Regulation of carbohydrate uptake and adenylate cyclase activity mediated by the enzymes II of the phosphoenolpyruvate: sugar phosphotransferase system in Escherichia coli

Shaping bacterial gene expression by physiological and proteome allocation constraints

Networks of molecular regulators are often the primary objects of focus in the study of gene regulation, with the machinery of protein synthesis tacitly relegated to the background. Shifting focus to the constraints imposed by the allocation of protein synthesis flux reveals surprising ways in which the actions of molecular regulators are shaped by physiological demands. Using carbon catabolite repression as a case study, we describe how physiological constraints are sensed through metabolic fluxes and how flux-controlled regulation gives rise to simple empirical relations between protein levels and the rate of cell growth.

A Class IV Adenylate Cyclase, CyaB, Is Required for Capsule Polysaccharide Production and Biofilm Formation in Vibrio parahaemolyticus

Cyclic AMP (cAMP) receptor protein (CRP), encoded by crp, is a global regulator that is activated by cAMP, a second messenger synthesized by a class I adenylate cyclase (AC-I) encoded by cyaA in Escherichia coli. cAMP-CRP is required for growth on nonpreferred carbon sources and is a global regulator. We constructed in-frame nonpolar deletions of the crp and cyaA homologs in Vibrio parahaemolyticus and found that the Δcrp mutant did not grow in minimal media supplemented with nonpreferred carbon sources, but the ΔcyaA mutant grew similarly to the wild type. Bioinformatics analysis of the V. parahaemolyticus genome identified a 181-amino-acid protein annotated as a class IV adenylate cyclase (AC-IV) named CyaB, a member of the CYTH protein superfamily. AC-IV phylogeny showed that CyaB was present in Gammaproteobacteria and Alphaproteobacteria as well as Planctomycetes and Archaea. Only the bacterial CyaB proteins contained an N-terminal motif, HFxxxxExExK, indicative of adenylyl cyclase activity. Both V. parahaemolyticus cyaA and cyaB genes functionally complemented an E. coli ΔcyaA mutant. The Δcrp and ΔcyaB ΔcyaA mutants showed defects in growth on nonpreferred carbon sources and in swimming and swarming motility, indicating that cAMP-CRP is an activator. The ΔcyaA and ΔcyaB single mutants had no defects in these phenotypes, indicating that AC-IV complements AC-I. Capsule polysaccharide and biofilm production assays showed significant defects in the Δcrp, ΔcyaBΔcyaA, and ΔcyaB mutants, whereas the ΔcyaA strain behaved similarly to the wild type. This is consistent with a role of cAMP-CRP as an activator of these phenotypes and establishes a cellular role for AC-IV in capsule and biofilm formation, which to date has been unestablished. IMPORTANCE Here, we characterized the roles of CRP and CyaA in V. parahaemolyticus, showing that cAMP-CRP is an activator of metabolism, motility, capsule production, and biofilm formation. These results are in contrast to cAMP-CRP in V. cholerae, which represses capsule and biofilm formation. Previously, only an AC-I CyaA had been identified in Vibrio species. Our data showed that an AC-IV CyaB homolog is present in V. parahaemolyticus and is required for optimal growth. The data demonstrated that CyaB is essential for capsule production and biofilm formation, uncovering a physiological role of AC-IV in bacteria. The data showed that the cyaB gene was widespread among Vibrionaceae species and several other Gammaproteobacteria, but in general, its phylogenetic distribution was limited. Our phylogenetic analysis also demonstrated that in some species the cyaB gene was acquired by horizontal gene transfer.

Competition for nutritional resources masks the true frequency of bacterial mutants

Background

It is widely assumed that all mutant microorganisms present in a culture are able to grow and form colonies, provided that they express the features required for selection. Unlike wild-type Escherichia coli, PHO-constitutive mutants overexpress alkaline phosphatase and hence can hydrolyze glycerol-2-phosphate (G2P) to glycerol and form colonies on plates having G2P as the sole carbon source. These mutations mostly occur in the pst operon. However, the frequency of PHO-constitutive colonies on the G2P selective plate is exceptionally low.

Results

We show that the rate in which spontaneous PHO-constitutive mutations emerge is about 8.0 × 10⁻⁶/generation, a relatively high rate, but the growth of most existing mutants is inhibited by their neighboring wild-type cells. This inhibition is elicited only by non-mutant viable bacteria that can take up and metabolize glycerol formed by the mutants. Evidence indicates that the few mutants that do form colonies derive from microclusters of mutants on the selective plate. A mathematical model that describes the fate of the wild-type and mutant populations under these circumstances supports these results.

Conclusion

This scenario in which neither the wild-type nor the majority of the mutants are able to grow resembles an unavoidable “tragedy of the commons” case which results in the collapse of the majority of the population. Cooperation between rare adjacent mutants enables them to overcome the competition and eventually form mutant colonies. The inhibition of PHO-constitutive mutants provides an example of mutant frequency masked by orders of magnitude due to a competition between mutants and their ancestral wild-type cells. Similar “tragedy of the commons-like” cases may occur in other settings and should be taken into consideration while estimating true mutant frequencies and mutation rates.

Supplementary Information

The online version contains supplementary material available at (doi:10.1186/s12915-020-00913-1).

Resource allocation and metabolism: the search for governing principles

Elucidating strategies of resource allocation and metabolism is crucial for a better understanding of microbial phenotypes. In particular, uncovering the governing principles underlying these processes would be a crucial step for achieving a central aim of systems microbiology, which is to quantitatively predict phenotypes of microbial cells or entire populations in diverse conditions. Here, some of the key concepts for understanding cellular resource allocation and metabolism that have been suggested over the past years are reviewed. In particular, recent experimental studies that have shown how phenotypic patterns from orthogonal genetic and environmental perturbations can help to differentiate between competing hypotheses and their respective predictions are discussed. Phenomenological models have proven to be a valuable addition to genome-scale models, capable of making quantitative predictions with only few parameters and having aided the identification of molecular mechanisms.

