Functional citric acid cycle in an arcA mutant of Escherichia coli during growth with nitrate under anoxic conditions

Enterohaemorrhagic E. coli utilizes host- and microbiota-derived L-malate as a signaling molecule for intestinal colonization

The mammalian gastrointestinal tract is a complex environment that hosts a diverse microbial community. To establish infection, bacterial pathogens must be able to compete with the indigenous microbiota for nutrients, as well as sense the host environment and modulate the expression of genes essential for colonization and virulence. Here, we found that enterohemorrhagic Escherichia coli (EHEC) O157:H7 imports host- and microbiota-derived L-malate using the DcuABC transporters and converts these substrates into fumarate to fuel anaerobic fumarate respiration during infection, thereby promoting its colonization of the host intestine. Moreover, L-malate is important not only for nutrient metabolism but also as a signaling molecule that activates virulence gene expression in EHEC O157:H7. The complete virulence-regulating pathway was elucidated; the DcuS/DcuR two-component system senses high L-malate levels and transduces the signal to the master virulence regulator Ler, which in turn activates locus of enterocyte effacement (LEE) genes to promote EHEC O157:H7 adherence to epithelial cells of the large intestine. Disruption of this virulence-regulating pathway by deleting either dcuS or dcuR significantly reduced colonization by EHEC O157:H7 in the infant rabbit intestinal tract; therefore, targeting these genes and altering physiological aspects of the intestinal environment may offer alternatives for EHEC infection treatment.

An Oxidative Central Metabolism Enables Salmonella to Utilize Microbiota-Derived Succinate

The mucosal inflammatory response induced by Salmonella serovar Typhimurium creates a favorable niche for this gut pathogen. Conventional wisdom holds that S. Typhimurium undergoes an incomplete tricarboxylic acid (TCA) cycle in the anaerobic mammalian gut. One change during S. Typhimurium-induced inflammation is the production of oxidized compounds by infiltrating neutrophils. We show that inflammation-derived electron acceptors induce a complete, oxidative TCA cycle in S. Typhimurium, allowing the bacteria to compete with the microbiota for colonization. A complete TCA cycle facilitates utilization of the microbiota-derived fermentation product succinate as a carbon source. S. Typhimurium succinate utilization genes contribute to efficient colonization in conventionally raised mice, but provide no growth advantage in germ-free mice. Mono-association of gnotobiotic mice with Bacteroides, a major succinate producer, restores succinate utilization in S. Typhimurium. Thus, oxidative central metabolism enables S. Typhimurium to utilize a variety of carbon sources, including microbiota-derived succinate.

C4-Dicarboxylate Utilization in Aerobic and Anaerobic Growth

C4-dicarboxylates and the C4-dicarboxylic amino acid l-aspartate support aerobic and anaerobic growth of Escherichia coli and related bacteria. In aerobic growth, succinate, fumarate, D- and L-malate, L-aspartate, and L-tartrate are metabolized by the citric acid cycle and associated reactions. Because of the interruption of the citric acid cycle under anaerobic conditions, anaerobic metabolism of C4-dicarboxylates depends on fumarate reduction to succinate (fumarate respiration). In some related bacteria (e.g., Klebsiella), utilization of C4-dicarboxylates, such as tartrate, is independent of fumarate respiration and uses a Na+-dependent membrane-bound oxaloacetate decarboxylase. Uptake of the C4-dicarboxylates into the bacteria (and anaerobic export of succinate) is achieved under aerobic and anaerobic conditions by different sets of secondary transporters. Expression of the genes for C4-dicarboxylate metabolism is induced in the presence of external C4-dicarboxylates by the membrane-bound DcuS-DcuR two-component system. Noncommon C4-dicarboxylates like l-tartrate or D-malate are perceived by cytoplasmic one-component sensors/transcriptional regulators. This article describes the pathways of aerobic and anaerobic C4-dicarboxylate metabolism and their regulation. The citric acid cycle, fumarate respiration, and fumarate reductase are covered in other articles and discussed here only in the context of C4-dicarboxylate metabolism. Recent aspects of C4-dicarboxylate metabolism like transport, sensing, and regulation will be treated in more detail. This article is an updated version of an article published in 2004 in EcoSal Plus. The update includes new literature, but, in particular, the sections on the metabolism of noncommon C4-dicarboxylates and their regulation, on the DcuS-DcuR regulatory system, and on succinate production by engineered E. coli are largely revised or new.

