Isolation and characterization of Salmonella typhimurium glyoxylate shunt mutants

Parkinson's disease kinase LRRK2 coordinates a cell-intrinsic itaconate-dependent defence pathway against intracellular Salmonella

Cell-intrinsic defences constitute the first line of defence against intracellular pathogens. The guanosine triphosphatase RAB32 orchestrates one such defence response against the bacterial pathogen Salmonella, through delivery of antimicrobial itaconate. Here we show that the Parkinson's disease-associated leucine-rich repeat kinase 2 (LRRK2) orchestrates this defence response by scaffolding a complex between RAB32 and aconitate decarboxylase 1, which synthesizes itaconate from mitochondrial precursors. Itaconate delivery to Salmonella-containing vacuoles was impaired and Salmonella replication increased in LRRK2-deficient cells. Loss of LRRK2 also restored virulence of a Salmonella mutant defective in neutralizing this RAB32-dependent host defence pathway in mice. Cryo-electron tomography revealed tether formation between Salmonella-containing vacuoles and host mitochondria upon Salmonella infection, which was significantly impaired in LRRK2-deficient cells. This positions LRRK2 centrally within a host defence mechanism, which may have favoured selection of a common familial Parkinson's disease mutant allele in the human population.

Itaconate is an effector of a Rab GTPase cell-autonomous host defense pathway against Salmonella

The guanosine triphosphatase (GTPase) Rab32 coordinates a cell-intrinsic host defense mechanism that restricts the replication of intravacuolar pathogens such as Salmonella Here, we show that this mechanism requires aconitate decarboxylase 1 (IRG1), which synthesizes itaconate, a metabolite with antimicrobial activity. We find that Rab32 interacts with IRG1 on Salmonella infection and facilitates the delivery of itaconate to the Salmonella-containing vacuole. Mice defective in IRG1 rescued the virulence defect of a S. enterica serovar Typhimurium mutant specifically defective in its ability to counter the Rab32 defense mechanism. These studies provide a link between a metabolite produced in the mitochondria after stimulation of innate immune receptors and a cell-autonomous defense mechanism that restricts the replication of an intracellular bacterial pathogen.

Salmonella enterica's "Choice": Itaconic Acid Degradation or Bacteriocin Immunity Genes

Itaconic acid is an immunoregulatory metabolite produced by macrophages in response to pathogen invasion. It also exhibits antibacterial activity because it is an uncompetitive inhibitor of isocitrate lyase, whose activity is required for the glyoxylate shunt to be operational. Some bacteria, such as Yersinia pestis, encode enzymes that can degrade itaconic acid and therefore eliminate this metabolic inhibitor. Studies, primarily with Salmonella enterica subspecies enterica serovar Typhimurium, have demonstrated the presence of similar genes in this pathogen and the importance of these genes for the persistence of the pathogen in murine hosts. This minireview demonstrates that, based on Blast searches of 1063 complete Salmonella genome sequences, not all Salmonella serovars possess these genes. It is also shown that the growth of Salmonella isolates that do not possess these genes is sensitive to the acid under glucose-limiting conditions. Interestingly, most of the serovars without the three genes, including serovar Typhi, harbor DNA at the corresponding genomic location that encodes two open reading frames that are similar to bacteriocin immunity genes. It is hypothesized that these genes could be important for Salmonella that finds itself in strong competition with other Enterobacteriacea in the intestinal tract—for example, during inflammation.

