Mannitol transport in Streptococcus mutans.

A hexitol-inducible, phosphoenolpyruvate-dependent phosphotransferase system was demonstrated in Streptococcus mutans. Cell-free extracts obtained from mannitol-grown cells from a representative strain of each of the five S. mutans serotypes (AHT, BHT, C-67-1, 6715, and LM7) were capable of converting mannitol to mannitol-1-phosphate by a reaction which required phosphoenolpyruvate and Mg2+. Mannitol and sorbitol phosphotransferase activities were found in cell-free extracts prepared from cells grown on the respective substrate, but neither hexitol phosphotransferase activity was present in extracts obtained from cells grown on other substrates examined. A heat-stable, low-molecular-weight component was partially purified from glucose-grown cells and found to stimulate the mannitol phosphotransferase system. Divalent cations Mn2+ and Ca2+ partially replaced Mg2+, while Zn2+ was found to be highly inhibitory.     

Carbohydrate uptake in the oral pathogen Streptococcus mutans: mechanisms and regulation by protein phosphorylation

Streptococcus mutans is the primary etiological agent of dental caries in man and other animals. This organism and other related oral streptococci use carbohydrates almost exclusively as carbon and energy sources, fermenting them primarily to lactic acid which initiates erosion of tooth surfaces. Investigations over the past decade have shown that the major uptake mechanism for most carbohydrates in S. mutans is the phosphoenolpyruvate (PEP)-dependent phosphotransferase system (PTS), although non-PTS systems have also been identified for glucose and sucrose. Regulation of sugar uptake occurs by induction/repression and inducer exclusion mechanisms in S. mutans, but apparently not by inducer expulsion as is found in some other streptococci. In addition, ATP-dependent protein kinases have also been identified in S. mutans and other oral streptococci, and a regulatory function for at least one of these has been postulated. Among a number of proteins that are phosphorylated by these enzymes, the predominant soluble protein substrate is the general phospho-carrier protein of the PTS, HPr, as had previously been observed in a variety of Gram-positive bacteria. Recent results have provided evidence for a role for ATP-dependent phosphorylation of HPr in the coordination of sugar uptake and its catabolism in S. mutans. In this review, these results are summarized, and directions for future research in this area are discussed.     

Construction of scrA::lacZ gene fusions to investigate regulation of the sucrose PTS of Streptococcus mutans

The scrA gene coding for sucrose EnzymeII of the phosphoenolpyruvate dependent phosphotransferase system previously isolated from Streptococcus mutans was fused in vitro to the promoterless lacZ' gene to monitor the expression of the scrA gene. The scrA::lacZ gene fusion was introduced back into S. mutans GS-5IS3 by two independent transformation procedures involving either linear or plasmid DNA to produce both scrA and scrA+ mutants. These mutants should prove useful for analyzing the regulation of sucrose transport in S. mutans.     

Regulation of glycolysis and sugar phosphotransferase activities in Streptococcus lactis: growth in the presence of 2-deoxy-D-glucose.

Streptococcus lactis K1 has the capacity to grow on many sugars, including sucrose and lactose, in the presence of high levels (greater than 500 mM) of 2-deoxy-D-glucose. Initially, growth of the organism was transiently halted by the addition of comparatively low concentrations (less than 0.5 mM) of the glucose analog to the culture. Inhibition was coincident with (i) rapid accumulation of 2-deoxy-D-glucose 6-phosphate (ca. 120 mM) and preferential utilization of phosphoenolpyruvate via the mannose:phosphotransferase system, (ii) depletion of phosphorylated glycolytic intermediates, and (iii) a 60% reduction in intracellular ATP concentration. During the 5- to 10-min period of bacteriostasis the intracellular concentration of 2-deoxy-D-glucose 6-phosphate rapidly declined, and the concentrations of glycolytic intermediates were restored to near-normal levels. When growth resumed, the cell doubling time (Td) and the steady-state levels of 2-deoxy-D-glucose 6-phosphate maintained by the cells were dependent upon the medium concentration of 2-deoxy-D-glucose. Resistance of S. lactis K1 to the potentially toxic analog was a consequence of negative regulation of the mannose:phosphotransferase system by two independent mechanisms. The first, short-term response occurred immediately after the initial "overshoot" accumulation of 2-deoxy-D-glucose 6-phosphate, and this mechanism reduced the activity (fine control) of the mannose:phosphotransferase system. The second, long-term mechanism resulted in repression of synthesis (coarse control) of enzyme IImannose. The two regulatory mechanisms reduced the rate of 2-deoxy-D-glucose translocation via the mannose:phosphotransferase system and minimized the activity of the phosphoenolpyruvate-dependent futile cycle of the glucose analog (J. Thompson and B. M. Chassy, J. Bacteriol. 151:1454-1465, 1982). Phosphoenolpyruvate was thus conserved for transport of the growth sugar and for generation of ATP required for biosynthetic and work functions of the growing cell.     