Comprehensive analysis of central carbon metabolism illuminates connections between nutrient availability, growth rate, and cell morphology in Escherichia coli

Bacterial morphology is a complex trait that is highly sensitive to changes in the environment. For heterotrophic organisms, such as Escherichia coli, increases in nutrient levels are frequently accompanied by several-fold increases in both size and growth rate. Despite the dramatic nature of these changes, how alterations in nutrient availability translate into changes in growth and morphology remains a largely open question. To understand the signaling networks coupling nutrient availability with size and shape, we examined the impact of deletions in the entirety of non-essential central carbon metabolic genes on E. coli growth rate and cell size. Our data reveal the presence of multiple metabolic nodes that play important yet distinctive roles in dictating biosynthetic capacity and shaping cell morphology. Specifically, perturbations of acetyl-CoA metabolism impact cell size and division through changes in fatty acid synthesis. Additionally, we identify a genetic pathway linking glucose levels to cell width through the signaling molecule cyclic-AMP. Together our findings highlight a surprising diversity of factors and mechanisms contributing to growth potential and cell morphology, providing a foundation for further studies.

Often taken for granted, the shape of bacterial cells is a complex trait that is highly sensitive to environmental perturbations. Nutrients in particular, strongly impact bacterial morphology together with growth rate. The ubiquitous, rod-shaped bacteria Escherichia coli increases both length and width several fold upon a shift from nutrient poor to nutrient rich medium, a change accompanied by an equally dramatic increase in growth rate. Central carbon metabolism is an obvious site for the integration of nutrient dependent signals that dictate cell size and shape. To develop a clearer picture of the molecular mechanisms coupling nutrient assimilation with cell growth and morphology, we screened the entirety of non-essential carbon metabolic genes for their contribution to growth rate and cell shape. Our data reveal the presence of multiple regulatory circuits coordinating different metabolic pathways with specific aspects of cell growth and morphology. Together, these data firmly establish a role for central carbon metabolism as an environmentally sensitive sculptor of bacterial cells.

Nutrient Composition Promotes Switching between Pellicle and Bottom Biofilm in Salmonella

Salmonella is one of the most frequently reported causes of foodborne illness worldwide. Non-typhoidal serovars cause gastroenteritis in humans. Salmonella can grow on surfaces forming biofilms, contributing to its persistence since biofilms are difficult to eradicate due to the high resistance to antimicrobials and disinfectants. It has been described that there are two crucial biofilm promoting factors in Salmonella: curli and cellulose. The expression of both factors is coordinately regulated by the transcriptional regulator CsgD. Most biofilm studies of Salmonella have been performed by growing bacteria in low osmolarity rich medium and low temperature (25°C). In such conditions, the biofilm is formed at the air–liquid interface (pellicle biofilm). Remarkably, when Salmonella grow in minimal medium, biofilm formation switches from the air–liquid interface to the solid–liquid interface (bottom biofilm). In this report, the switching between pellicle and bottom biofilm has been characterized. Our data indicate that curli, but not cellulose, is crucial for the formation of both kinds of biofilms. In minimal medium, conditions promoting formation of bottom biofilm, a high transcriptional expression of csgD and consequently of the genes involved in the synthesis of curli and cellulose was detected. The nutritional status of the cells seems to be pivotal for the spatial distribution of the biofilms formed. When bacteria is growing in minimal medium the addition of amino acids downregulates the expression of csgB and causes the switch between bottom and pellicle biofilm. The crosstalk between general metabolism and biofilm formation is also highlighted by the fact that the metabolic sensor cAMP modulates the type of biofilm generated by Salmonella. Moreover, cAMP regulates transcriptional expression of csgD and stimulates pellicle biofilm formation, suggesting that the physiological conditions define the type of biofilm formed by Salmonella. The consequences of the switching between pellicle and bottom biofilm during either infection or survival in natural environments remain undercover.

Rethinking the roles of CRP, cAMP, and sugar-mediated global regulation in the Vibrionaceae

Many proteobacteria modulate a suite of catabolic genes using the second messenger cyclic 3', 5'-AMP (cAMP) and the cAMP receptor protein (CRP). Together, the cAMP-CRP complex regulates target promoters, usually by activating transcription. In the canonical model, the phosphotransferase system (PTS), and in particular the EIIA(Glc) component for glucose uptake, provides a mechanistic link that modulates cAMP levels depending on glucose availability, resulting in more cAMP and activation of alternative catabolic pathways when glucose is unavailable. Within the Vibrionaceae, cAMP-CRP appears to play the classical role in modulating metabolic pathways; however, it also controls functions involved in natural competence, bioluminescence, pheromone signaling, and colonization of animal hosts. For this group of marine bacteria, chitin is an ecologically relevant resource, and chitin's monomeric sugar N-acetylglucosamine (NAG) supports robust growth while also triggering regulatory responses. Recent studies with Vibrio fischeri indicate that NAG and glucose uptake share EIIA(Glc), yet the responses of cAMP-CRP to these two carbon sources are starkly different. Moreover, control of cAMP levels appears to be more dominantly controlled by export and degradation. Perhaps more surprisingly, although CRP may require cAMP, its activity can be controlled in response to glucose by a mechanism independent of cAMP levels. Future studies in this area promise to shed new light on the role of cAMP and CRP.

Growth on glucose decreases cAMP-CRP activity while paradoxically increasing intracellular cAMP in the light-organ symbiont Vibrio fischeri

Proteobacteria often co-ordinate responses to carbon sources using CRP and the second messenger cyclic 3', 5'-AMP (cAMP), which combine to control transcription of genes during growth on non-glucose substrates as part of the catabolite-repression response. Here we show that cAMP-CRP is active and important in Vibrio fischeri during colonization of its host squid Euprymna scolopes. Moreover, consistent with a classical role in catabolite repression, a cAMP-CRP-dependent reporter showed lower activity in cells grown in media amended with glucose rather than glycerol. Surprisingly though, intracellular cAMP levels were higher in glucose-grown cells. Mutant analyses were consistent with predictions that CyaA was responsible for cAMP generation, that the EIIA(Glc) component of glucose transport could enhance cAMP production and that the phophodiesterases CpdA and CpdP consumed intracellular and extracellular cAMP respectively. However, the observation of lower cAMP levels in glycerol-grown cells seemed best explained by changes in cAMP export, via an unknown mechanism. Our data also indicated that cAMP-CRP activity decreased during growth on glucose independently of crp's native transcriptional regulation or cAMP levels. We speculate that some unknown mechanism, perhaps carbon-source-dependent post-translational modulation of CRP, may help control cAMP-CRP activity in V.fischeri.

