Regulation of methyl beta-galactoside permease activity in pts and crr mutants of Salmonella typhimurium

Escherichia coli is unable to produce pyrroloquinoline quinone (PQQ)

Many bacteria can synthesize the cofactor pyrroloquinoline quinone (PQQ), a cofactor of several dehydrogenases, including glucose dehydrogenase (GCD). Among the enteric bacteria, Klebsiella pneumoniae has been shown to contain the genes required for PQQ biosynthesis. Escherichia coli and Salmonella typhimurium were thought to be unable to synthesize PQQ but it has been reported that strain EF260, a derivative of E. coli FB8, can synthesize PQQ after mutation and can oxidize glucose to gluconate via the GCD/PQQ pathway (F. Biville, E. Turlin & F. Gasser, 1991, J Gen Microbiol 137, 1775-1782). We have re-investigated this claim and conclude that it is most likely erroneous. (i) Strain EF260, isolated originally by Biville and coworkers, was unable to synthesize a holo-enzyme GCD unless PQQ was supplied to the growth medium. No GCD activity could be detected in membrane fractions. (ii) The amount of PQQ detected in the growth medium of EF260 was very low and not very different from that found in a medium with its parent strain or in a medium containing no cells. (iii) EF260 cells were unable to produce gluconate from glucose via the PQQ/GCD pathway. (iv) Introduction of a gcd::Cm deletion in EF260, eliminating GCD, did not affect glucose metabolism. This suggested a pathway for glucose metabolism other than the PQQ/GCD pathway. (v) Glucose uptake and metabolism in EF260 involved a low-affinity transport system of unknown identity, followed most likely by phosphorylation via glucokinase. It is concluded that E. coli cannot synthesize PQQ and that it lacks genes required for PQQ biosynthesis.

Cra-mediated regulation of Escherichia coli adenylate cyclase

In Escherichia coli, expression of certain genes and operons, including the fructose operon, is controlled by Cra, the pleiotropic catabolite repressor/activator protein formerly known as FruR. In this study we have demonstrated that cra mutant strains synthesize 10-fold less cAMP than isogenic wild-type strains, specifically when grown in fructose-containing minimal media. The glucose-specific IIA protein (IIAglc) of the phosphotransferase system, which activates adenylate cyclase when phosphorylated, is largely dephosphorylated in cra but not wild-type strains growing under these conditions. Dephosphorylation of IIAglc in cra strains apparently results from enhanced fructose operon transcription and fructose uptake. These conclusions were supported by showing that fructose-grown cra strains possess 2.5-fold higher fructose-1-phosphate kinase activity than fructose-grown wild-type strains. Moreover, artificially increasing fructose operon expression in cells transporting fructose dramatically decreased the activity of adenylate cyclase. The results establish that Cra indirectly regulates the activity of adenylate cyclase by controlling the expression of the fructose operon in cells growing with fructose as the sole carbon source.

Regulation of Escherichia coli adenylate cyclase activity during hexose phosphate transport

In Escherichia coli, cAMP levels vary with the carbon source used in the culture medium. These levels are dependent on the cellular concentration of phosphorylated EnzymeIIAglc, a component of the glucose-phosphotransferase system, which activates adenylate cyclase (AC). When cells are grown on glucose 6-phosphate (Glc6P), the cAMP level is particularly low. In this study, we investigated the mechanism leading to the low cAMP level when Glc6P is used as the carbon source, i.e. the mechanism preventing the activation of AC by phosphorylated EnzymeIIAglc. Glc6P is transported via the Uhp system which is inducible by extracellular Glc6P. The Uhp system comprises a permease UhpT and three proteins UhpA, UhpB and UhpC which are necessary for uhpT gene transcription. Controlled expression of UhpT in the absence of the regulatory proteins (UhpA, UhpB and UhpC) allowed us to demonstrate that (i) the Uhp regulatory proteins do not prevent the activation of AC by direct interaction with EnzymeIIAglc and (ii) an increase in the amount of UhpT synthesized (corresponding to an increase in the amount of Glc6P transported) correlates with a decrease in the cAMP level. We present data indicating that Glc6P per se or its degradation is unlikely to be responsible for the low cAMP level. It is concluded that the level of cAMP in the cell is determined by the flux of Glc6P through UhpT.