Regulation of sugar transport via the multiple sugar metabolism operon of Streptococcus mutans by the phosphoenolpyruvate phosphotransferase system.

In this report, we provide evidence that the transport of sugars in Streptococcus mutans via the multiple sugar metabolism system is regulated by the phosphoenolpyruvate phosphotransferase system. A ptsI-defective mutant (DC10), when grown on the multiple sugar metabolism system substrate raffinose, exhibited reduced growth, transport, and glycolytic activity with raffinose relative to the parent strain BM71. Inhibition of [3H]raffinose uptake was also observed in both BM71 and DC10 with increasing concentrations of glucose and the glucose analogs alpha-methyl glucoside and 2-deoxyglucose.     

The phosphoenolpyruvate:sugar phosphotransferase system and biofilms in gram-positive bacteria

This review will examine the connection between the bacterial phosphoenolpyruvate:sugar phosphotransferase system and biofilms. We will consider both the primary role of the phosphoenolpyruvate:sugar phosphotransferase system in sugar uptake by biofilm cells and its possible role in regulatory processes in cells growing as biofilms, and in establishment and maintenance of these biofilms.     

Novel phosphoenolpyruvate-dependent futile cycle in Streptococcus lactis: 2-deoxy-D-glucose uncouples energy production from growth.

The addition of 2-deoxy-D-glucose to cultures of Streptococcus lactis 133 that were growing exponentially on sucrose or lactose reduced the growth rate by ca. 95%. Inhibition did not occur with glucose or mannose as the growth sugar. The reduction in growth rate was concomitant with rapid accumulation of the analog in phosphorylated form (2-deoxy-D-glucose 6-phosphate) via the phosphoenolpyruvate-dependent mannose:phosphotransferase system. Within 5 min the intracellular 2-deoxy-D-glucose 6-phosphate concentration reached a steady-state level of greater than 100 mM. After maximum accumulation of the sugar phosphate, the rate of sucrose metabolism (glycolysis) decreased by only 30%, but the cells were depleted of fructose-1,6-diphosphate. The addition of glucose to 2-deoxy-D-glucose 6-phosphate preloaded cells caused expulsion of 2-deoxy-D-glucose and a resumption of normal growth. S. lactis 133 contained an intracellular Mg2+-dependent, fluoride-sensitive phosphatase which hydrolyzed 2-deoxy-D-glucose 6-phosphate (and glucose 6-phosphate) to free sugar and inorganic phosphate. Because of continued dephosphorylation and efflux of the non-metabolizable analog, the maintenance of the intracellular 2-deoxy-D-glucose 6-phosphate pool during growth stasis was dependent upon continued glycolysis. This steady-state condition represented a dynamic equilibrium of: (i) phosphoenolpyruvate-dependent accumulation of 2-deoxy-D-glucose 6-phosphate, (ii) intracellular dephosphorylation, and (iii) efflux of free 2-deoxy-D-glucose. This sequence of events constitutes a futile cycle which promotes the dissipation of phosphoenolpyruvate. We conclude that 2-deoxy-D-glucose functions as an uncoupler by dissociating energy production from growth in S. lactis 133.     