Biochemical characterization of a nitrogen-type phosphotransferase system reveals that enzyme EI(Ntr) integrates carbon and nitrogen signaling in Sinorhizobium meliloti

In Sinorhizobium meliloti, catabolite repression is influenced by a noncanonical nitrogen-type phosphotransferase system (PTS(Ntr)). In this PTS(Ntr), the protein HPr is phosphorylated on histidine-22 by the enzyme EI(Ntr) and the flux of phosphate through this residue onto downstream proteins leads to an increase in succinate-mediated catabolite repression (SMCR). In order to explore the molecular determinants of HPr phosphorylation by EI(Ntr), both proteins were purified and the activity of EI(Ntr) was measured. Experimentally determined kinetic parameters of EI(Ntr) activity were significantly slower than those determined for the carbohydrate-type EI in Escherichia coli. Enzymatic assays showed that glutamine, a signal of nitrogen availability in many Gram-negative bacteria, strongly inhibits EI(Ntr). Binding experiments using the isolated GAF domain of EI(Ntr) (EIGAF) showed that it is the domain responsible for detection of glutamine. EI(Ntr) activity was not affected by α-ketoglutarate, and no binding between the EIGAF and α-ketoglutarate could be detected. These data suggest that in S. melilloti, EI(Ntr) phosphorylation of HPr is regulated by signals from both carbon metabolism (phosphoenolpyruvate) and nitrogen metabolism (glutamine).

Coordination of bacterial proteome with metabolism by cyclic AMP signalling

The cyclic AMP (cAMP)-dependent catabolite repression effect in Escherichia coli is among the most intensely studied regulatory processes in biology. However, the physiological function(s) of cAMP signalling and its molecular triggers remain elusive. Here we use a quantitative physiological approach to show that cAMP signalling tightly coordinates the expression of catabolic proteins with biosynthetic and ribosomal proteins, in accordance with the cellular metabolic needs during exponential growth. The expression of carbon catabolic genes increased linearly with decreasing growth rates upon limitation of carbon influx, but decreased linearly with decreasing growth rate upon limitation of nitrogen or sulphur influx. In contrast, the expression of biosynthetic genes showed the opposite linear growth-rate dependence as the catabolic genes. A coarse-grained mathematical model provides a quantitative framework for understanding and predicting gene expression responses to catabolic and anabolic limitations. A scheme of integral feedback control featuring the inhibition of cAMP signalling by metabolic precursors is proposed and validated. These results reveal a key physiological role of cAMP-dependent catabolite repression: to ensure that proteomic resources are spent on distinct metabolic sectors as needed in different nutrient environments. Our findings underscore the power of quantitative physiology in unravelling the underlying functions of complex molecular signalling networks.

Selective regulation of ptsG expression by Fis. Formation of either activating or repressing nucleoprotein complex in response to glucose

Transcription of ptsG encoding glucose-specific permease, enzyme IICB(Glc), in Escherichia coli is initiated from two promoters, P1 and P2. ptsG transcription is repressed by Mlc, a glucose-inducible regulator of carbohydrate metabolism. The regulation of ptsG P1 transcription is also under positive control by cyclic AMP receptor protein and cyclic AMP complex (CRP.cAMP) as observed in other Mlc regulon. We report here that Fis, one of the nucleoid-associated proteins, plays a key role in glucose induction of Mlc regulon. ptsG transcription was induced when wild-type cells were grown in the presence of glucose. However, in a fis mutant, the basal level of ptsG transcription was higher but decreased when cells were grown in the presence of glucose, which implies the possibility of regulatory interactions among Fis, Mlc, and CRP.cAMP. Footprinting experiments with various probes and transcription assays revealed that Fis assists both Mlc repression and CRP.cAMP activation of ptsG P1 through the formation of Fis.CRP.Mlc or Fis.CRP nucleoprotein complexes at ptsG P1 promoter depending on the availability of glucose in the growth medium. ptsG P2 transcription was inhibited by Fis and Mlc. Tighter Mlc repression and enhanced CRP.cAMP activation of ptsG P1 by Fis enable cells to regulate Mlc regulon efficiently by selectively controlling the concentration of enzyme IICB(Glc) that modulates Mlc activity.

Soluble sugar permeases of the phosphotransferase system in Escherichia coli: evidence for two physically distinct forms of the proteins in vivo

The bacterial phosphoenolpyruvate-dependent phosphotransferase system (PTS) consists of a set of cytoplasmic energy-coupling proteins and various integral membrane permeases/sugar phosphotransferases, each specific for a different sugar. We have conducted biochemical analyses of three PTS permeases (enzymes II), the glucose permease (IIGlc), the mannitol permease (IIMtl) and the mannose permease (IIMan). These enzymes each catalyse two vectorial/chemical reactions, sugar phosphorylation using phosphoenolpyruvate (PEP) as the phosphoryl donor, dependent on enzyme I, HPr and IIA as well as IIBC (the PEP reaction), and transphosphorylation using a sugar phosphate (glucose-6-P for IIGlc and IIMan; mannitol-1-P for IIMtl) as the phosphoryl donor, dependent only on IIBC (the TP reaction). When crude extracts of French-pressed or osmotically shocked Escherichia coli cells are centrifuged in an ultracentrifuge at high speed, 5-20% of the enzyme II activity remains in the high-speed supernatant, and passage through a gel filtration column gives two activity peaks, one in the void volume exhibiting high PEP-dependent and TP activities, and a second included peak with high PEP-dependent activity and high (IIMan), moderate (IIGlc) or negligible (IIMtl) TP activities. Both log and stationary phase cells exhibit comparable relative amounts of pelletable and soluble enzyme II activities, but long-term exposure of cells to chloramphenicol results in selective loss of the soluble fraction with retention of much of the pelleted activity concomitant with extensive protein degradation. Short-term exposure of cells to chloramphenicol results in increased activities in both fractions, possibly because of increased lipid association, with more activation in the soluble fraction than in the pelleted fraction. Western blot analyses show that the soluble IIGlc exhibits a subunit size of about 45 kDa, and all three soluble enzymes II elute from the gel filtration column with apparent molecular weights of 40-50 kDa. We propose that enzymes II of the PTS exist in two physically distinct forms in the E. coli cell, one tightly integrated into the membrane and one either soluble or loosely associated with the membrane. We also propose that the membrane-integrated enzymes II are largely dimeric, whereas the soluble enzymes II, retarded during passage through a gel filtration column, are largely monomeric.