Biotin and Lipoic Acid: Synthesis, Attachment, and Regulation

Two vitamins, biotin and lipoic acid, are essential in all three domains of life. Both coenzymes function only when covalently attached to key metabolic enzymes. There they act as "swinging arms" that shuttle intermediates between two active sites (= covalent substrate channeling) of key metabolic enzymes. Although biotin was discovered over 100 years ago and lipoic acid 60 years ago, it was not known how either coenzyme is made until recently. In Escherichia coli the synthetic pathways for both coenzymes have now been worked out for the first time. The late steps of biotin synthesis, those involved in assembling the fused rings, were well described biochemically years ago, although recent progress has been made on the BioB reaction, the last step of the pathway in which the biotin sulfur moiety is inserted. In contrast, the early steps of biotin synthesis, assembly of the fatty acid-like "arm" of biotin were unknown. It has now been demonstrated that the arm is made by using disguised substrates to gain entry into the fatty acid synthesis pathway followed by removal of the disguise when the proper chain length is attained. The BioC methyltransferase is responsible for introducing the disguise, and the BioH esterase is responsible for its removal. In contrast to biotin, which is attached to its cognate proteins as a finished molecule, lipoic acid is assembled on its cognate proteins. An octanoyl moiety is transferred from the octanoyl acyl carrier protein of fatty acid synthesis to a specific lysine residue of a cognate protein by the LipB octanoyltransferase followed by sulfur insertion at carbons C-6 and C-8 by the LipA lipoyl synthetase. Assembly on the cognate proteins regulates the amount of lipoic acid synthesized, and, thus, there is no transcriptional control of the synthetic genes. In contrast, transcriptional control of the biotin synthetic genes is wielded by a remarkably sophisticated, yet simple, system, exerted through BirA, a dual-function protein that both represses biotin operon transcription and ligates biotin to its cognate proteins.

Genome-wide analysis of the response to nitric oxide in uropathogenic Escherichia coli CFT073

Uropathogenic Escherchia coli (UPEC) is the causative agent of urinary tract infections. Nitric oxide (NO) is a toxic water-soluble gas that is encountered by UPEC in the urinary tract. Therefore, UPEC probably requires mechanisms to detoxify NO in the host environment. Thus far, flavohaemoglobin (Hmp), an NO denitrosylase, is the only demonstrated NO detoxification system in UPEC. Here we show that, in E. coli strain CFT073, the NADH-dependent NO reductase flavorubredoxin (FlRd) also plays a major role in NO scavenging. We generated a mutant that lacks all known and candidate NO detoxification pathways (Hmp, FlRd and the respiratory nitrite reductase, NrfA). When grown and assayed anaerobically, this mutant expresses an NO-inducible NO scavenging activity, pointing to the existence of a novel detoxification mechanism. Expression of this activity is inducible by both NO and nitrate, and the enzyme is membrane-associated. Genome-wide transcriptional profiling of UPEC grown under anaerobic conditions in the presence of nitrate (as a source of NO) highlighted various aspects of the response of the pathogen to nitrate and NO. Several virulence-associated genes are upregulated, suggesting that host-derived NO is a potential regulator of UPEC virulence. Chromatin immunoprecipitation and sequencing was used to evaluate the NsrR regulon in CFT073. We identified 49 NsrR binding sites in promoter regions in the CFT073 genome, 29 of which were not previously identified in E. coli K-12. NsrR may regulate some CFT073 genes that do not have homologues in E. coli K-12.

