Enhanced Glutamic Acid Production by a H+-ATPase-Defective Mutant ofCorynebacterium glutamicum

Contributions of the anaplerotic reaction enzymes pyruvate carboxylase and phosphoenolpyruvate carboxylase to l-lysine production in Corynebacterium glutamicum

Anaplerotic reactions catalyzed by pyruvate carboxylase (PC) and phosphoenolpyruvate carboxylase (PEPC) have important roles in the production of l-lysine to replenish oxaloacetic acid (OAA) in Corynebacterium glutamicum. However, the relative contributions of these enzymes to l-lysine production in C. glutamicum are not fully understood. In this study, using a parent strain (P) carrying a feedback inhibition-resistant aspartokinase with the T311I mutation, we constructed a PC gene-deleted mutant strain (PΔPC) and a PEPC gene-deleted mutant strain (PΔPEPC). Although the growth of both mutant strains was comparable to the growth of strain P, the maximum l-lysine production in strains PΔPC and PΔPEPC decreased by 14% and 49%, respectively, indicating that PEPC strongly contributed to OAA supply. l-Lysine production in strain PΔPC slightly decreased during the logarithmic phase, while production during the early stationary phase was comparable to production in strain P. By contrast, strain PΔPEPC produced l-lysine in an amount comparable to the production of strain P during the logarithmic phase; l-lysine production after the early stationary phase was completely stopped in strain PΔPEPC. These results indicate that OAA is supplied by both PC and PEPC during the logarithmic phase, while only PEPC can continuously supply OAA after the logarithmic phase.

Cofactor Metabolic Engineering of Escherichia coli for Aerobic L-Malate Production with Lower CO2 Emissions

Escherichia coli has been engineered for L-malate production via aerobic cultivation. However, the maximum yield obtained through this mode is inferior to that of anaerobic fermentation due to massive amounts of CO2 emissions. Here, we aim to address this issue by reducing CO2 emissions of recombinant E. coli during aerobic L-malate production. Our findings indicated that NADH oxidation and ATP-synthesis-related genes were down-regulated with 2 g/L of YE during aerobic cultivations of E. coli E23, as compared to 5 g/L of YE. Then, E23 was engineered via the knockout of nuoA and the introduction of the nonoxidative glycolysis (NOG) pathway, resulting in a reduction of NAD+ and ATP supplies. The results demonstrate that E23 (ΔnuoA, NOG) exhibited decreased CO2 emissions, and it produced 21.3 g/L of L-malate from glucose aerobically with the improved yield of 0.43 g/g. This study suggests that a restricted NAD+ and ATP supply can prompt E. coli to engage in incomplete oxidization of glucose, leading to the accumulation of metabolites instead of utilizing them in cellular respiration.

H+-Translocating Membrane-Bound Pyrophosphatase from Rhodospirillum rubrum Fuels Escherichia coli Cells via an Alternative Pathway for Energy Generation

Inorganic pyrophosphatases (PPases) catalyze an essential reaction, namely, the hydrolysis of PP_i, which is formed in large quantities as a side product of numerous cellular reactions. In the majority of living species, PP_i hydrolysis is carried out by soluble cytoplasmic PPase (S-PPases) with the released energy dissipated in the form of heat. In Rhodospirillum rubrum, part of this energy can be conserved by proton-pumping pyrophosphatase (H⁺-PPase^Rru) in the form of a proton electrochemical gradient for further ATP synthesis. Here, the codon-harmonized gene hppa^Rru encoding H⁺-PPase^Rru was expressed in the Escherichia coli chromosome. We demonstrate, for the first time, that H⁺-PPase^Rru complements the essential native S-PPase in E. coli cells. ¹³C-MFA confirmed that replacing native PPase to H⁺-PPase^Rru leads to the re-distribution of carbon fluxes; a statistically significant 36% decrease in tricarboxylic acid (TCA) cycle fluxes was found compared with wild-type E. coli MG1655. Such a flux re-distribution can indicate the presence of an additional method for energy generation (e.g., ATP), which can be useful for the microbiological production of a number of compounds, the biosynthesis of which requires the consumption of ATP.