Expression of ptsG, the gene for the major glucose PTS transporter in Escherichia coli, is repressed by Mlc and induced by growth on glucose

The gene for the glucose-specific transporter of the phosphotransferase system, ptsG, is expressed from two promoters separated by 141 bp. The expression of the major, shorter transcript is very strongly dependent upon cAMP/CAP. However, unlike other CAP-activated genes, the expression of ptsG is higher in glucose media than in glycerol, implying that ptsG is controlled by a glucose-inducible regulator. A mutation in the mlc gene greatly enhances ptsG expression in a glycerol-grown culture but has no effect on ptsG expression during growth on glucose. The mlc gene encodes a transcriptional regulator that has been shown to affect the expression of manXYZ and malT. ptsG mRNA levels are lower in the mlc strain grown on glucose than in the same strain grown on glycerol. This is presumably because of the greater catabolite repression in the glucose culture than in glycerol. The final level of expression of ptsG in a mlc+ strain in glucose is a compromise between specific induction by glucose and generalized catabolite repression. The result is that ptsG expression is very similar in glucose-grown cultures of wild-type and mlc strains. The Mlc protein binds to two sites centred at -6 and -175 upstream of the major ptsG transcript. CAP binds at -40.5 compared with this site, typical of class II CAP-regulated promoters, and the binding of CAP and Mlc is co-operative.

Quantification of the regulation of glycerol and maltose metabolism by IIAGlc of the phosphoenolpyruvate-dependent glucose phosphotransferase system in Salmonella typhimurium

The amount of IIAGlc, one of the proteins of the phosphoenolpyruvate:glucose phosphotransferase system (PTS), was modulated over a broad range with the help of inducible expression plasmids in Salmonella typhimurium. The in vivo effects of different levels of IIAGlc on glycerol and maltose metabolism were studied. The inhibition of glycerol uptake, by the addition of a PTS sugar, was sigmoidally related to the amount of IIAGlc. For complete inhibition of glycerol uptake, a minimal ratio of about 3.6 mol of IIAGlc to 1 mol of glycerol kinase (tetramer) was required. Varying the level of IIAGlc (from 0 to 1,000% of the wild-type level) did not affect the growth rate on glycerol, the rate of glycerol uptake, or the synthesis of glycerol kinase. In contrast, the growth rate on maltose, the rate of maltose uptake, and the synthesis of the maltose-binding protein increased two- to fivefold with increasing levels of IIAGlc. In the presence of cyclic AMP, the maximal levels were obtained at all IIAGlc concentrations. The synthesis of the MalK protein, the target of IIAGlc, was not affected by varying the levels of IIAGlc. The inhibition of maltose uptake was sigmoidally related to the amount of IIAGlc. For complete inhibition of maltose uptake by a PTS sugar, a ratio of about 18 mol of IIAGlc to 1 mol of MalK protein (taken as a dimer) was required.

Effect of Bdellovibrio bacteriovorus infection on the phosphoenolpyruvate:sugar phosphotransferase system in Escherichia coli: evidence for activation of cytoplasmic proteolysis

Intact cells of Bdellovibrio bacteriovorus strain 109J were found to be incapable of taking up 14C-methyl alpha-glucoside, mannitol or fructose, and extracts derived from these cells exhibited negligible activities of the protein components of the phosphoenolpyruvate:sugar phosphotransferase system (PTS). Escherichia coli strain ML35 cells exhibited high in vivo sugar uptake activities that were progressively lost over a period of 2 h at 30 degrees C following the entry of B. bacteriovorus into the periplasm of E. coli. In vitro complementation assays revealed that the E. coli PTS enzymes, enzyme I, HPr, and the glucose- and mannitol-specific enzymes II, were all lost almost in parallel with the disappearance of uptake activity. Thus, loss of activity in vivo was not due to membrane leakiness, energy depletion, or preferential inhibition or inactivation of any one protein component of the PTS. Instead, loss of PTS activity was attributed to digestion of the protein constituents of the system by proteases present in the cytoplasm of the host cell after bdellovibrio entry. Both ethylenediaminetetraacetate and phenylmethylsulphonyl fluoride partially protected against inactivation in vitro, and the two inhibitors together gave full protection, suggesting that both metallo- and seryl-proteases were responsible for the inactivation. Protease activity increased progressively with time following bdellovibrio entry and appeared to degrade the E. coli PTS enzymes in vivo. Preliminary evidence suggested that the proteases responsible for PTS enzyme degradation may be encoded by the B. bacteriovorus chromosome.

Properties of the glucose phosphotransferase system of Clostridium acetobutylicum NCIB 8052

The glucose phosphotransferase system (PTS) of Clostridium acetobutylicum was studied by using cell extracts. The system exhibited a Km for glucose of 34 microM, and glucose phosphorylation was inhibited competitively by mannose and 2-deoxyglucose. The analogs 3-O-methylglucoside and methyl alpha-glucoside did not inhibit glucose phosphorylation significantly. Activity showed no dependence on Mg2+ ions or on pH in the range 6.0 to 8.0. The PTS comprised both soluble and membrane-bound proteins, which interacted functionally with the PTSs of Clostridium pasteurianum, Bacillus subtilis, and Escherichia coli. In addition to a membrane-bound enzyme IIGlc, sugar phosphorylation assays in heterologous systems incorporating extracts of pts mutants of other organisms provided evidence for enzyme I, HPr, and IIIGlc components. The HPr was found in the soluble fraction of C. acetobutylicum extracts, whereas enzyme I, and probably also IIIGlc, was present in both the soluble and membrane fractions, suggesting a membrane location in the intact cell.

Uncommon pathways of metabolism among lactic acid bacteria

A small number of lactic acid bacteria possess the ability to derive energy from organic molecules not utilized by the vast majority of representatives of this large group of microorganisms. Thus, strains of Lactobacillus casei and enterococci readily grow at the expense of substrates such as gluconate, malate and pentitols. Transport of gluconate and pentitols is catalysed by phosphotransferase systems unique to these bacteria. Similarly, the initial steps in pentitol dissimilation are mediated by enzymes found only in Lb. casei and Streptococcus avium.