Control of glucose metabolism by the enzymes of the glucose phosphotransferase system in Salmonella typhimurium

The quantitative role of the phosphoenolpyruvate:glucose phosphotransferase system (glucose phosphotransferase system) in glucose uptake and metabolism, and phosphotransferase-system-mediated regulation of glycerol uptake, was studied in vivo in Salmonella typhimurium. Expression plasmids were constructed which contained the genes encoding enzyme I (ptsI), HP (ptsH), IIAGlc (crr), and IICBGlc (ptsG) of the glucose phosphotransferase system behind inducible promoters. These plasmids allowed the controlled expression of each of the glucose phosphotransferase system proteins from about 30% to about 300% of its wild-type level. When enzyme I, HPr or IIAGlc were modulated between 30% and 300% of their wild-type value, hardly any effects on the growth rate on glucose, the glucose oxidation rate, the rate of methyl alpha-D-glucopyranoside (a glucose analog) uptake or the phosphotransferase-system-mediated inhibition of glycerol uptake by methyl alpha-D-glucopyranoside were observed. Employing the method of metabolic control analysis, it was shown that the enzyme flux control coefficients of these phosphotransferase system components on the different measured processes were close to zero. The enzyme flux control coefficient of IICBGlc on growth on glucose or glucose oxidation was also close to zero. In contrast, the enzyme flux control coefficient of IICBGlc on the flux through the glucose phosphotransferase system (transport and phosphorylation) was 0.72. The experimentally determined enzyme flux control coefficients allowed us to calculate the flux control coefficients of the phosphoenolpyruvate/pyruvate and methyl alpha-D-glucopyranoside/methyl alpha-D-glucopyranoside 6-phosphate couples and the process control coefficients of the phosphotransfer reactions of the glucose phosphotransferase system. We discuss the implications of these values and the possible control points in the glucose phosphotransferase system.

The catalytic domain of Escherichia coli K-12 adenylate cyclase as revealed by deletion analysis of the cya gene

In Escherichia coli, adenylate cyclase activity is regulated by phosphorylated EnzymeIIA(Glc), a component of the phosphotransferase system for glucose transport. In strains deficient in EnzymeIIA(Glc), cAMP levels are very low. Adenylate cyclase containing the D414N substitution produces a low level of cAMP and it has been proposed that D414 may be involved in the process leading to activation by EnzymeIIA(Glc). In this work, spontaneous secondary mutants producing large amounts of cAMP in strains deficient in EnzymeIIA(Glc) were obtained. The secondary mutations were all deletions located in the cya gene around the D414N mutation, generating adenylate cyclases truncated at the carboxyl end. Among them, a 48 kDa protein (half the size of wild-type adenylate cyclase) was shown to produce ten times more cAMP than wild-type adenylate cyclase in strains deficient in EnzymeIIA(Glc). In addition, this protein was not regulated in strains grown on glucose and diauxic growth was abolished. This allowed the definition of a catalytic domain that is not regulated by the phosphotransferase system and produces levels of cAMP similar to that of regulated wild-type adenylate cyclase in wild-type strains grown in the absence of glucose. Further analysis allowed the characterization of the COOH-terminal regulatory domain, which is proposed to be inhibitory to the activity of the catalytic domain.

Analysis of mutations that uncouple transport from phosphorylation in enzyme IIGlc of the Escherichia coli phosphoenolpyruvate-dependent phosphotransferase system