Effect of short-term light irradiation with varying energy densities on the activities of nitrifiers in wastewater

Microalgal-bacterial consortium (MBC) process has been proposed as an alternative to conventional activated sludge process for nitrogen removal from wastewater. As one of the most influencing parameters, light irradiation effects on microalgae have been extensively investigated. However, light influence on the performance of nitrifiers in activated sludge and its mechanism remains unclear. In this study, the effects of three factors (light irradiation power, irradiation time and sludge concentration) on activities and physiological characteristics of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) were systematically studied through both the Design of Experiments driven response surface methodology (RSM) approach and light-nitrification kinetic modeling. Results indicated that light irradiation with the specific light energy density (Es) at 0.0203-0.1571 kJ·mg^-1 VSS (80-160 W/400-1000 μmol·m^-2·s^-1, 2.0-5.0 h and 2750-4250 mg·L^-1) stimulated the relative AOB activities (rAOB) by 120.0%. This was supported by the increased electron transport system activity, key enzyme activity (AMO) , gene expression (amoA) and energy generation (ATP consumption) in the light treatment. Moreover, further Es increasing up to 0.18 kJ·mg^-1 VSS inhibited both AOB and NOB activities. The inhibition was ascribed to the joint light responses of metabolic disorders and lipid peroxidation. The findings enhance our understanding of nitrifiers' physiological responses to short-term light irradiation, and promote the development of MBC as a sustainable approach for wastewater treatment.

Regulation of glycolytic flux and overflow metabolism depending on the source of energy generation for energy demand

Overflow metabolism is a common phenomenon observed at higher glycolytic flux in many bacteria, yeast (known as Crabtree effect), and mammalian cells including cancer cells (known as Warburg effect). This phenomenon has recently been characterized as the trade-offs between protein costs and enzyme efficiencies based on coarse-graining approaches. Moreover, it has been recognized that the glycolytic flux increases as the source of energy generation changes from energetically efficient respiration to inefficient respiro-fermentative or fermentative metabolism causing overflow metabolism. It is highly desired to clarify the metabolic regulation mechanisms behind such phenomena. Metabolic fluxes are located on top of the hierarchical regulation systems, and represent the outcome of the integrated response of all levels of cellular regulation systems. In the present article, we discuss about the different levels of regulation systems for the modulation of fluxes depending on the growth rate, growth condition such as oxygen limitation that alters the metabolism towards fermentation, and genetic perturbation affecting the source of energy generation from respiration to respiro-fermentative metabolism in relation to overflow metabolism. The intracellular metabolite of the upper glycolysis such as fructose 1,6-bisphosphate (FBP) plays an important role not only for flux sensing, but also for the regulation of the respiratory activity either directly or indirectly (via transcription factors) at higher growth rate. The glycolytic flux regulation is backed up (enhanced) by unphosphorylated EIIA and HPr of the phosphotransferase system (PTS) components, together with the sugar-phosphate stress regulation, where the transcriptional regulation is further modulated by post-transcriptional regulation via the degradation of mRNA (stability of mRNA) in Escherichia coli. Moreover, the channeling may also play some role in modulating the glycolytic cascade reactions.

Current advance in bioconversion of methanol to chemicals

Methanol has become an attractive substrate for biotechnological applications due to its abundance and low-price. Chemicals production from methanol could alleviate the environmental concerns, costs, and foreign dependency associated with the use of petroleum feedstock. Recently, a growing fraction of research has focused on metabolites production using methanol as sole carbon and energy source or as co-substrate with carbohydrates by native or synthetic methylotrophs. In this review, we summarized the recent significant progress in native and synthetic methylotrophs and their application for methanol bioconversion into various products. Moreover, strategies for improvement of methanol metabolism and new perspectives on the generation of desired products from methanol were also discussed, which will benefit for the development of a methanol-based economy.

NADPH metabolism: a survey of its theoretical characteristics and manipulation strategies in amino acid biosynthesis

Reduced nicotinamide adenine nucleotide phosphate (NADPH), which is one of the key cofactors in the metabolic network, plays an important role in the biochemical reactions, and physiological function of amino acid-producing strains. The manipulation of NADPH availability and form is an efficient and easy method of redirecting the carbon flux to the amino acid biosynthesis in industrial strains. In this review, we survey the metabolic mode of NADPH. Furthermore, we summarize the research developments in the understanding of the relationship between NADPH metabolism and amino acid biosynthesis. Detailed strategies to manipulate NADPH availability are addressed based on this knowledge. Finally, the uses of NADPH manipulation strategies to enhance the metabolic function of amino acid-producing strains are discussed.