Protein phosphorylation and allosteric control of inducer exclusion and catabolite repression by the bacterial phosphoenolpyruvate: sugar phosphotransferase system

The bacterial phosphotransferase system (PTS) functions in a variety of regulatory capacities. One of the best characterized of these is the process by which the PTS regulates inducer uptake and catabolite repression. Early genetic and physiological evidence supported a mechanism whereby the phosphorylation state of an enzyme of the PTS, the enzyme III specific for glucose (IIIGlc), allosterically inhibits the activities of a number of permeases and catabolic enzymes, the lactose, galactose, melibiose, and maltose permeases, as well as glycerol kinase. Extensive biochemical evidence now supports this model. Evidence is also available showing that substrate binding to those target proteins enhances their affinities for IIIGlc. In the case of the lactose permease, this positively cooperative interaction represents a well documented example of transmembrane signaling, demonstrated both in vivo and in vitro. Although the PTS-mediated regulation of cyclic AMP synthesis (catabolite repression) is not as well defined from a mechanistic standpoint, a model involving allosteric activation of adenylate cyclase by phospho-IIIGlc, together with the evidence supporting it, is presented. These regulatory mechanisms may prove to be operative in gram-positive as well as gram-negative bacteria, but the former organisms may have introduced variations on the theme by covalently attaching IIIGlc-like moieties to some of the target permeases and catabolic enzymes. It appears likely that the general process of PTS-catalyzed protein phosphorylation-dephosphorylation will prove to be important to the regulation of numerous bacterial physiological processes, including chemotaxis, intermediary metabolism, gene transcription, and virulence.

Evidence against direct involvement of cyclic GMP or cyclic AMP in bacterial chemotactic signaling

Defects in phosphotransferase chemotaxis in cya and cpd mutants previously cited as evidence of a cyclic GMP or cyclic AMP intermediate in signal transduction were not reproduced in a study of chemotaxis in Escherichia coli and Salmonella typhimurium. In cya mutants, which lack adenylate cyclase, the addition of cyclic AMP was required for synthesis of proteins that were necessary for phosphotransferase transport and chemotaxis. However, the induced cells retained normal phosphotransferase chemotaxis after cyclic AMP was removed. Phosphotransferase chemotaxis was normal in a cpd mutant of S. typhimurium that has elevated levels of cyclic GMP and cyclic AMP. S. typhimurium crr mutants are deficient in enzyme III glucose, which is a component of the glucose transport system, and a regulator of adenylate cyclase. After preincubation with cyclic AMP, the crr mutants were deficient in enzyme II glucose-mediated transport and chemotaxis, but other chemotactic responses were normal. It is concluded that cyclic GMP does not determine the frequency of tumbling and is probably not a component of the transduction pathway. The only known role of cyclic AMP is in the synthesis of some proteins that are subject to catabolite repression.

Enterotoxin A production in Staphylococcus aureus: inhibition by glucose

In this study, we investigated the relationship between carbohydrate metabolism and repression of staphylococcus enterotoxin A (SEA) in Staphylococcus aureus 196E and a pleiotrophic mutant derived from strain 196E. The mutant, designated at strain 196E-MA, lacked a functional phosphoenolpyruvate phosphotransferase system (PTS). The mutant produced acid, under aerobic conditions, from only glucose and glycerol. The parent strain contained an active PTS, and aerobically produced acid from a large number of carbohydrates. Prior growth in glucose led to repression of SEA synthesis in the parent strain; addition to the casamino acids enterotoxin production medium (CAS) led to more severe repression of toxin synthesis. The repression was not related to pH decreases produced by glucose metabolism. When S. aureus 196E was grown in the absence of glucose, there was inhibition of toxin production as glucose level was increased in CAS. The inhibition was related to pH decrease and was unlike the repression observed with glucose-grown strain 196E. The inhibition of SEA synthesis in mutant strain 196E-MA was approximately the same in cells grown with or without glucose and was pH related. Repression of SEA synthesis similar to that seen with glucose-grown S. aureus 196E could not be demonstrated in the mutant. In addition, glucose-grown S. aureus 196E neither synthesized beta-galactosidase nor showed respiratory activity with certain tricarboxylic acid (TCA) cycle compounds. Glucose-grown strain 196E-MA, however, did not show suppressed respiration of TCA cycle compounds; beta-galactosidase was not synthesized because the mutant lacked a functional PTS.(ABSTRACT TRUNCATED AT 250 WORDS)

Bacterial phosphotransferase system: regulation of the glucose and mannose enzymes II by sulfhydryl oxidation

We have investigated the effect of oxidizing agents on methyl alpha-glucoside phosphorylation by the Escherichia coli phosphotransferase system (PTS). Oxidizing agents inhibited methyl alpha-glucoside phosphorylation at low methyl alpha-glucoside concentrations, and the degree of inhibition was shown to decrease with increasing concentrations of methyl alpha-glucoside. Results of studies with mutant bacteria and substrate analogues of the glucose and mannose enzymes II showed that contrary to the interpretation of Robillard and Konings [Robillard, G. T., & Konings, W. N. (1981) Biochemistry 20, 5025-5032] the apparent change in the Km value for methyl alpha-glucoside phosphorylation induced by sulfhydryl oxidation is not due to the formation of a low-affinity, oxidized form of the glucose enzyme II. Rather, the results are explained by the presence of two phosphotransferase systems that phosphorylate methyl alpha-glucoside with different affinities and that are differentially sensitive to oxidizing agents. The low Km system corresponds to the glucose enzyme II, which is strongly inhibited by potassium ferricyanide, phenazine methosulfate, and plumbagin. The high Km system corresponds to the mannose enzyme II, which is less sensitive to inhibition by these oxidizing agents. This differential sensitivity to inhibition by oxidizing agents can account for the apparent Km change for methyl alpha-glucoside phosphorylation reported by Robillard and Konings. The physiological significance of sulfhydryl oxidation in the enzymes II of the PTS has yet to be ascertained.

Regulation of glycerol uptake by the phosphoenolpyruvate-sugar phosphotransferase system in Bacillus subtilis

Enteric bacteria have been previously shown to regulate the uptake of certain carbohydrates (lactose, maltose, and glycerol) by an allosteric mechanism involving the catalytic activities of the phosphoenolpyruvate-sugar phosphotransferase system. In the present studies, a ptsI mutant of Bacillus subtilis, possessing a thermosensitive enzyme I of the phosphotransferase system, was used to gain evidence for a similar regulatory mechanism in a gram-positive bacterium. Thermoinactivation of enzyme I resulted in the loss of methyl alpha-glucoside uptake activity and enhanced sensitivity of glycerol uptake to inhibition by sugar substrates of the phosphotransferase system. The concentration of the inhibiting sugar which half maximally blocked glycerol uptake was directly related to residual enzyme I activity. Each sugar substrate of the phosphotransferase system inhibited glycerol uptake provided that the enzyme II specific for that sugar was induced to a sufficiently high level. The results support the conclusion that the phosphotransferase system regulates glycerol uptake in B. subtilis and perhaps in other gram-positive bacteria.