Regulation of Lactose Transport by the Phosphoenolpyruvate-Sugar Phosphotransferase System in Membrane Vesicles of Escherichia coli

Regulation of lactose uptake by the phosphoenolpyruvate-sugar phosphotransferase system (PTS) has been demonstrated in membrane vesicles of Escherichia coli strain ML308-225. Substrates of the phosphotransferase system inhibited D-lactate energized uptake of lactose but did not inhibit uptake of either L-alanine or L-proline. This inhibition was reversed by intravesicular (but not extravesicular) phosphoenolpyruvate. Lactose uptake was also inhibited by enzyme III^glc preparations that were shocked into the vesicles, and this inhibition was reversed by phosphoenolpyruvate. Intravesicular HPr and enzyme I stimulated methyl α-glucoside uptake but did not inhibit or stimulate lactose accumulation. Vesicles maintained at 0°C for several days partially lost 1) the ability to take up lactose, 2) the ability to accumulate PTS substrates, and 3) PTS-mediated regulation. Phosphoenolpyruvate addition restored all of these activities. These results support a mechanism in which the relative proportions of phosphorylated and nonphosphorylated forms of a phosphotransferase constituent regulate the activity of the lactose permcase.     

The glucose transport system of the hyperthermophilic anaerobic bacterium Thermotoga neapolitana

The glucose transport system of the extremely thermophilic anaerobic bacterium Thermotoga neapolitana was studied with the nonmetabolizable glucose analog 2-deoxy-D-glucose (2-DOG). T. neapolitana accumulated 2-DOG against a concentration gradient in an intracellular free sugar pool that was exchangeable with external source of energy, such as pyruvate, and was inhibited by arsenate and gramicidin D. There was no phosphoenolpyruvate-dependent phosphorylation of glucose, 2-DOG, or fructose by cell extracts or toluene-treated cells, indicating the absence of a phosphoenolpyruvate:sugar phosphotransferase system. These data indicate that D-glucose is taken up by T. neapolitana via an active transport system that is energized by an ion gradient generated by ATP, derived from substrate-level phosphorylation.     

Nature and properties of hexitol transport systems in Escherichia coli.

In Escherichia coli K-12 the naturally occurring hexitols D-mannitol, D-glucitol, and galactitol are taken up and phosphorylated via three distinct transport systems by a mechanism called either group translocation or vectorial phosphorylation. For every system, a membrane-bound enzyme II-complex of the phosphoenolpyruvate-dependent phosphotransferase system has been found, each requiring phosphoenolpyruvate, enzyme I, and HPr or alternatively P-HPr as the phosphate donor. Cells with a constitutive synthesis of all hexitol transport systems but with low P-HPr levels have very low transport and phosphorylating activities in vivo, although 40 to 90% of the enzyme II-complex activities are detected in cell extracts of such mutants. No indications for additional hexitol transport systems, especially for systems able to transport and accumulate free hexitols as in Klebsiella aerogenes, have been found. Substrate Km, and Vmax of the three transport systems for several hexitols and hexitol analogues have been determined by growth rates, transport activities, and in vitro phosphorylating activities. Each system was found to take up several hexitols, but only one hexitol serves as the inducer. This inducer invariably is the substrate with the highest affinity. Since bacterial transport systems, as a general rule, seem to have a relatively broad substrate specificity, in contrast to a more restricted inducer specificity, we propose to name the system inducible by D-mannitol and coded by the gene mtlA the D-mannitol transport system, the system inducible by D-glucitol and coded by gutA the D-glucitol transport system, and the system inducible by galactitol and coded by gatA the galactitol transport system.     