Tricarboxylic Acid Cycle and Glyoxylate Bypass

The tricarboxylic acid (TCA) cycle plays two essential roles in metabolism. First, under aerobic conditions the cycle is responsible for the total oxidation of acetyl-CoA that is derived mainly from the pyruvate produced by glycolysis. Second, TCA cycle intermediates are required in the biosynthesis of several amino acids. Although the TCA cycle has long been considered a "housekeeping" pathway in Escherichia coli and Salmonella enterica, the pathway is highly regulated at the transcriptional level. Much of this control is exerted in response to respiratory conditions. The TCA cycle gene-protein relationship and mutant phenotypes have been well studied, although a few loose ends remain. The realization that a "shadow" TCA cycle exists that proceeds through methylcitrate has cleared up prior ambiguities. The glyoxylate bypass has long been known to be essential for growth on carbon sources such as acetate or fatty acids because this pathway allowsnet conversion of acetyl-CoA to metabolic intermediates. Strains lacking this pathway fail to grow on these carbon sources, since acetate carbon entering the TCA cycle is quantitatively lost as CO2 resulting in the lack of a means to replenish the dicarboxylic acids consumed in amino acid biosynthesis. The TCA cycle gene-protein relationship and mutant phenotypes have been well studied, although the identity of the small molecule ligand that modulates transcriptional control of the glyoxylate cycle genes by binding to the IclR repressor remains unknown. The activity of the cycle is also exerted at the enzyme level by the reversible phosphorylation of the TCA cycle enzyme isocitrate dehydrogenase catalyzed by a specific kinase/phosphatase to allow isocitratelyase to compete for isocitrate and cleave this intermediate to glyoxylate and succinate.

Biotin and Lipoic Acid: Synthesis, Attachment, and Regulation

Two vitamins, biotin and lipoic acid, are essential in all three domains of life. Both coenzymes function only when covalently attached to key metabolic enzymes. There they act as "swinging arms" that shuttle intermediates between two active sites (= covalent substrate channeling) of key metabolic enzymes. Although biotin was discovered over 100 years ago and lipoic acid was discovered 60 years ago, it was not known how either coenzyme is made until recently. In Escherichia coli the synthetic pathways for both coenzymes have now been worked out for the first time. The late steps of biotin synthesis, those involved in assembling the fused rings, were well described biochemically years ago, although recent progress has been made on the BioB reaction, the last step of the pathway, in which the biotin sulfur moiety is inserted. In contrast, the early steps of biotin synthesis, assembly of the fatty acid-like "arm" of biotin, were unknown. It has now been demonstrated that the arm is made by using disguised substrates to gain entry into the fatty acid synthesis pathway followed by removal of the disguise when the proper chain length is attained. The BioC methyltransferase is responsible for introducing the disguise and the BioH esterase for its removal. In contrast to biotin, which is attached to its cognate proteins as a finished molecule, lipoic acid is assembled on its cognate proteins. An octanoyl moiety is transferred from the octanoyl-ACP of fatty acid synthesis to a specific lysine residue of a cognate protein by the LipB octanoyl transferase, followed by sulfur insertion at carbons C6 and C8 by the LipA lipoyl synthetase. Assembly on the cognate proteins regulates the amount of lipoic acid synthesized, and thus there is no transcriptional control of the synthetic genes. In contrast, transcriptional control of the biotin synthetic genes is wielded by a remarkably sophisticated, yet simple, system exerted through BirA, a dual-function protein that both represses biotin operon transcription and ligates biotin to its cognate protein.