Interactions in vivo between IIIGlc of the phosphoenolpyruvate:sugar phosphotransferase system and the glycerol and maltose uptake systems of Salmonella typhimurium

Our previous studies indicated that the ability of phosphoenolpyruvate:sugar phosphotransferase system (PTS) substrates to inhibit the uptake of glycerol or maltose in Salmonella typhimurium is dependent on the relative cellular content of the PTS-sensitive uptake system and of the PTS protein IIIGlc. Our present study confirms and extends those observations. The maltose and glycerol uptake systems are rendered (wholly or partially) insensitive to PTS inhibition by the presence of a second PTS-sensitive uptake system (respectively that for glycerol or maltose) and its substrate. Both the second PTS-sensitive uptake system and its substrate were needed for this protective effect. Galactose and the galactose permease (a PTS-insensitive transport system) did not have any effect on PTS-mediated inhibition of the maltose uptake system. The protective effect of the second PTS-sensitive uptake system and its substrate is counteracted by increasing the cellular levels of IIIGlc. Overproduction of IIIGlc in crr-plasmid-containing strains renders the glycerol and maltose uptake systems hypersensitive to inhibition by PTS substrates. We interpret our results on the basis of a stoichiometric interaction between IIIGlc and a PTS-sensitive uptake system, in which the IIIGlc--transport-system complex is inactive. Competition between two PTS-sensitive transport systems for formation of inactive complex with IIIGlc lowers the free intracellular concentration of IIIGlc resulting in a mutual protective effect against inhibition by IIIGlc.

Adenylate cyclase is required for chemotaxis to phosphotransferase system sugars by Escherichia coli

We report that in Escherichia coli, chemotaxis to sugars transported by the phosphotransferase system is mediated by adenylate cyclase, the nucleotide cyclase linked to the phosphotransferase system. We conclude that adenylate cyclase is required in this chemotaxis pathway because mutations in the cyclase gene (cya) eliminate or impair the response to phosphotransferase system sugars, even though other components of the phosphotransferase system known to be required for the detection of these sugars are relatively unaffected by such mutations. Moreover, merely supplying the mutant bacteria with the products of this enzyme, cyclic AMP and cyclic GMP, does not restore the chemotactic response. Because a residual chemotactic response is observed in certain strains with residual cyclic GMP synthesis but no cyclic AMP synthesis, it appears that the guanylate cyclase activity rather than the adenylate cyclase activity of the enzyme may be required for chemotaxis to sugars transported by the phosphotransferase system. Mutations in the cyclic nucleotide phosphodiesterase gene, which increase the level of both cyclic AMP and cyclic GMP, also reduce chemotaxis to these sugars. Therefore, it appears that control of the level of a cyclic nucleotide is critical for the chemotactic response to phosphotransferase system sugars.

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.

Role of inducer exclusion in preferential utilization of glucose over melibiose in diauxic growth of Escherichia coli

The role of inducer exclusion in diauxic growth of Escherichia coli on glucose and melibiose was investigated. The amounts of glucose and melibiose in the culture medium were determined during the diauxie. Glucose was consumed during the first growth cycle of the diauxie, and melibiose was consumed during the second cycle. The addition of adenosine 3',5'-cyclic monophosphate to the culture medium released both transient and catabolite repressions on the melibiose operon by glucose. Biphasic growth without a transient lag phase was observed in the presence of adenosine 3',5'-cyclic monophosphate. Preferential utilization of glucose over melibiose was observed even under such conditions. Thus, it is clear that inducer exclusion alone is sufficient to ensure the preferential utilization of glucose over melibiose. Similar results were obtained from a glucose-lactose diauxie. Inducer exclusion itself was not affected by adenosine 3',5'-cyclic monophosphate.

Regulation of methyl-beta-d-thiogalactopyranoside-6-phosphate accumulation in Streptococcus lactis by exclusion and expulsion mechanisms

Starved cells of Streptococcus lactis ML3 (grown previously on galactose, lactose, or maltose) accumulated methyl-beta-D-thiogalactopyranoside (TMG) by the lactose:phosphotransferase system. More than 98% of accumulated sugar was present as a phosphorylated derivative, TMG-6-phosphate (TMG-6P). When a phosphotransferase system sugar (glucose, mannose, 2-deoxyglucose, or lactose) was added to the medium simultaneously with TMG, the beta-galactoside was excluded from the cells. Galactose enhanced the accumulation of TMG-6P. Glucose, mannose, lactose, or maltose plus arginine, was added to a suspension of TMG-6P-loaded cells of S. lactis ML3, elicited rapid expulsion of intracellular solute. The material recovered in the medium was exclusively free TMG. Expulsion of galactoside required both entry and metabolism of an appropriate sugar, and intracellular dephosphorylation of TMG-6P preceded efflux of TMG. The rate of dephosphorylation of TMG-6P by permeabilized cells was increased two-to threefold by adenosine 5'-triphosphate but was strongly inhibited by fluoride. S. lactis ML3 (DGr) was derived from S. lactis ML3 by positive selection for resistance to 2-deoxy-D-glucose and was defective in the enzyme IIMan component of the glucose:phosphotransferase system. Neither glucose nor mannose excluded TMG from cells of S. lactic ML3 (DGr), and these two sugars failed to elicit TMG expulsion from preloaded cells of the mutant strain. Accumulation of TMG-6P by S. lactis ML3 can be regulation by two independent mechanisms whose activities promote exclusion or expulsion of galactoside from the cell.

Regulation of the synthesis of adenylate cyclase in Escherichia coli by the cAMP -- cAMP receptor protein complex

The synthesis of the adenylate cyclase [ATP pyrophosphatelyase-(cyclizing), E.C. 4.6.1.1.] of Escherichia coli, appears to be regulated negatively by the cAMP receptor protein, CRP. This conclusion is based on a comparison of adenylate cyclase activities measured in vitro with the rates of cAMP synthesis by intact bacteria. The activity of adenylate cyclase, depending on conditions of growth, is also regulated by CRP; this effect, however, is indirect insofar as it is mediated by a protein or proteins under CRP control.