Mutations that uncouple glucose transport from phosphorylation were isolated in plasmid-encoded Escherichia coli enzyme IIGlc of the phosphoenolpyruvate-dependent phosphotransferase system (PTS). The uncoupled enzymes IIGlc were able to transport glucose in the absence of the general phosphoryl-carrying proteins of the PTS, enzyme I and HPr, although with relatively low affinity. Km values of the uncoupled enzymes IIGlc for glucose ranged from 0.5 to 2.5 mM, 2 orders of magnitude higher than the value of normal IIGlc. Most of the mutant proteins were still able to phosphorylate glucose and methyl alpha-glucoside (a non-metabolizable glucose analog specific for IIGlc), indicating that transport and phosphorylation are separable functions of the enzyme. Some of the uncoupled enzymes IIGlc transported glucose with a higher rate and lower apparent Km in a pts+ strain than in a delta ptsHI strain lacking the general proteins enzyme I and HPr. Since the properties of these uncoupled enzymes IIGlc in the presence of PTS-mediated phosphoryl transfer resembled those of wild-type IIGlc, these mutants appeared to be conditionally uncoupled. Sequencing of the mutated ptsG genes revealed that all amino acid substitutions occurred in a hydrophilic segment within the hydrophobic N-terminal part of IIGlc. These results suggest that this hydrophilic loop is involved in binding and translocation of the sugar substrate.

Structural and functional relationships between Pasteurella multocida and enterobacterial adenylate cyclases

The Pasteurella multocida adenylate cyclase gene has been cloned and expressed in Escherichia coli. The primary structure of the protein (838 amino acids) deduced from the corresponding nucleotide sequence was compared with that of E. coli. The two enzymes have similar molecular sizes and, based on sequence conservation at the protein level, are likely to be organized in two functional domains: the amino-terminal catalytic domain and the carboxy-terminal regulatory domain. It was shown that P. multocida adenylate cyclase synthesizes increased levels of cyclic AMP in E. coli strains deficient in the catabolite gene activator protein compared with wild-type strains. This increase does not occur in strains deficient in both the catabolite gene activator protein and enzyme III-glucose, indicating that a protein similar to E. coli enzyme III-glucose is involved in the regulation of P. multocida adenylate cyclase. It also indicates that the underlying process leading to enterobacterial adenylate cyclase activation has been conserved through evolution.

Energetics of glucose uptake in a Salmonella typhimurium mutant containing uncoupled enzyme IIGlc

Uncoupled enzyme IIGlc of the phosphoenolpyruvate (PEP): glucose phosphotransferase system (PTS) in Salmonella typhimurium is able to catalyze glucose transport in the absence of PEP-dependent phosphorylation. We have studied the energetics of glucose uptake catalyzed by this uncoupled enzyme IIGlc. The molar growth yields on glucose of two strains cultured anaerobically in glucose-limited chemostat- and batch cultures were compared. Strain PP799 transported and phosphorylated glucose via an intact PTS, while strain PP952 took up glucose exclusively via uncoupled enzyme IIGlc, followed by ATP-dependent phosphorylation by glucokinase. Thus the strains were isogenic except for the mode of uptake and phosphorylation of the growth substrate. PP799 and PP952 exhibited similar YGlc values. Assuming equal YATP values for both strains this result indicated that there were no energetic demands for glucose uptake via uncoupled enzyme IIGlc.

Transport of trehalose in Salmonella typhimurium

We have studied trehalose uptake in Salmonella typhimurium and the possible involvement of the phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS) in this process. Two transport systems could recognize and transport trehalose, the mannose PTS and the galactose permease. Uptake of trehalose via the latter system required that it be expressed constitutively (due to a galR or galC mutation). Introduction of a ptsM mutation, resulting in a defective IIMan/IIIMan system, in S. typhimurium strains that grew on trehalose abolished growth on trehalose. A ptsG mutation, eliminating IIGlc of the glucose PTS, had no effect. In contrast, a crr mutation that resulted in the absence of IIIGlc of the glucose PTS prevented growth on trehalose. The inability of crr and also cya mutants to grow on trehalose was due to lowered intracellular cyclic AMP synthesis, since addition of extracellular cyclic AMP restored growth. Subsequent trehalose metabolism could be via a trehalose phosphate hydrolase, if trehalose phosphate was formed via the PTS, or trehalase. Trehalose-grown cells contained trehalase activity, but we could not detect phosphoenolpyruvate-dependent phosphorylation of trehalose in toluenized cells.