Salmonella enterica Requires Lipid Metabolism Genes To Replicate in Proinflammatory Macrophages and Mice

To survive and replicate during infection, pathogens utilize different carbon and energy sources depending on the nutritional landscape of their host microenvironment. Salmonella enterica serovar Typhimurium is an intracellular bacterial pathogen that occupies diverse cellular niches. While it is clear that Salmonella Typhimurium requires access to glucose during systemic infection, data on the need for lipid metabolism are mixed. We report that Salmonella Typhimurium strains lacking lipid metabolism genes were defective for systemic infection of mice. Bacterial lipid import, β-oxidation, and glyoxylate shunt genes were required for tissue colonization upon oral or intraperitoneal inoculation. In cultured macrophages, lipid import and β-oxidation genes were required for bacterial replication and/or survival only when the cell culture medium was supplemented with nonessential amino acids. Removal of glucose from tissue culture medium further enhanced these phenotypes and, in addition, conferred a requirement for glyoxylate shunt genes. We also observed that Salmonella Typhimurium needs lipid metabolism genes in proinflammatory but not anti-inflammatory macrophages. These results suggest that during systemic infection, the Salmonella Typhimurium that relies upon host lipids to replicate is within proinflammatory macrophages that have access to amino acids but not glucose. An improved understanding of the host microenvironments in which pathogens have specific metabolic requirements may facilitate the development of targeted approaches to treatment.

Mapping Salmonella typhimurium pathways using 13C metabolic flux analysis

In the last years, Salmonella has been extensively studied not only due to its importance as a pathogen, but also as a host to produce pharmaceutical compounds. However, the full exploitation of Salmonella as a platform for bioproduct delivery has been hampered by the lack of information about its metabolism. Genome-scale metabolic models can be valuable tools to delineate metabolic engineering strategies as long as they closely represent the actual metabolism of the target organism. In the present study, a ¹³C-MFA approach was applied to map the fluxes at the central carbon pathways of S. typhimurium LT2 growing at glucose-limited chemostat cultures. The experiments were carried out in a 2L bioreactor, using defined medium enriched with 20% ¹³C-labeled glucose. Metabolic flux distributions in central carbon pathways of S. typhimurium LT2 were estimated using OpenFLUX2 based on the labeling pattern of biomass protein hydrolysates together with biomass composition. The results suggested that pentose phosphate is used to catabolize glucose, with minor fluxes through glycolysis. In silico simulations, using Optflux and pFBA as simulation method, allowed to study the performance of the genome-scale metabolic model. In general, the accuracy of in silico simulations was improved by the superimposition of estimated intracellular fluxes to the existing genome-scale metabolic model, showing a better fitting to the experimental extracellular fluxes, whereas the intracellular fluxes of pentose phosphate and anaplerotic reactions were poorly described.

One ligand, two regulators and three binding sites: How KDPG controls primary carbon metabolism in Pseudomonas

Effective regulation of primary carbon metabolism is critically important for bacteria to successfully adapt to different environments. We have identified an uncharacterised transcriptional regulator; RccR, that controls this process in response to carbon source availability. Disruption of rccR in the plant-associated microbe Pseudomonas fluorescens inhibits growth in defined media, and compromises its ability to colonise the wheat rhizosphere. Structurally, RccR is almost identical to the Entner-Doudoroff (ED) pathway regulator HexR, and both proteins are controlled by the same ED-intermediate; 2-keto-3-deoxy-6-phosphogluconate (KDPG). Despite these similarities, HexR and RccR control entirely different aspects of primary metabolism, with RccR regulating pyruvate metabolism (aceEF), the glyoxylate shunt (aceA, glcB, pntAA) and gluconeogenesis (pckA, gap). RccR displays complex and unusual regulatory behaviour; switching repression between the pyruvate metabolism and glyoxylate shunt/gluconeogenesis loci depending on the available carbon source. This regulatory complexity is enabled by two distinct pseudo-palindromic binding sites, differing only in the length of their linker regions, with KDPG binding increasing affinity for the 28 bp aceA binding site but decreasing affinity for the 15 bp aceE site. Thus, RccR is able to simultaneously suppress and activate gene expression in response to carbon source availability. Together, the RccR and HexR regulators enable the rapid coordination of multiple aspects of primary carbon metabolism, in response to levels of a single key intermediate.