Catabolite repression during single and multiple induction in Escherichia coli

Intracellular concentration of cAMP regulates the synthesis of enzymes sensitive to catabolite repression. The relationship between the single and multiple induction of beta-galactosidase (EC 3.2.1.23), L-tryptophanase (EC 4.1.99.1), D-serine deaminase (EC 4.2.1.14), L-asparaginase (EC 3.5.1.1) and L-malate dehydrogenase (EC 1.1.1.37) was studied and the effect of cAMP level on the induction in Escherichia coli Crookes (ATCC 8739) was investigated. A varying degree of catabolite repression was observed during induction of individual enzymes induced separately on different energy sources. The synthesis of l-tryptophanase was most sensitive, whereas l-asparaginase was not influenced at all. Exogenous cAMP was found to overcome partially the catabolite repression of beta-galactosidase and D-serine deaminase, both during single induction. The synthesis of l-malate dehydrogenase was negatively influenced by the multiple induction even in the presence of cAMP; on the other hand, the synthesis of l-tryptophanase was stimulated, independently of the level of the exogenous cAMP. Similarly, the activity of L-asparaginase slightly but significantly increased during the multiple induction of all five enzymes; here too the activity increase did not depend on exogenous cAMP.

Genetic locus, distant from ptsM, affecting enzyme IIA/IIB function in Escherichia coli K-12

Most strains of Escherichia coli K-12 are unable to use the enzyme IIA/IIB (enzyme IIMan) complex of the phosphoenolpyruvate:sugar phosphotransferase system (PTS) in anaerobic growth and therefore cannot utilize glucosamine anaerobically. Introduction into these strains of a ptsG mutation, which eliminates activity of the enzyme IIIGlc/IIB' complex of the PTS, resulted in inability to grow anaerobically on glucose and mannose. Derivative strains able to grow anaerobically on glucosamine had mutations at a locus close to man, the gene coding for phosphomannose isomerase, and had higher enzyme IIA/IIB activities during anaerobic growth than did the parental strain. These results establish a locus affecting function of enzyme IIA/IIB that maps distant from ptsM, the probable structural gene for enzyme IIB.

Melibiose transport of Escherichia coli

Transport of [3H]melibiose, prepared from [3H]raffinose, was investigated in Escherichia coli. Na+ stimulated the transport of melibiose via the melibiose system, whereas Li+ inhibited it. Kinetic parameters of melibiose transport were determined. The Kt values were 0.57 mM in the absence of Na+ or Li+, 0.27 mM in the presence of 10 mM NaCl, and 0.29 mM in the presence of 10 mM LiCl. The Vmax values were 40 and 46 nmol/min per mg of protein in the absence and in the presence of NaCl and 18 nmol/min per mg of protein in the presence of LiCl. Melibiose transport via the melibiose system was temperature sensitive in a wild-type strain of Escherichia coli and was not inhibited by lactose. On the other hand, melibiose uptake via the lactose system was not temperature sensitive, was inhibited by lactose, and was not affected by Na+ and Li+. Methyl-beta-D-thiogalactoside, a substrate for both systems, inhibited the transport of melibiose via both systems.

Fine control of adenylate cyclase by the phosphoenolpyruvate:sugar phosphotransferase systems in Escherichia coli and Salmonella typhimurium

Inhibition of cellular adenylate cyclase activity by sugar substrates of the phosphoenolpyruvate-dependent phosphotransferase system was reliant on the activities of the protein components of this enzyme system and on a gene designated crrA. In bacterial strains containing very low enzyme I activity, inhibition could be elicited by nanomolar concentrations of sugar. An antagonistic effect between methyl alpha-glucoside and phosphoenolpyruvate was observed in permeabilized Escherichia coli cells containing normal activities of the phosphotransferase system enzymes. In contrast, phosphoenolpyruvate could not overcome the inhibitory effect of this sugar in strains deficient for enzyme I or HPr. Although the in vivo sensitivity of adenylate cyclase to inhibition correlated with sensitivity of carbohydrate permease function to inhibition in most strains studied, a few mutant strains were isolated in which sensitivity of carbohydrate uptake to inhibition was lost and sensitivity of adenylate cyclase to regulation was retained. These results are consistent with the conclusions that adenylate cyclase and the carbohydrate permeases were regulated by a common mechanism involving phosphorylation of a cellular constituent by the phosphotransferase system, but that bacterial cells possess mechanisms for selectively uncoupling carbohydrate transport from regulation.

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.

Catalytic activities associated with the enzymes II of the bacterial phosphotransferase system

The phosphotransferase system (PTS) in Escherichia coli is a multifunctional, multicomponent enzyme system. Its primary functions deal with carbon source acquisition, while its secondary functions are concerned with the regulation of bacterial physiology. The primary functions of the system include 1) extracellular detection, 2) unidirectional and exchange transmembrane transport, and 3) phosphoenolpyruvate-dependent and sugar phosphate-dependent phosphorylation of the sugar substrates of the system. The secondary functions include 1) regulation of the activities of adenylate cyclase and various non-PTS permeases and 2) regulation of the induced synthesis of several PTS enzymes. Both the primary and secondary functions appear to be elicited by the binding of a sugar substrate to an Enzyme II complex. One of these integral transmembrane enzymes, the mannitol Enzyme II (IImtl), has been solubilized with detergent, purified to homogeneity, and reconstituted in an artificial membrane system. The molecular weight of this protein, IImtl, is 60,000 daltons. It possesses an extracellular sugar binding site and distinct intracellular combining sites for sugar phosphate and phospho-HPr. An essential sulfhydryl group and an antibody combining site are localized to the cytoplasmic surface of the enzyme, while a dextran combining site is localized to the external surface. Preliminary experiments suggest that the different functions of the Enzyme IImtl can be dissected by genetic and biochemical techniques. These studies emphasize the functional complexity of the PTS and its integral membrane protein constituents.