Inducer exclusion in Escherichia coli by non-PTS substrates: the role of the PEP to pyruvate ratio in determining the phosphorylation state of enzyme IIAGlc

The main mechanism causing catabolite repression in Escherichia coli is the dephosphorylation of enzyme IIA^Glc, one of the enzymes of the phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS). The PTS is involved in the uptake of a large number of carbohydrates that are phosphorylated during transport, phosphoenolpyruvate (PEP) being the phosphoryl donor. Dephosphorylation of enzyme IIA^Glc causes inhibition of uptake of a number of non-PTS carbon sources, a process called inducer exclusion. In this paper, we show that dephosphorylation of enzyme IIA^Glc is not only caused by the transport of PTS carbohydrates, as has always been thought, and that an additional mechanism causing dephosphorylation exists. Direct monitoring of the phosphorylation state of enzyme IIA^Glc also showed that many carbohydrates that are not transported by the PTS caused dephosphorylation during growth. In the case of glucose 6-phosphate, it was shown that transport and the first metabolic step are not involved in the dephosphorylation of enzyme IIA^Glc, but that later steps in the glycolysis are essential. Evidence is provided that the [PEP]–[pyruvate] ratio, the driving force for the phosphorylation of the PTS proteins, determines the phosphorylation state of enzyme IIA^Glc. The implications of these new findings for our view on catabolite repression and inducer exclusion are discussed.     

Regulation of carbohydrate transport activities in Salmonella typhimurium: use of the phosphoglycerate transport system to energize solute uptake.

The phosphoglycerate transport system was employed to supply energy-depleted, lysozyme-treated Salmonella typhimurium cells with a continuous intracellular source of phosphoenolpyruvate. When the cells had been induced to high levels of the phosphoglycerate transport system, a low extracellular concentration of phosphoenolpyruvate (0.1 mM) half maximally stimulated uptake of methyl alpha-glucoside via the phosphoenolpyruvate:sugar phosphotransferase system. If the phosphoglycerate transport system was not induced before energy depletion, 100 times this concentration of phosphoenolpyruvate was required for half-maximal stimulation. Phosphoenolpyruvate could not be replaced by other energy sources if potassium fluoride (an inhibitor of enolase) was present. Inhibition of [14C]-glycerol uptake into energy-depleted cells by methyl alpha-glucoside was demonstrated. A concentration of phosphoenolpyruvate which stimulated methyl alpha-glucoside accumulation counteracted the inhibitory effect of the glucoside. In the presence of potassium fluoride, phosphoenolpyruvate could not be replaced by other energy sources. Inhibition of glycerol uptake by methyl alpha-glucoside in intact untreated cells was also counteracted by phosphoenolpyruvate, but several energy sources were equally effective; potassium fluoride was without effect. These and other results were interpreted in terms of a mechanism in which the relative proportions of the phosphorylated and nonphosphorylated forms of a cell constituent influence the activity of the glycerol transport system.     

Lactose metabolism in Streptococcus lactis: phosphorylation of galactose and glucose moieties in vivo.

Starved cells of Streptococcus lactis ML3 grown previously on lactose, galactose, or maltose were devoid of adenosine 5'-triphosphate contained only three glycolytic intermediates: 3-phosphoglycerate, 2-phosphoglycerate, and phosphoenolpyruvate (PEP). The three metabolites (total concentration, ca 40 mM) served as the intracellular PEP potential for sugar transport via PEP-dependent phosphotransferase systems. When accumulation of [14C]lactose by iodoacetate-inhibited starved cells was abolished within 1 s of commencement of transport, a phosphorylated disaccharide was identified by autoradiography. The compound was isolated by ion-exchange (borate) chromatography, and enzymatic analysis showed that the derivative was 6-phosphoryl-O-beta-D-galactopyranosyl (1 leads to 4')-alpha-D-glucopyranose (lactose 6-phosphate). After maximum lactose uptake (ca. 15 mM in 15 s) the cells were collected by membrane filtration and extracted with trichloroacetic acid. Neither free nor phosphorylated lactose was detected in cell extracts, but enzymatic analysis revealed high levels of galactose 6-phosphate and glucose 6-phosphate. The starved organisms rapidly accumulated glucose, 2-deoxy-D-glucose, methyl-beta-D-thiogalactopyranoside, and o-nitrophenyl-beta-D-galactopyranoside in phosphorylated form to intracellular concentrations of 32, 32, 42, and 38.5 mM, respectively. In contrast, maximum accumulation of lactose (ca. 15 mM) was only 40 to 50% that of the monosaccharides. From the stoichiometry of PEP-dependent lactose transport and the results of enzymatic analysis, it was concluded that (i) ca. 60% of the PEP potential was utilized via the lactose phosphotransferase system for phosphorylation of the galactosyl moiety of the disaccharide, and (ii) the residual potential (ca. 40%) was consumed during phosphorylation of the glucose moiety.     