C4-Dicarboxylate Degradation in Aerobic and Anaerobic Growth

C4-dicarboxylates, like succinate, fumarate, L- and D-malate, tartrate, and the C4-dicarboxylic amino acid aspartate, support aerobic and anaerobic growth of Escherichia coli and related bacteria and can serve as carbon and energy sources. In aerobic growth, the C4-dicarboxylates are oxidized in the citric acid cycle. Due to the interruption of the citric acid cycle under anaerobic conditions, anaerobic metabolism of the C4-dicarboxylates depends on fumarate reduction to succinate. In some related bacteria (e.g., Klebsiella), degradation of C4-dicarboxylates, like tartrate, uses a different mechanism and pathway. It requires the functioning of an Na+-dependent and membrane-associated oxaloacetate decarboxylase. Due to the incomplete function of the citric acid cycle in anaerobic growth, succinate supports only aerobic growth of E. coli. This chapter describes the pathways of and differences in aerobic and anaerobic C4-dicarboxylate metabolism and the physiological consequences. The citric acid cycle, fumarate respiration, and fumarate reductase are discussed here only in the context of aerobic and anaerobic C4-dicarboxylate metabolism. Some recent aspects of C4-dicarboxylate metabolism, such as transport and sensing of C4-dicarboxylates, and their relationships are treated in more detail.

Metabolic Engineering of Escherichia coli for Production of Mixed-Acid Fermentation End Products

Mixed-acid fermentation end products have numerous applications in biotechnology. This is probably the main driving force for the development of multiple strains that are supposed to produce individual end products with high yields. The process of engineering Escherichia coli strains for applied production of ethanol, lactate, succinate, or acetate was initiated several decades ago and is still ongoing. This review follows the path of strain development from the general characteristics of aerobic versus anaerobic metabolism over the regulatory machinery that enables the different metabolic routes. Thereafter, major improvements for broadening the substrate spectrum of E. coli toward cheap carbon sources like molasses or lignocellulose are highlighted before major routes of strain development for the production of ethanol, acetate, lactate, and succinate are presented.

Metabolic regulation analysis of wild-type and arcA mutant Escherichia coli under nitrate conditions using different levels of omics data

It is of practical interest to investigate the effect of nitrates on bacterial metabolic regulation of both fermentation and energy generation, as compared to aerobic and anaerobic growth without nitrates. Although gene level regulation has previously been studied for nitrate assimilation, it is important to understand this metabolic regulation in terms of global regulators. In the present study, therefore, we measured gene expression using DNA microarrays, intracellular metabolite concentrations using CE-TOFMS, and metabolic fluxes using the (13)C-labeling technique for wild-type E. coli and the ΔarcA (a global regulatory gene for anoxic response control, ArcA) mutant to compare the metabolic state under nitrate conditions to that under aerobic and anaerobic conditions without nitrates in continuous culture conditions at a dilution rate of 0.2 h(-1). In wild-type, although the measured metabolite concentrations changed very little among the three culture conditions, the TCA cycle and the pentose phosphate pathway fluxes were significantly different under each condition. These results suggested that the ATP production rate was 29% higher under nitrate conditions than that under anaerobic conditions, whereas the ATP production rate was 10% lower than that under aerobic conditions. The flux changes in the TCA cycle were caused by changes in control at the gene expression level. In ΔarcA mutant, the TCA cycle flux was significantly increased (4.4 times higher than that of the wild type) under nitrate conditions. Similarly, the intracellular ATP/ADP ratio increased approximately two-fold compared to that of the wild-type strain.

Dissimilatory iron reduction in Escherichia coli: identification of CymA of Shewanella oneidensis and NapC of E. coli as ferric reductases