Here we show how Pseudomonas controls multiple different primary carbon metabolism pathways by sensing levels of KDPG, an Entner Doudoroff (ED) pathway intermediate. KDPG binds to two highly similar transcription factors; the ED regulator HexR and the previously uncharacterised protein RccR. RccR inversely controls the glyoxylate shunt, gluconeogenesis and pyruvate metabolism, suppressing the first two pathways as pyruvate metabolism genes are expressed, and vice versa. This complex regulation is enabled by two distinct RccR-binding consensus sequences in the RccR regulon promoters. KDPG binding simultaneously increases RccR affinity for the glyoxylate shunt and gluconeogenesis promoters, and releases repression of pyruvate metabolism. This elegant two-regulator circuit allows Pseudomonas to rapidly respond to carbon source availability by sensing a single key intermediate, KDPG.

Nutritional and metabolic requirements for the infection of HeLa cells by Salmonella enterica serovar Typhimurium

Salmonella is the causative agent of a spectrum of human and animal diseases ranging from gastroenteritis to typhoid fever. It is a food - and water - borne pathogen and infects via ingestion followed by invasion of intestinal epithelial cells and phagocytic cells. In this study we employed a mutational approach to define the nutrients and metabolic pathways required by Salmonella enterica serovar Typhimurium during infection of a human epithelial cell line (HeLa). We deleted the key glycolytic genes, pfkA and pfkB to show that S. Typhimurium utilizes glycolysis for replication within HeLa cells; however, glycolysis was not absolutely essential for intracellular replication. Using S. Typhimurium strains deleted for genes encoding components of the phosphotransferase system and glucose transport, we show that glucose is a major substrate required for the intracellular replication of S. Typhimurium in HeLa cells. We also deleted genes encoding enzymes involved in the utilization of gluconeogenic substrates and the glyoxylate shunt and show that neither of these pathways were required for intracellular replication of S. Typhimurium within HeLa cells.

An incomplete TCA cycle increases survival of Salmonella Typhimurium during infection of resting and activated murine macrophages

BACKGROUND

In comparison to the comprehensive analyses performed on virulence gene expression, regulation and action, the intracellular metabolism of Salmonella during infection is a relatively under-studied area. We investigated the role of the tricarboxylic acid (TCA) cycle in the intracellular replication of Salmonella Typhimurium in resting and activated macrophages, epithelial cells, and during infection of mice.

METHODOLOGY/PRINCIPAL FINDINGS

We constructed deletion mutations of 5 TCA cycle genes in S. Typhimurium including gltA, mdh, sdhCDAB, sucAB, and sucCD. We found that the mutants exhibited increased net intracellular replication in resting and activated murine macrophages compared to the wild-type. In contrast, an epithelial cell infection model showed that the S. Typhimurium ΔsucCD and ΔgltA strains had reduced net intracellular replication compared to the wild-type. The glyoxylate shunt was not responsible for the net increased replication of the TCA cycle mutants within resting macrophages. We also confirmed that, in a murine infection model, the S. Typhimurium ΔsucAB and ΔsucCD strains are attenuated for virulence.

CONCLUSIONS/SIGNIFICANCE

Our results suggest that disruption of the TCA cycle increases the ability of S. Typhimurium to survive within resting and activated murine macrophages. In contrast, epithelial cells are non-phagocytic cells and unlike macrophages cannot mount an oxidative and nitrosative defence response against pathogens; our results show that in HeLa cells the S. Typhimurium TCA cycle mutant strains show reduced or no change in intracellular levels compared to the wild-type. The attenuation of the S. Typhimurium ΔsucAB and ΔsucCD mutants in mice, compared to their increased net intracellular replication in resting and activated macrophages suggest that Salmonella may encounter environments within the host where a complete TCA cycle is advantageous.

Use of operon and gene fusions to study gene regulation in Salmonella

Coupling the expression of a gene with an easily assayable reporter gene provides a simple genetic trick for studying the regulation of gene expression. Two types of fusions between a gene and a reporter gene are possible. Operon fusions place the transcription of a reporter gene under the control of the promoter of a target gene, but the translation of the reporter gene and target gene are independent; gene fusions place the transcription and translation of a reporter gene under the control of a target gene, and result in a hybrid protein. Such fusions can be constructed in vitro using recombinant DNA techniques or in vivo using transposon derivatives. Many different transposon derivatives are available for constructing operon and gene fusions, but two extremely useful fusion vectors are (1) Mu derivatives that form operon and gene fusions to the lacZ gene, and (2) Tn5 derivative that forms gene fusions to the phoA gene.