Cofactor recycling for co-production of 1,3-propanediol and glutamate by metabolically engineered Corynebacterium glutamicum

Production of 1,3-propanediol (1,3-PDO) from glycerol is a promising route toward glycerol biorefinery. However, the yield of 1,3-PDO is limited due to the requirement of NADH regeneration via glycerol oxidation process, which generates large amounts of undesired byproducts. Glutamate fermentation by Corynebacterium glutamicum is an important oxidation process generating excess NADH. In this study, we proposed a novel strategy to couple the process of 1,3-PDO synthesis with glutamate production for cofactor regeneration. With the optimization of 1,3-PDO synthesis route, C. glutamicum can efficiently convert glycerol into 1,3-PDO with the yield of ~ 1.0 mol/mol glycerol. Co-production of 1,3-PDO and glutamate was also achieved which increased the yield of glutamate by 18% as compared to the control. Since 1,3-PDO and glutamate can be easily separated in downstream process, this study provides a potential green route for coupled production of 1,3-PDO and glutamate to enhance the economic viability of biorefinery process.

Enhanced Glucose Consumption and Organic Acid Production by Engineered Corynebacterium glutamicum Based on Analysis of a pfkB1 Deletion Mutant

In the analysis of a carbohydrate metabolite pathway, we found interesting phenotypes in a mutant strain of Corynebacterium glutamicum deficient in pfkB1, which encodes fructose-1-phosphate kinase. After being aerobically cultivated with fructose as a carbon source, this mutant consumed glucose and produced organic acid, predominantly l-lactate, at a level more than 2-fold higher than that of the wild-type grown with glucose under conditions of oxygen deprivation. This considerably higher fermentation capacity was unique for the combination of pfkB1 deletion and cultivation with fructose. In the metabolome and transcriptome analyses of this strain, marked intracellular accumulation of fructose-1-phosphate and significant upregulation of several genes related to the phosphoenolpyruvate:carbohydrate phosphotransferase system, glycolysis, and organic acid synthesis were identified. We then examined strains overexpressing several of the identified genes and demonstrated enhanced glucose consumption and organic acid production by these engineered strains, whose values were found to be comparable to those of the model pfkB1 deletion mutant grown with fructose. l-Lactate production by the ppc deletion mutant of the engineered strain was 2,390 mM (i.e., 215 g/liter) after 48 h under oxygen deprivation, which was a 2.7-fold increase over that of the wild-type strain with a deletion of ppc IMPORTANCE: Enhancement of glycolytic flux is important for improving microbiological production of chemicals, but overexpression of glycolytic enzymes has often resulted in little positive effect. That is presumably because the central carbon metabolism is under the complex and strict regulation not only transcriptionally but also posttranscriptionally, for example, by the ATP/ADP ratio. In contrast, we studied a mutant strain of Corynebacterium glutamicum that showed markedly enhanced glucose consumption and organic acid production and, based on the findings, identified several genes whose overexpression was effective in enhancing glycolytic flux under conditions of oxygen deprivation. These results will further understanding of the regulatory mechanisms of glycolytic flux and can be widely applied to the improvement of the microbial production of useful chemicals.

Branched-Chain Amino Acids

Branched-chain amino acids (BCAAs), viz., L-isoleucine, L-leucine, and L-valine, are essential amino acids that cannot be synthesized in higher organisms and are important nutrition for humans as well as livestock. They are also valued as synthetic intermediates for pharmaceuticals. Therefore, the demand for BCAAs in the feed and pharmaceutical industries is increasing continuously. Traditional industrial fermentative production of BCAAs was performed using microorganisms isolated by random mutagenesis. A collection of these classical strains was also scientifically useful to clarify the details of the BCAA biosynthetic pathways, which are tightly regulated by feedback inhibition and transcriptional attenuation. Based on this understanding of the metabolism of BCAAs, it is now possible for us to pursue strains with higher BCAA productivity using rational design and advanced molecular biology techniques. Additionally, systems biology approaches using augmented omics information help us to optimize carbon flux toward BCAA production. Here, we describe the biosynthetic pathways of BCAAs and their regulation and then overview the microorganisms developed for BCAA production. Other chemicals, including isobutanol, i.e., a second-generation biofuel, can be synthesized by branching the BCAA biosynthetic pathways, which are also outlined.