2-Ketobutyrate: a putative alarmone of Escherichia coli

2-ketobutyrate is synthesized from threonine by threonine deaminase (dehydratase) in E. coli. The effects of 2-ketobutyrate as a regulatory metabolite were studied in vivo. 2-ketobutyrate was shown to inhibit the phosphoenolpyruvate (PEP): sugar phosphotransferase system resulting in aspartate starvation, elevation of ppGpp endogenous pools, and cessation of growth in E. coli grown in glucose and related carbon sources. Accordingly, we propose that 2-ketobutyrate might serve as an alarmone whose concentration precisely governs the shift from anaerobic growth to aerobic growth in E. coli. Such shifts are common phenomena among the Enterobacteriaceae.

Phosphoenolpyruvate:sugar phosphotransferase system-mediated regulation of carbohydrate metabolism in Salmonella typhimurium

The crr mutation was shown to affect the phosphoenolpyruvate:sugar phosphotransferase system-mediated transient repression of the lac operon, intracellular cAMP levels, and sensitivity to inducer exclusion. Our results indicate that the presumed crr gene product, factor IIIGlc, plays a direct role in the regulation of inducer exclusion. We propose a mechanism in which inducer exclusion depends on both the level and state of phosphorylation of factor IIIGlc and the level of an inducer exclusion-sensitive transport system. The results of studies on the sensitivity to inducer exclusion of glycerol and maltose in cultures induced for short periods of time on these substrates (resulting in varying degrees of activity of the respective transport systems) support this model of inducer exclusion. Previously, the crp*-771 mutation has been shown to result in an altered cAMP receptor protein, which has a changed affinity for cAMP, and to affect the sensitivity for inducer exclusion of glycerol. Changes in other functions of the altered cAMP receptor protein were indicated by our results; these changes were in the roles of this protein in (i) the cAMP-dependent initiation of transcription of the lac operon and (ii) the regulation of intracellular cAMP levels and the export of cAMP. We propose that the crp*-771 mutation has an indirect effect in relieving inducer exclusion in repressed or hypersensitive strains, in which the crp*-771 mutation allows the synthesis of inducer exclusion-sensitive transport systems to higher levels than the levels found in strains containing wild-type cAMP receptor protein.

Multiple regulation of the activity of adenylate cyclase in Escherichia coli

We have studied the correlation between the activities of adenylate cyclase (ATP pyrophosphatelyase-(cyclizing); EC 4.6.1.1) and in vivo rates of synthesis and intracellular concentrations of adenosine 3',5' cyclic monophosphate (cAMP) under various growth conditions in wild-type Escherichia coli and in mutants lacking or overproducing the cAMP receptor protein (CAP). We showed that when wild-type bacteria are grown in the presence of a variety of carbon sources the intracellular concentrations of cAMP are inversely related to the adenylate cyclase activities determined in permeabilized cells, suggesting that the carbon source-dependent modulation of cAMP levels is not directly related to the regulation of adenylate cyclase activity. In mutants lacking functional CAP (crp) the in vivo rates of cAMP synthesis are several hundred-fold higher than in the wild-type parent without a parallel increase of adenylate cyclase activities. In a strain carrying multiple copies of the crp gene and overproducing CAP the activity of adenylate cyclase is severely inhibited, although the in vivo rate of cAMP synthesis is similar to the parental strain. We interpret these results as indicating that CAP controls mainly the activity rather than the synthesis of adenylate cyclase.

Glucose transport in Salmonella typhimurium and Escherichia coli

We have investigated the claim by Schweiger and coworkers [Eur. J. Biochem. 102(1979)231-236] that glucose transport in Escherichia coli is catalyzed mainly by an ATP-dependent transport system instead of the phosphoenolpyruvate:sugar phosphotransferase system. A major argument was the differential effect of 2,4-dinitrophenol on glucose uptake and the transport of its non-metabolizable analogue, methyl alpha-glucoside. Whereas the first was inhibited, the second was stimulated. When subsequent glucose metabolism is prevented by introducing mutations that eliminate glucose 6-phosphate metabolism, 2,4-dinitrophenol does not inhibit glucose transport. Although dinitrophenol inhibited in wild-type cells of E. coli and Salmonella typhimurium the uptake of 14C label in cells using [U-14C]glucose as a substrate, disappearance of glucose from the medium was not affected or only slightly affected. Since uptake represents a combination of transport and subsequent metabolism, retention of labelled material depends on the balance of incorporation of label in cellular material and efflux of labelled compounds. Our experiments show that inhibition of the uptake of labelled glucose by 2,4-dinitrophenol is not due to inhibition of transport as suggested by Schweiger and coworkers, but to increased efflux of labelled compounds such as acetate and pyruvate. In addition, incorporation of label in cellular material is lowered by dinitrophenol. Inhibition of uptake by dinitrophenol is found with many labelled sugars, including mannitol, galactose and glycerol, the transport of which is energized in quite different ways. We conclude that there is no need to postulate a novel ATP-driven system for glucose transport. All results can be explained with the phosphoenolpyruvate:glucose phosphotransferase system as the main if not sole glucose transport system.