Isocitrate lyase (AceA) is required for Salmonella persistence but not for acute lethal infection in mice

Isocitrate lyase is required for fatty acid utilization via the glyoxylate shunt. Although isocitrate lyase is essential for Salmonella persistence during chronic infection, it is dispensable for acute lethal infection in mice. Substrate availability in the phagosome appears to evolve over time, with increasing fatty acid dependence during chronic infection.

The cold-inducible icl gene encoding thermolabile isocitrate lyase of a psychrophilic bacterium, Colwellia maris

The gene encoding isocitrate lyase (ICL; EC 4.1.3.1) of a psychrophilic bacterium, Colwellia maris, was cloned and sequenced. The ORF of the gene (icl) was 1584 bp long, and the predicted gene product consisted of 528 aa (molecular mass 58150 Da) and showed low homology with the corresponding enzymes from other organisms. The analyses of amino acid content and primary structure of the C. maris ICL suggested that it possessed many features of a cold-adapted enzyme. Primer extension and Northern blot analyses revealed that two species of the icl mRNAs with differential lengths of 5'-untranslated regions (TS1 and TS2) were present, of which the 5' end (TS1 and TS2 sites) were G and A, located at 130 and 39 bases upstream of the translation start codon, respectively. The levels of TS1 and TS2 mRNAs were increased by both acetate and low temperature. The induction of icl expression by low temperature took place in the C. maris cells grown on succinate as the carbon source but not acetate. Furthermore, a similar manner of inductions was also found in the levels of the translation and the enzyme activity in cell-free extract. These results suggest that the icl gene, encoding thermolabile isocitrate lyase, of C. maris is important for acetate utilization and cold adaptation.

Chromosomal locus that affects pathogenicity of Rhodococcus fascians

The gram-positive plant pathogen Rhodococcus fascians provokes leafy gall formation on a wide range of plants through secretion of signal molecules that interfere with the hormone balance of the host. Crucial virulence genes are located on a linear plasmid, and their expression is tightly controlled. A mutant with a mutation in a chromosomal locus that affected virulence was isolated. The mutation was located in gene vicA, which encodes a malate synthase and is functional in the glyoxylate shunt of the Krebs cycle. VicA is required for efficient in planta growth in symptomatic, but not in normal, plant tissue, indicating that the metabolic requirement of the bacteria or the nutritional environment in plants or both change during the interaction. We propose that induced hyperplasia on plants represents specific niches for the causative organisms as a result of physiological alterations in the symptomatic tissue. Hence, such interaction could be referred to as metabolic habitat modification.

Regulation of acetate metabolism by protein phosphorylation in enteric bacteria

Growth of enteric bacteria on acetate as the sole source of carbon and energy requires operation of a particular anaplerotic pathway known as the glyoxylate bypass. In this pathway, two specific enzymes, isocitrate lyase and malate synthase, are activated to divert isocitrate from the tricarboxylic acid cycle and prevent the quantitative loss of acetate carbons as carbon dioxide. Bacteria are thus supplied with the metabolic intermediates they need for synthesizing their cellular components. The channeling of isocitrate through the glyoxylate bypass is regulated via the phosphorylation/dephosphorylation of isocitrate dehydrogenase, the enzyme of the tricarboxylic acid cycle which competes for a common substrate with isocitrate lyase. When bacteria are grown on acetate, isocitrate dehydrogenase is phosphorylated and, concomitantly, its activity declines drastically. Conversely, when cells are cultured on a preferred carbon source, such as glucose, the enzyme is dephosphorylated and recovers full activity. Such reversible phosphorylation is mediated by an unusual bifunctional enzyme, isocitrate dehydrogenase kinase/phosphatase, which contains both modifying and demodifying activities on the same polypeptide. The genes coding for malate synthase, isocitrate lyase, and isocitrate dehydrogenase kinase/phosphatase are located in the same operon. Their expression is controlled by a complex dual mechanism that involves several transcriptional repressors and activators. Recent developments have brought new insights into the nature and mode of action of these different regulators. Also, significant advances have been made lately in our understanding of the control of enzyme activity by reversible phosphorylation. In general, analyzing the physiological behavior of bacteria on acetate provides a valuable approach for deciphering at the molecular level the mechanisms of cell adaptation to the environment.