Control of glycolytic flux in directed biosynthesis of uridine-phosphoryl compounds through the manipulation of ATP availability

Adenosine triphosphate (ATP), the most important energy source for metabolic reactions and pathways, plays a vital role in control of metabolic flux. Considering the importance of ATP in regulation of the glycolytic pathway, the use of ATP-oriented manipulation is a rational and efficient route to regulate metabolic flux. In this paper, a series of efficient ATP-oriented regulation methods, such as changing ambient temperature and altering reduced nicotinamide adenine dinucleotide (NADH), was developed. To satisfy the different demand for ATP at different phases in directed biosynthesis of uridine-phosphoryl compounds, a multiphase ATP supply regulation strategy was also used to enhance to yield of target metabolites.

Platform engineering of Corynebacterium glutamicum with reduced pyruvate dehydrogenase complex activity for improved production of L-lysine, L-valine, and 2-ketoisovalerate

Exchange of the native Corynebacterium glutamicum promoter of the aceE gene, encoding the E1p subunit of the pyruvate dehydrogenase complex (PDHC), with mutated dapA promoter variants led to a series of C. glutamicum strains with gradually reduced growth rates and PDHC activities. Upon overexpression of the l-valine biosynthetic genes ilvBNCE, all strains produced l-valine. Among these strains, C. glutamicum aceE A16 (pJC4 ilvBNCE) showed the highest biomass and product yields, and thus it was further improved by additional deletion of the pqo and ppc genes, encoding pyruvate:quinone oxidoreductase and phosphoenolpyruvate carboxylase, respectively. In fed-batch fermentations at high cell densities, C. glutamicum aceE A16 Δpqo Δppc (pJC4 ilvBNCE) produced up to 738 mM (i.e., 86.5 g/liter) l-valine with an overall yield (YP/S) of 0.36 mol per mol of glucose and a volumetric productivity (QP) of 13.6 mM per h [1.6 g/(liter × h)]. Additional inactivation of the transaminase B gene (ilvE) and overexpression of ilvBNCD instead of ilvBNCE transformed the l-valine-producing strain into a 2-ketoisovalerate producer, excreting up to 303 mM (35 g/liter) 2-ketoisovalerate with a YP/S of 0.24 mol per mol of glucose and a QP of 6.9 mM per h [0.8 g/(liter × h)]. The replacement of the aceE promoter by the dapA-A16 promoter in the two C. glutamicum l-lysine producers DM1800 and DM1933 improved the production by 100% and 44%, respectively. These results demonstrate that C. glutamicum strains with reduced PDHC activity are an excellent platform for the production of pyruvate-derived products.

Mechanism of increased respiration in an H+-ATPase-defective mutant of Corynebacterium glutamicum

We previously reported that a spontaneous H(+)-ATPase-defective mutant of Corynebacterium glutamicum, F172-8, derived from C. glutamicum ATCC 14067, showed enhanced glucose consumption and respiration rates. To investigate the genome-based mechanism of enhanced respiration rate in such C. glutamicum mutants, A-1, an H(+)-ATPase-defective mutant derived from C. glutamicum ATCC 13032, which harbors the same point mutation as F172-8, was used in this study. A-1 showed similar fermentation profiles to F172-8 when cultured in a jar fermentor. Enzyme activity measurements, quantitative real-time PCR, and DNA microarray analysis suggested that A-1 enhanced malate:quinone oxidoreductase/malate dehydrogenase and l-lactate dehydrogenase/NAD(+)-dependent-lactate dehydrogenase coupling reactions, but not NADH dehydrogenase-II, for reoxidation of the excess NADH arising from enhanced glucose consumption. A-1 also up-regulated succinate dehydrogenase, which may result in the relief of excess proton-motive force (pmf) in the H(+)-ATPase mutant. In addition, the transcriptional level of cytochrome bd oxidase, but not cytochrome bc(1)-aa(3), also increased, which may help prevent the excess pmf generation caused by enhanced respiration. These results indicate that C. glutamicum possesses intriguing strategies for coping with NADH over-accumulation. Furthermore, these mechanisms are different from those in Escherichia coli, even though the two species use similar strategies to prevent excess pmf generation.