Over geological time scales, microbial reduction of chelated Fe(III) or Fe(III) minerals has profoundly affected today's composition of our bio- and geosphere. However, the electron transfer reactions that are specific and defining for dissimilatory iron(III)-reducing (DIR) bacteria are not well understood. Using a synthetic biology approach involving the reconstruction of the putative electron transport chain of the DIR bacterium Shewanella oneidensis MR-1 in Escherichia coli, we showed that expression of cymA was necessary and sufficient to convert E. coli into a DIR bacterium. In intact cells, the Fe(III)-reducing activity was limited to Fe(III) NTA as electron acceptor. In vitro biochemical analysis indicated that CymA, which is a cytoplasmic membrane-associated tetrahaem c-type cytochrome, carries reductase activity towards Fe(III) NTA, Fe(III) citrate, as well as to AQDS, a humic acid analogue. The in vitro specific activities of Fe(III) citrate reductase and AQDS reductase of E. coli spheroplasts were 10x and 30x higher, respectively, relative to the specific rates observed in intact cells, suggesting that access of chelated and insoluble forms of Fe(III) and AQDS is restricted in whole cells. Interestingly, the E. coli CymA orthologue NapC also carried ferric reductase activity. Our data support the argument that the biochemical mechanism of Fe(III) reduction per se was not the key innovation leading to environmental relevant DIR bacteria. Rather, the evolution of an extension of the electron transfer pathway from the Fe(III) reductase CymA to the cell surface via a system of periplasmic and outer membrane cytochrome proteins enabled access to diffusion-impaired electron acceptors.

Production of L-carnitine by secondary metabolism of bacteria

The increasing commercial demand for L-carnitine has led to a multiplication of efforts to improve its production with bacteria. The use of different cell environments, such as growing, resting, permeabilized, dried, osmotically stressed, freely suspended and immobilized cells, to maintain enzymes sufficiently active for L-carnitine production is discussed in the text. The different cell states of enterobacteria, such as Escherichia coli and Proteus sp., which can be used to produce L-carnitine from crotonobetaine or D-carnitine as substrate, are analyzed. Moreover, the combined application of both bioprocess and metabolic engineering has allowed a deeper understanding of the main factors controlling the production process, such as energy depletion and the alteration of the acetyl-CoA/CoA ratio which are coupled to the end of the biotransformation. Furthermore, the profiles of key central metabolic activities such as the TCA cycle, the glyoxylate shunt and the acetate metabolism are seen to be closely interrelated and affect the biotransformation efficiency. Although genetically modified strains have been obtained, new strain improvement strategies are still needed, especially in Escherichia coli as a model organism for molecular biology studies. This review aims to summarize and update the state of the art in L-carnitine production using E. coli and Proteus sp, emphasizing the importance of proper reactor design and operation strategies, together with metabolic engineering aspects and the need for feed-back between wet and in silico work to optimize this biotransformation.

Role of gluconeogenesis and the tricarboxylic acid cycle in the virulence of Salmonella enterica serovar Typhimurium in BALB/c mice

In Salmonella enterica serovar Typhimurium, the Cra protein (catabolite repressor/activator) regulates utilization of gluconeogenic carbon sources by activating transcription of genes in the gluconeogenic pathway, the glyoxylate bypass, the tricarboxylic acid (TCA) cycle, and electron transport and repressing genes encoding glycolytic enzymes. A serovar Typhimurium SR-11 Deltacra mutant was recently reported to be avirulent in BALB/c mice via the peroral route, suggesting that gluconeogenesis may be required for virulence. In the present study, specific SR-11 genes in the gluconeogenic pathway were deleted (fbp, glpX, ppsA, and pckA), and the mutants were tested for virulence in BALB/c mice. The data show that SR-11 does not require gluconeogenesis to retain full virulence and suggest that as yet unidentified sugars are utilized by SR-11 for growth during infection of BALB/c mice. The data also suggest that the TCA cycle operates as a full cycle, i.e., a sucCD mutant, which prevents the conversion of succinyl coenzyme A to succinate, and an DeltasdhCDA mutant, which blocks the conversion of succinate to fumarate, were both attenuated, whereas both an SR-11 DeltaaspA mutant and an SR-11 DeltafrdABC mutant, deficient in the ability to run the reductive branch of the TCA cycle, were fully virulent. Moreover, although it appears that SR-11 replenishes TCA cycle intermediates from substrates present in mouse tissues, fatty acid degradation and the glyoxylate bypass are not required, since an SR-11 DeltafadD mutant and an SR-11 DeltaaceA mutant were both fully virulent.