Characterization, gene cloning and expression of isocitrate lyase involved in the assimilation of one-carbon compounds in Hyphomicrobium methylovorum GM2

Isocitrate lyase of an obligate methylotrophic bacterium, Hyphomicrobium methylovorum GM2, was purified to homogeneity and characterized. The enzyme is a homotetramer of identical 62-kDa subunits. After the enzyme had been incubated at temperatures up to 25 degrees C for 30 min, no loss of activity was observed. The enzyme was stable in the pH range of 7.5-9.0. Maximum activity was observed at pH 7.5 and around 45 degrees C. The Km value for Ds-isocitrate was 0.51 mM. The activity required Mg2+ and was inhibited by oxalate, succinate and glycolate. The gene encoding the isocitrate lyase and its flanking regions were isolated from H. methylovorum GM2. Nucleotide sequencing of recombinant plasmids revealed that the isocitrate lyase gene codes for a 540-amino-acid protein. The amino acid sequence of the enzyme is similar to those of the enzymes from Escherichia coli (40% identity) and cucumber (37% identity). The recombinant plasmid, which was constructed by ligation of the cloned gene and an expression vector, pKK223-3, was introduced into E. coli HB101. The transformed E. coli cells expressed isocitrate lyase, which was indistinguishable from the purified H. methylovorum GM2 isocitrate lyase on analysis by SDS/PAGE.

Integration host factor amplifies the induction of the aceBAK operon of Escherichia coli by relieving IclR repression

A binding site for integration host factor (IHF) was identified upstream of the aceBAK promoter. Under inducing conditions, IHF activates aceB::lacZ expression by opposing IclR repression. In contrast, IHF has little effect on aceB::lacZ expression under repressing conditions. The ability of IHF to relieve repression under inducing but not repressing conditions allows this protein to amplify the induction of aceBAK.

Characterization of the isocitrate lyase gene from Corynebacterium glutamicum and biochemical analysis of the enzyme

Isocitrate lyase is a key enzyme in the glyoxylate cycle and is essential as an anaplerotic enzyme for growth on acetate as a carbon source. It is assumed to be of major importance in carbon flux control in the amino acid-producing organism Corynebacterium glutamicum. In crude extracts of C. glutamicum, the specific activities of isocitrate lyase were found to be 0.01 U/mg of protein after growth on glucose and 2.8 U/mg of protein after growth on acetate, indicating tight regulation. The isocitrate lyase gene, aceA, was isolated, subcloned, and characterized. The predicted gene product of aceA consists of 432 amino acids (M(r), 47,228) and shows up to 57% identity to the respective enzymes from other organisms. Downstream of aceA, a gene essential for thiamine biosynthesis was identified. Overexpression of aceA in C. glutamicum resulted in specific activities of 0.1 and 7.4 U/mg of protein in minimal medium containing glucose and acetate, respectively. Inactivation of the chromosomal aceA gene led to an inability to grow on acetate and to the absence of any detectable isocitrate lyase activity. Isocitrate lyase was purified to apparent homogeneity and subjected to biochemical analysis. The native enzyme was shown to be a tetramer of identical subunits, to exhibit an ordered Uni-Bi mechanism of catalysis, and to be effectively inhibited by 3-phosphoglycerate, 6-phosphogluconate, phosphoenolpyruvate, fructose-1,6-bisphosphate, and succinate.