Physiology and global gene expression of a Corynebacterium glutamicum ΔF(1)F(O)-ATP synthase mutant devoid of oxidative phosphorylation

A mutant of Corynebacterium glutamicum ATCC 13032 with a deletion of the atpBEFHAGDC genes encoding F(1)F(O)-ATP synthase was characterized. Whereas no growth was observed with acetate as sole carbon source, the ΔF(1)F(O) mutant reached 47% of the growth rate and 65% of the biomass of the wild type during shake-flask cultivation in glucose minimal medium. Initially, the mutant strain showed a strongly increased glucose uptake rate accompanied by a high oxygen consumption rate and pyruvate secretion into the medium. When oxygen became limiting, the glucose consumption rate was reduced below that of the wild type and pyruvate was consumed again. The ΔF(1)F(O) mutant had increased levels of b- and d-type cytochromes and a significantly increased proton motive force. Transcription of genes involved in central carbon metabolism was essentially unchanged, whereas genes for cytochrome bd oxidase, pyruvate:quinone oxidoreductase, oxidative stress response, and others showed increased mRNA levels. On the other hand, genes for amino acid biosynthesis and ribosomal proteins as well as many genes involved in transport displayed decreased mRNA levels. Several of the transcriptional changes were reflected at the protein level, but there were also discrepancies between the mRNA and protein levels suggesting some kind of posttranscriptional regulation. The results prove for the first time that F(1)F(O)-ATP synthase and oxidative phosphorylation are in general not essential for growth of C. glutamicum.

Development and experimental verification of a genome-scale metabolic model for Corynebacterium glutamicum

BACKGROUND

In silico genome-scale metabolic models enable the analysis of the characteristics of metabolic systems of organisms. In this study, we reconstructed a genome-scale metabolic model of Corynebacterium glutamicum on the basis of genome sequence annotation and physiological data. The metabolic characteristics were analyzed using flux balance analysis (FBA), and the results of FBA were validated using data from culture experiments performed at different oxygen uptake rates.

RESULTS

The reconstructed genome-scale metabolic model of C. glutamicum contains 502 reactions and 423 metabolites. We collected the reactions and biomass components from the database and literatures, and made the model available for the flux balance analysis by filling gaps in the reaction networks and removing inadequate loop reactions. Using the framework of FBA and our genome-scale metabolic model, we first simulated the changes in the metabolic flux profiles that occur on changing the oxygen uptake rate. The predicted production yields of carbon dioxide and organic acids agreed well with the experimental data. The metabolic profiles of amino acid production phases were also investigated. A comprehensive gene deletion study was performed in which the effects of gene deletions on metabolic fluxes were simulated; this helped in the identification of several genes whose deletion resulted in an improvement in organic acid production.

CONCLUSION

The genome-scale metabolic model provides useful information for the evaluation of the metabolic capabilities and prediction of the metabolic characteristics of C. glutamicum. This can form a basis for the in silico design of C. glutamicum metabolic networks for improved bioproduction of desirable metabolites.

ATP in current biotechnology: regulation, applications and perspectives

Adenosine tri-phosphate (ATP), the most important energy source for metabolic reactions and pathways, plays a vital role in the growth of industrial strain and the production of target metabolites. In this review, current advances in manipulating ATP in industrial strains, including altering NADH availability, and regulating NADH oxidation pathway, oxygen supply, proton gradient, the electron transfer chain activity and the F(0)F(1)-ATPase activity, are summarized and discussed. By applying these strategies, optimal product concentrations, yields and productivity in industrial biotechnology have been achieved. Furthermore, the mechanisms by which ATP extends the substrate utilization spectra and enhances the ability to challenge harsh environmental stress have been elucidated. Finally, three critical issues related to ATP manipulation have been addressed.