Function, attachment and synthesis of lipoic acid in Escherichia coli

A series of genetic, biochemical, and physiological studies in Escherichia coli have elucidated the unusual pathway whereby lipoic acid is synthesized. Here we describe the results of these investigations as well as the functions of enzyme proteins that are modified by covalent attachment of lipoic acid and the enzymes that catalyze the modification reactions. Some aspects of the synthesis and attachment mechanisms have strong parallels in the pathways used in synthesis and attachment of biotin and these are compared and contrasted. Homologues of the lipoic acid metabolism proteins are found in all branches of life, save the Archea, and thus these findings seem to have wide biological relevance.

Hierarchical control of anaerobic gene expression in Escherichia coli K-12: the nitrate-responsive NarX-NarL regulatory system represses synthesis of the fumarate-responsive DcuS-DcuR regulatory system

Hierarchical control ensures that facultative bacteria preferentially use the available respiratory electron acceptor with the most positive standard redox potential. Thus, nitrate is used before other electron acceptors such as fumarate for anaerobic respiration. Nitrate regulation is mediated by the NarX-NarL two-component system, which activates the transcription of operons encoding nitrate respiration enzymes and represses the transcription of operons for other anaerobic respiratory enzymes, including enzymes involved in fumarate respiration. These are fumarate reductase (encoded by the frdABCD operon), fumarase B, which generates fumarate from malate, and the DcuB permease for fumarate, malate, and aspartate. The transcription of the corresponding structural genes is activated by the DcuS-DcuR two-component system in response to fumarate or its dicarboxylate precursors. We report results from preliminary transcription microarray experiments that revealed two previously unknown members of the NarL regulon: the aspA gene encoding aspartate-ammonia lyase, which generates fumarate; and the dcuSR operon encoding the dicarboxylate-responsive regulatory system. We measured beta-galactosidase expression from monocopy aspA-lacZ, frdA-lacZ, and dcuS-lacZ operon fusions in response to added nitrate and fumarate and with respect to the dcuR and narL genotypes. Nitrate, acting through the NarX-NarL regulatory system, repressed the transcription of all three operons. Only frdA-lacZ expression, however, was responsive to added fumarate or a dcuR(+) genotype. Phospho-NarL protein protected operator sites in the aspA and dcuS promoter regions from DNase I cleavage in vitro. The overall results are consistent with the hypothesis that nitrate represses frdA operon transcription not only directly, by repressing frdA promoter activity, but also indirectly, by repressing dcuS promoter activity.

Impact of global transcriptional regulation by ArcA, ArcB, Cra, Crp, Cya, Fnr, and Mlc on glucose catabolism in Escherichia coli

Even though transcriptional regulation plays a key role in establishing the metabolic network, the extent to which it actually controls the in vivo distribution of metabolic fluxes through different pathways is essentially unknown. Based on metabolism-wide quantification of intracellular fluxes, we systematically elucidated the relevance of global transcriptional regulation by ArcA, ArcB, Cra, Crp, Cya, Fnr, and Mlc for aerobic glucose catabolism in batch cultures of Escherichia coli. Knockouts of ArcB, Cra, Fnr, and Mlc were phenotypically silent, while deletion of the catabolite repression regulators Crp and Cya resulted in a pronounced slow-growth phenotype but had only a nonspecific effect on the actual flux distribution. Knockout of ArcA-dependent redox regulation, however, increased the aerobic tricarboxylic acid (TCA) cycle activity by over 60%. Like aerobic conditions, anaerobic derepression of TCA cycle enzymes in an ArcA mutant significantly increased the in vivo TCA flux when nitrate was present as an electron acceptor. The in vivo and in vitro data demonstrate that ArcA-dependent transcriptional regulation directly or indirectly controls TCA cycle flux in both aerobic and anaerobic glucose batch cultures of E. coli. This control goes well beyond the previously known ArcA-dependent regulation of the TCA cycle during microaerobiosis.