Physical mapping of the scattered methionine genes on the Escherichia coli chromosome

Methionine is an important amino acid which acts not only as a substrate for protein elongation but also as the initiator of protein synthesis. The genes of the met regulon, which consists of 10 biosynthetic genes (metA, metB, metC, metE, metF, metH, metK, metL, metQ, and metX), two regulatory genes (metJ and metR), and the methionyl tRNA synthetase gene (metG), are scattered throughout the chromosome. The only linked genes are metK and metX at 63.6 min, metE and metR at 86.3 min, and the metJBLF gene cluster at 89 min. metBL form the only met operon.

Specific interactions between the IclR repressor of the acetate operon of Escherichia coli and its operator

The positions of interference points between the IclR repressor of the acetate operon of Escherichia coli and its specific operator were examined. The number and nature of nucleotides essential to repressor binding were determined by scanning populations of DNA previously methylated at guanine residues by dimethyl sulfate, or depurinated by treatment with formic acid, or depyrimidated by treatment with hydrazine. A total of 46 nucleotides, distributed almost equally between the two strands of the operator region, were found to be functionally important, although to a varying extent. These are clustered in two successive domains which expand from nucleotide -54 to nucleotide -27 and can organize in a palindrome-like structure containing a large proportion of A and T residues.

Cloning, sequencing and distribution of the Salmonella typhimurium LT2 sialidase gene, nanH, provides evidence for interspecies gene transfer

The Salmonella typhimurium LT2 sialidase (neuraminidase, EC 3.2.1.18) structural gene, nanH, has been cloned and sialidase overproduced from multicopy plasmids in Escherichia coli. Sialidase expression was regulated positively by cAMP. In contrast, certain Tn1000 insertions located upstream of nanH coding sequences reduced sialidase activity. A nanH chromosomal insertion mutation constructed by marker exchange demonstrated a single sialidase gene copy in S. typhimurium LT2. The complete nucleotide sequence of nanH, encoding a 41,300 dalton polypeptide, was determined and the derived primary structure was similar to sialidases from Clostridium perfringens, Clostridium sordellii, Bacteroides fragilis, and Trypanosoma cruzi. Comparative sequence analysis, including codon usage and secondary structure predictions, indicated that the S. typhimurium and clostridial sialidases are homologous, strongly suggestive of an interspecies gene transfer event. At least two primary sequence motifs of the bacterial enzymes were detected in influenza A virus sialidases. The predicted secondary structure of the bacterial enzymes was strikingly similar to viral sialidase. From the population distribution of nanH detected within a collection of salmonellae, it was apparent that S. typhimurium obtained its nanH copy most recently from Salmonella arizonae. S. typhimurium LT2 is thus a genetic mosaic that differs from other strains of even the same serotype by nanH plus potentially additional characters linked to nanH. These results have relevance to the evolution and function of sialidases in pathogenic microbes, and to the origin of the sialic acids.

Overproduction and characterization of the iclR gene product of Escherichia coli K-12 and comparison with that of Salmonella typhimurium LT2

The iclR gene of Escherichia coli K-12, which encodes a regulatory protein (repressor) for the aceBAK operon, is located between that operon and metH in the 91-min region of the chromosome. The iclR gene was cloned and expressed in a coupled T7 RNA polymerase/promoter system and the gene product was identified by specific binding to a fragment containing the aceBAK operator region. The iclR gene product is a polypeptide of 274 amino acids (aa) with a calculated Mr of 29,741. Comparison of the deduced IclR aa sequence to that of Salmonella typhimurium revealed that the two IclR repressors exhibit 89% identity. A possible helix-turn-helix motif characteristic of DNA-binding proteins was found within the IclR sequence. A search in protein data banks revealed that IclR has a score of similarity of 43.7% with GylR, a transcriptional regulator of the glycerol operon of Streptomyces coelicolor.