Analysis of regulatory mechanisms controlling the activity of the hexitol transport systems in Escherichia coli K12.

Summary The transport systems (enzymeII-complexes of the PEP-dependent sugar: phosphotransferase system) coded for the mtl and in the gut (srl) operon of E. coli K12 have been shown to be the pacemaker enzymes in the catabolism of the two hexitols D-mannitol and D-glucitol, respecitively. As for other pacemaker enzymes their activity is regulated in a complex way: (i) via competitive inhibition by analogues. (ii) via non-competitive (feedback) inhibition by the simultaneous, rapid uptake of a number of structurally related or non-related carbohydrates, regardless if these are transported by group translocation, active transport or facilitated diffusion. This type of inhibition is strongly reinforced, if the inhibitory carbohydrates are converted efficiently into hexose-phosphates at the same time. Among these, predominantly D-fructose-6-P seems to act as a feedback inhibitor for the hexitol specific enzymeII-complexes; (iii) inhibition of hexitol-phosphate accumulation by rapid exchange with and/or chase out of the cell by D-glucose-6-P. The influence of additional parameters (PEP level, PHPr level) and indications for the existence of further mechanisms controlling the activity of hexitol and other carbohydrate transport systems will be discussed, as will be the part the inhibitory mechanisms described above play in the phenomena of transient repression and inducer exclusion.     

Distribution of the phosphoenolpyruvate:glucose phosphotransferase system in fermentative bacteria.

A number of selected fermentative bacteria were surveyed for the presence of the phosphoenolpyruvate:glucose phosphotransferase system, with particular attention to those organisms which ferment glucose by pathways other than the Embden-Meyerhof-Parnas pathway. The phosphoenolpyruvate:glusoe phosphotransferase system was found in all homofermentative lactic acid bacteria tested that ferment glucose via the Embden-Meyerhof-Parnas pathway, but in none of a group of heterofermentative species of Lactobacillus or Leuconostoc, which ferment glucose via the phosphoketolase pathway. A phosphoenolpyruvate:glucose phosphotransferase system was also absent in Zymomonas mobilis, which ferments glucose via an anaerobic Entner-Doudoroff pathway. It thus appears that the phosphotransferase mode of glucose transport is limited to bacteria with the Embden-Meyerhof-Parnas mode of glucose fermentation.     

Transport and Metabolism of Lactose, Glucose, and Galactose in Homofermentative Lactobacilli

A number of species of lactobacilli were examined for their ability to ferment both the glucose and galactose moieties of lactose. Lactobacillus helveticus strains metabolized both the glucose and galactose moieties, whereas L. bulgaricus, L. lactis, and L. acidophilus strains metabolized only the glucose moiety and released galactose into the growth medium. All four species tested contained β-galactosidase activity, and no significant phospho-β-galactosidase activity was observed. L. bulgaricus and L. helveticus had a phosphoenolpyruvate (PEP):glucose phosphotransferase system for the uptake of glucose, but no evidence for a PEP:lactose phosphotransferase or PEP:galactose phosphotransferase system was obtained.     

Involvement of the central loop of the lactose permease of Escherichia coli in its allosteric regulation by the glucose-specific enzyme IIA of the phosphoenolpyruvate-dependent phosphotransferase system.