Characterization of factor IIIGLc in catabolite repression-resistant (crr) mutants of Salmonella typhimurium

crr mutants of Salmonella typhimurium are thought to be defective in the regulation of adenylate cyclase and a number of transport systems by the phosphoenolpyruvate-dependent sugar phosphotransferase system, crr mutants are also defective in the enzymatic activity of factor IIIGlc (IIIGlc), a protein component of the phosphotransferase system involved in glucose transport. Therefore, it has been proposed that IIIGlc is the primary effector of phosphotransferase system-mediated regulation of cell metabolism. We characterized crr mutants with respect to the presence and function of IIIGlc by using an immunochemical approach. All of the crr mutants tested had low (0 to 30%) levels of IIIGlc compared with wild-type cells, as determined by rocket immunoelectrophoresis. The IIIGlc isolated from one crr mutant was investigated in more detail and showed abnormal aggregation behavior, which indicated a structural change in the protein. These results supported the hypothesis that a crr mutation directly affects IIIGlc, probably by altering the structural gene of IIIGlc. Several crr strains which appeared to be devoid of IIIGlc in immunoprecipitation assays were still capable of in vitro phosphorylation and transport of methyl alpha-glucoside. This phosphorylation activity was sensitive to specific anti-IIIGlc serum. Moreover, the membranes of crr mutants, as well as those of wild-type cells, contained a protein that reacted strongly with our anti-IIIGlc serum. We propose that S. typhimurium contains a membrane-bound form of IIIGlc which may be involved in phosphotransferase system activity.

Defective enzyme II-BGlc of the phosphoenolpyruvate:sugar phosphotransferase system leading to uncoupling of transport and phosphorylation in Salmonella typhimurium

Transport and phosphorylation of glucose via enzymes II-A/II-B and II-BGlc of the phosphoenolpyruvate:sugar phosphotransferase system are tightly coupled in Salmonella typhimurium. Mutant strains (pts) that lack the phosphorylating proteins of this system, enzyme I and HPr, are unable to transport or to grow on glucose. From ptsHI deletion strains of S. typhimurium, mutants were isolated that regained growth on and transport of glucose. Several lines of evidence suggest that these Glc+ mutants have an altered enzyme II-BGlc as follows. (i) Insertion of a ptsG::Tn10 mutation (resulting in a defective II-BGlc) abolished growth on and transport of glucose in these Glc+ strains. Introduction of a ptsM mutation, on the other hand, which abolishes II-A/II-B activity, had no effect. (ii) Methyl alpha-glucoside transport and phosphorylation (specific for II-BGlc) was lowered or absent in ptsH+,I+ transductants of these Glc+ strains. Transport and phosphorylation of other phosphoenolpyurate:sugar phosphotransferase system sugars were normal. (iii) Membranes isolated from these Glc+ mutants were unable to catalyze transphosphorylation of methyl alpha-glucoside by glucose 6-phosphate, but transphosphorylation of mannose by glucose 6-phosphate was normal. (iv) The mutation was in the ptsG gene or closely linked to it. We conclude that the altered enzyme II-BGlc has acquired the capacity to transport glucose in the absence of phosphoenolpyruvate:sugar phosphotransferase system-mediated phosphorylation. However, the affinity for glucose decreased at least 1,000-fold as compared to the wild-type strain. At the same time the mutated enzyme II-BGlc lost the ability to catalyze the phosphorylation of its substrates via IIIGlc.