Regulation of the glyoxylate bypass operon: cloning and characterization of iclR

In Escherichia coli, expression of the glyoxylate bypass operon appears to be controlled, in part, by the product of iclR+. Mutations in iclR have been found to yield constitutive expression of this operon, suggesting that iclR+ encodes a repressor protein. We have cloned iclR+ by taking advantage of its tight genetic linkage with the glyoxylate bypass operon. The clone complemented a mutant allele of iclR in trans, restoring an inducible phenotype for this operon. Deletion analysis identified a region of ca. 900 base pairs that was necessary and sufficient for complementation. The nucleotide sequence of the insert was then determined. Translation of this sequence revealed an open reading frame capable of encoding a protein with Mr 29,741 preceded by a potential Shine-Dalgarno ribosome-binding site. The deduced amino acid sequence includes a region at the amino terminus that may form a helix-turn-helix motif, a structure found in many DNA-binding domains.

Characterization of phosphoenolpyruvate synthase mutants in Salmonella typhimurium

The enteric bacteria are able to grow by utilizing three-carbon compounds (pyruvate, lactate, and alanine) as sole carbon sources only if they have a functional phosphoenolpyruvate synthase (PEP synthase). PEP synthase catalyzes the phosphorylation of pyruvate to PEP with the hydrolysis of ATP to AMP. This anaplerotic reaction is needed for the synthesis of carbohydrates and citric acid cycle intermediates that are essential for continued cell growth. Insertion mutants were isolated in Salmonella typhimurium that specifically lack the ability to grow on three-carbon compounds. These mutants also fail to utilize acetate as a sole carbon source. Enzyme assays were performed and the results showed that these mutants contain no PEP synthase activity. By using bacteriophage P22, the pps mutations isolated in this study were found to be contransducible with genetic markers in both the aroD and btuC genes. Three-factor crosses pinpointed the order of these genes and their distances with respect to each other. One of the mutants carries a pps::lac operon fusion. This fusion was used to explore the transcriptional regulation of the pps gene. A functional copy of the pps gene is required for its own induction. The pps gene is also under catabolite repression, but the addition of adenosine 3',5'-cyclic monophosphate (cyclic AMP) to cells grown in the presence of glucose does not relieve this repression. These results indicate that the synthesis of PEP synthase is regulated in a more complex manner than has been previously recognized.

Altered transcriptional patterns affecting several metabolic pathways in strains of Salmonella typhimurium which overexpress the fructose regulon

Expression of beta-galactosidase in transcriptional fusions with the pps gene (encoding phosphoenolpyruvate [PEP] synthase), the aceBAK operon (encoding malate synthase, isocitrate lyase, and isocitrate dehydrogenase kinase, respectively), and the phs operon (encoding either thiosulfate reductase or a regulatory protein controlling its expression) was studied in Salmonella typhimurium. beta-Galactosidase synthesis in these strains was repressible either by growth in the presence of glucose or by the presence of a fruR mutation, which resulted in the constitutive expression of the fructose (fru) regulon. Five enzymes of gluconeogenesis (PEP synthase, PEP carboxykinase, isocitrate lyase, malate synthase, and fructose-1,6-diphosphatase) were shown to be repressed either by growth in the presence of glucose or the fruR mutation, while the glycolytic enzymes, enzyme I and enzymes II of the phosphotransferase system as well as phosphofructokinase, were induced either by growth in the presence of glucose or the fruR mutation. Overexpression of the cloned fru regulon genes (not including fruR) resulted in parallel repression of representative gluconeogenic, Krebs cycle, and glyoxylate shunt enzymes. Studies with temperature-sensitive mutants of S. typhimurium which synthesized heat-labile IIIFru proteins provided evidence that this protein plays a role in the regulation of gluconeogenic substrate utilization. Other mutant analyses revealed a complex relationship between fru gene expression and the expression of genes encoding gluconeogenic enzymes. Taken together, the results suggest that a number of genes encoding catabolic, biosynthetic, and amphibolic enzymes in enteric bacteria are transcriptionally regulated by a complex catabolite repression/activation mechanism which may involve enzyme IIIFru of the phosphotransferase system as one component of the regulatory system.