Allosteric regulation of several sugar transport systems such as those specific for lactose, maltose and melibiose in Escherichia coli (inducer exclusion) is mediated by the glucose-specific enzyme IIA (IIAGlc) of the phosphoenolpyruvate:sugar phosphotransferase system (PTS). Deletion mutations in the cytoplasmic N and C termini of the lactose permease protein, LacY, and replacement of all cysteine residues in LacY with other residues did not prevent IIAGlc-mediated inhibition of lactose uptake, but several point and insertional mutations in the central cytoplasmic loop of this permease abolished transport regulation and IIAGlc binding. The results substantiate the conclusion that regulation of the lactose permease in E. coli by the PTS is mediated by a primary interaction of IIAGlc with the central cytoplasmic loop of the permease.     

Enzymes II of the phosphotransferase system do not catalyze sugar transport in the absence of phosphorylation.

In Salmonella typhimurium, glucose, mannose, and fructose are normally transported and phosphorylated by the phosphoenolpyruvate:sugar phosphotransferase system. We have investigated the transport of these sugars and their non-metabolizable analogs in mutant strains lacking the phospho-carrier proteins of the phosphoenolpyruvate:sugar phosphotransferase system, the enzymes I and HPr, to determine whether the sugar-specific, membrane-bound components of the phosphonenolpyruvate: sugar phosphotransferase system, the enzymes II, can catalyze the uptake of these sugars in the absence of phosphorylation. This process does not occur. We have also isolated mutant strains which lack enzyme I and HPr, but have regained the ability to grow on mannose or fructose. These mutants contained elevated levels of mannokinase (fructokinase). In addition, growth on mannose required constitutive synthesis of the galactose permease. When strains were constructed which lacked the galactose permease, they were unable to grow even on high concentrations of mannose, although elevated levels of mannokinase (fructokinase) were present. These results substantiate the conclusion that the enzymes II of the phosphoenolpyruvate:sugar phosphotransferase system are unable to carry out facilitated diffusion.     

Thermophysiology of Streptococcus mutans and related lactic-acid bacteria

The temperature ranges for growth of Streptococcus mutans GS-5 and S. sobrinus 6715 were found to be very narrow, from about 30 to 47 °C, with optimal growth around 37 °C. Thus, the organisms showed little potential to grow in the environment outside of the animal host. In contrast wider ranges were found for Enterococcus hirae, S. rattus and S. sanguis. Detailed study of S. mutans GS-5 showed that energetic coupling, reflected in yields of biomass per mol of glucose utilized, were not greatly affected by changes in temperature within the growth range. However, since glycolysis occurred over a wider temperature range (about 10 to 52 °C) than growth, yield values dropped to zero at temperatures above or below the growth range. The temperature range for glycolysis could be related to temperature sensitivity of the phosphoenolpyruvate: sugar phosphotransferase system for sugar uptake. F-ATPases were active over a similar range of temperatures, but with a broad optimal range from about 30 to 50 °C. Proton permeability of S. mutans increased steadily with temperature in a manner similar to that of other mesophilic bacteria, such as Escherichia coli. Growth of the bacteria in media supplemented with various fatty acids had major effects on proton permeabilities but the effects were not well reflected by changes in growth or glycolysis of the bacteria. The overall conclusions were that S. mutans is a typical mesophile in relation to membrane and catabolic functions but its narrow temperature range for growth is related to temperature sensitivities of anabolic systems.     

Phosphoenolpyruvate-dependent phosphorylation of alpha-methylglucoside in Streptococcus sanguis ATCC 10556

Spontaneous mutants defective in some undefined membrane components of the phosphoenolpyruvate:glucose phosphotransferase system were isolated by plating cells of Streptococcus sanguis ATCC 10556 onto an agar containing lactose and 10 mM 2-deoxyglucose. Toluenized cells of these mutants were defective in their ability to catalyse the phosphoenolpyruvate-dependent phosphorylation of 2-deoxyglucose but were still able to phosphorylate alpha-methylglucoside. The phosphorylation of alpha-methylglucoside was essentially dependent on phosphoenolpyruvate and required the presence of both soluble and membrane components. It was concluded that S. sanguis possessed two different phosphoenolpyruvate:glucose phosphotransferase systems.