converts glucose and fructose to gluconolactone and sorbitol. Thus, Z. mobilis can form and accumulate the compatible solute sorbitol from a natural carbon source, sucrose, in order to overcome osmotic stress in high-sugar media. No other major compatible solute (betaine, proline, glutamate, or trehalose) was detected.

Zymomonas mobilis cell viability: measurement method comparison

Comparison of three different cell viability methods: slide count, plate count and methylene blue staining techniques, applied on Zymomonas mobilis cultures, was performed. The slide technique proved to be faster and more accurate than the plate count method, and both of them far more reliable than the standard methylene blue method which constantly overestimated the Zymomonas cell viability. The slide technique is advantageous also because it gives information on the cell morphology changes, notably the abnormal cell elongation, in the ethanol fermentation.

Use of the tac promoter and lacIq for the controlled expression of Zymomonas mobilis fermentative genes in Escherichia coli and Zymomonas mobilis

The Zymomonas mobilis genes encoding alcohol dehydrogenase I (adhA), alcohol dehydrogenase II (adhB), and pyruvate decarboxylase (pdc) were overexpressed in Escherichia coli and Z. mobilis by using a broad-host-range vector containing the tac promoter and the lacIq repressor gene. Maximal IPTG (isopropyl-beta-D-thiogalactopyranoside) induction of these plasmid-borne genes in Z. mobilis resulted in a 35-fold increase in alcohol dehydrogenase I activity, a 16.7-fold increase in alcohol dehydrogenase II activity, and a 6.3-fold increase in pyruvate decarboxylase activity. Small changes in the activities of these enzymes did not affect glycolytic flux in cells which are at maximal metabolic activity, indicating that flux under these conditions is controlled at some other point in metabolism. Expression of adhA, adhB, or pdc at high specific activities (above 8 IU/mg of cell protein) resulted in a decrease in glycolytic flux (negative flux control coefficients), which was most pronounced for pyruvate decarboxylase. Growth rate and flux are imperfectly coupled in this organism. Neither a twofold increase in flux nor a 50% decline from maximal flux caused any immediate change in growth rate. Thus, the rates of biosynthesis and growth in this organism are not limited by energy generation in rich medium.