Changes in enzyme activities at the pyruvate node in glutamate-overproducing Corynebacterium glutamicum

Glutamate is industrially produced by fermentation using Corynebacterium glutamicum. The key factor for efficient glutamate production by this microorganism has been considered to be a metabolic change at the 2-oxoglutarate dehydrogenase (ODH) branch point caused by a decrease in ODH activity under glutamate-overproducing conditions. However, this change would be insufficient because the ODH branch is merely the final branch in the glutamate biosynthetic pathway, and efficient glutamate production requires a balanced supply of acetyl-CoA and oxaloacetate (OAA), which are condensed to form a precursor of glutamate, namely, citrate. Therefore, there must be another (other) change(s) in metabolic flux. In this study, we demonstrated that a decrease in pyruvate dehydrogenase (PDH) activity catalyzes the conversion of pyruvate to acetyl-CoA. It is speculated that carbon flux from pyruvate to acetyl-CoA decreases under glutamate-overproducing conditions. Furthermore, an increase in pyruvate carboxylase (PC) activity, which catalyzes the reaction of pyruvate to OAA, is evident under glutamate-overproducing conditions, except under biotin-limited condition, which may lead to an increase in carbon flux from pyruvate to OAA. These data suggest that a novel metabolic change occurs at the pyruvate node, leading to a high yield of glutamate through adequate partitioning of the carbon flux.

Cytoplasmic proteome reference map for a glutamic acid-producing Corynebacterium glutamicum ATCC 14067

We constructed a cytoplasmic proteome reference map for a glutamic acid producing Corynebacterium glutamicum ATCC 14067 by 2-DE and protein identification by MALDI-TOF-MS and PMF using genome database of the type strain ATCC 13032. The map allowed us to identify 166 protein spots representing 139 different proteins. A considerable strain difference was observed in the proteomic images between strains ATCC 14067 and ATCC 13032 grown under the glutamic acid production conditions, suggesting the importance of strain-specific reference map for proteomic analysis.

A comparative proteomic approach to understand the adaptations of an H+ -ATPase-defective mutant of Corynebacterium glutamicum ATCC14067 to energy deficiencies

F172-8, an H(+)-ATPase-defective mutant of the glutamic acid-producing bacterium Corynebacterium glutamicum ATCC 14067, exhibits enhanced rates of glucose consumption and respiration compared to the parental strain when cultured in a biotin-rich medium with glucose as the carbon source. We conducted a comparative proteomic analysis to clarify the mechanism by which the enhanced glucose metabolism in this mutant is established using a proteome reference map for strain ATCC 14067. A comparison of the proteomes of the two strains revealed the up-regulated expression of the several important enzymes such as pyruvate kinase (Pyk), malate:quinone oxidoreductase (Mqo), and malate dehydrogenase (Mdh) in the mutant. Because Pyk activates glycolysis in response to cellular energy shortages in this bacterium, its increased expression may contribute to the enhanced glucose metabolism of the mutant. A unique reoxidation system has been suggested for NADH in C. glutamicum consisting of coupled reactions between Mqo and Mdh, together with the respiratory chain; therefore, the enhanced expression of both enzymes might contribute to the reoxidation of NADH during increased respiration. The proteomic analysis allowed the identification of unique physiological changes associated with the H(+)-ATPase defect in F172-8 and contributed to the understanding of the adaptations of C. glutamicum to energy deficiencies.

Alterations of cellular physiology in Escherichia coli in response to oxidative phosphorylation impaired by defective F1-ATPase

The physiological changes in an F1-ATPase-defective mutant of Escherichia coli W1485 growing in a glucose-limited chemostat included a decreased growth yield (60%) and increased specific rates of both glucose consumption (168%) and respiration (171%). Flux analysis revealed that the mutant showed approximately twice as much flow in glycolysis but only an 18% increase in the tricarboxylic acid (TCA) cycle, owing to the excretion of acetate, where most of the increased glycolytic flux was directed. Genetic and biochemical analyses of the mutant revealed the downregulation of many TCA cycle enzymes, including citrate synthase, and the upregulation of the pyruvate dehydrogenase complex in both transcription and enzyme activities. These changes seemed to contribute to acetate excretion in the mutant. No transcriptional changes were observed in the glycolytic enzymes, despite the enhanced glycolysis. The most significant alterations were found in the respiratory-chain components. The total activity of NADH dehydrogenases (NDHs) and terminal oxidases increased about twofold in the mutant, which accounted for its higher respiration rate. These changes arose primarily from the increased (3.7-fold) enzyme activity of NDH-2 and an increased amount of cytochrome bd in the mutant. Transcriptional upregulation appeared to be involved in these phenomena. As NDH-2 cannot generate an electrochemical gradient of protons and as cytochrome bd is inferior to cytochrome bo3 in this ability, the mutant was able to recycle NADH at a higher rate than the parent and avoid generating an excess proton-motive force. We discuss the physiological benefits of the alterations in the mutant.