Separation of four components of the phosphoenolpyruvate: glucose phosphotransferase system in Vibrio parahaemolyticus

Four classes of Vibrio parahaemolyticus mutants defective in the phosphoenolpyruvate: glucose phosphotransferase system (PTS) are described. They were phenotypically different, and were defective in different PTS components. The components designated tentatively as II, I, III, and H were separated by gel filtration of a wild-type extract. Component II, which was specific for glucose and found in the particulate fraction, is probably membrane-bound, glucose-specific enzyme II. Both components I and H were soluble proteins, and the latter was relatively heat-stable. Component I was required for phosphorylation of glucose, trehalose, fructose, mannose, and mannitol. Component H was also required for phosphorylating all the above sugars except fructose. These and some additional findings strongly suggest that components I and H correspond to enzyme I and HPr, respectively. Component III, a soluble heat-stable protein, may be equivalent to the sugar-specific factor III found in other organisms, although it seems to participate in phosphorylating two sugars, glucose and trehalose. There were evidences that mutants defective in components I and III were deficient in cyclic adenosine 3',5'-monophosphate synthesis under certain conditions.

Evolution of propanediol utilization in Escherichia coli: mutant with improved substrate-scavenging power

Wild-type strains of Escherichia coli are unable to use L-1,2-propanediol as a carbon and energy source. A series of mutants, able to grow on this compound at progressively faster rates, had been isolated by repeated transfers to a medium containing 20 mM L-1,2-propanediol. These strains synthesize at high constitutive levels a propanediolmicotinamide adenine dinucleotide oxidoreductase, an enzyme serving as a lactaldehyde during L-fucose fermentation by wild type cells. In this study, a mutant that can grow rapidly on the novel carbon source was subjected to further selection in a medium containing L-1,2-propanediol never exceeding 0.5 mM to obtain a derivative that has an increased power to extract the substrate from the medium. The emerging mutant exhibited four changes at the enzymatic level: (i) fuculose 1-phosphate aldolase activity is lost; (ii) the constitutive propanediol oxidoreductase activity is increased in its level; (iii) lactaldehyde dehydrogenase becomes constitutive and shows an elevated specific activity in crude extracts; and (iv) at low concentrations of propanediol, the facilitated diffusion across the cell membrane is enhanced. Changes two to four seem to act in concert in the trapping of propanediol by hastening its rate of entry and conversion to an ionized metabolite, lactate.

The cyclic 3',5'-adenosine monophosphate receptor protein and regulation of cyclic 3',5'-adenosine monophosphate synthesis in Escherichia coli

Rates of synthesis of cyclic 3',5'-adenosine monophosphate (cAMP) were measured in cultures of Escherichia coli aerating without a carbon source. This technique provides a representative measure of adenylate cyclase activity in the absence of inhibition caused by transport of the carbon source. Adenylate cyclase activity was found to vary more than 20-fold depending on the carbon source that had been available during growth. Synthesis of cAMP in cells aerating in the absence of the carbon source was highest when cells had been grown with glucose or fructose which inhibit adenylate cyclase activity severely. Synthesis of cAMP was much lower when cells had been grown with glycerol or succinate which cause only minimal inhibition of the activity. The variation in cAMP synthesis due to different carbon sources requires a functional cAMP receptor protein (CRP). Crp- mutants synthesize cAMP at comparable rates regardless of the carbon source that afforded growth. A novel mutant of E. coli having a CRP no longer dependent on cAMP has been isolated and characterized. Adenylate cyclase activity in this mutant no longer responds normally to variations in the carbon source.

Insulin action on Escherichia coli. Regulation of the adenylate cyclase and phosphotransferase enzymes

Insulin on Escherichia coli was studied using wild type E. coli B/r and K12 strains and a number of phosphoenolpyruvate phosphotransferase mutants. In vivo, the effects of insulin on the differential rate of tryptophanase synthesis, the rate of alpha-methylglucoside uptake and the rate of growth on glucose were determined in E. coli B/r. In vitro, the effect of insulin on the adenylate cyclase and the phosphotransferase activities was determined using toluenized cell preparations of E. coli B/r, E. coli K12 and phosphotransferase mutant strains. The specificity of insulin action on E. coli was determined using glucagon, vasopressin and somatropin as well as insulin antisera. Results show the specific action of insulin on E. coli, inhibiting tryptophanase induction and adenylate cyclase activity, while stimulating growth on glucose and uptake and phosphorylation of alpha-methylglucoside.

Permease-specific mutations in Salmonella typhimurium and Escherichia coli that release the glycerol, maltose, melibiose, and lactose transport systems from regulation by the phosphoenolpyruvate:sugar phosphotransferase system

Several carbohydrate permease systems in Salmonella typhimurium and Escherichia coli are sensitive to regulation by the phosphoenolpyruvate:sugar phosphotransferase system. Mutant Salmonella strains were isolated in which individual transport systems had been rendered insensitive to regulation by sugar substrates of the phosphotransferase system. In one such strain, glycerol uptake was insensitive to regulation; in another, the maltose transport system was resistant to inhibition; and in a third, the regulatory mutation specifically rendered the melibiose permease insensitive to regulation. An analogous mutation in E. coli abolished inhibition of the transport of beta-galactosides via the lactose permease system. The mutations were mapped near the genes which code for the affected transport proteins. The regulatory mutations rendered utilization of the particular carbohydrates resistant to inhibition and synthesis of the corresponding catabolic enzymes partially insensitive to repressive control by sugar substrates of the phosphotransferase system. Studies of repression of beta-galactosidase synthesis in E. coli were conducted with both lactose and isopropyl beta-thiogalactoside as exogenous sources of inducer. Employing high concentrations of isopropyl beta-thiogalactoside, repression of beta-galactosidase synthesis was not altered by the lactose-specific transport regulation-resistant mutation. By contrast, the more severe repression observed with lactose as the exogenous source of inducer was partially abolished by this regulatory mutation. The results support the conclusions that several transport systems, including the lactose permease system, are subject to allosteric regulation and that inhibition of inducer uptake is a primary cause of the repression of catabolic enzyme synthesis.

Mycoplasma phosphoenolpyruvate-dependent sugar phosphotransferase system: purification and characterization of enzyme I

The Mycoplasma phosphoenolpyruvate-dependent sugar phosphotransferase system consists of three components: a membrane-bound enzyme II, a soluble phosphocarrier protein (HPr), and a soluble enzyme I. The soluble enzyme I was purified by ammonium sulfate fractionation; Bio-Gel P-10 gel filtration; acid precipitation; diethylaminoethyl-Bio-Gel A; and Bio-Gel HTP column chromatography. The enzyme I was shown to be homogeneous by electrophoresis in a pH 8.9 non-sodium dodecyl sulfate gel and by isoelectric focusing. Whereas the protein moved as a single component in both the non-sodium dodecyl sulfate gel and isoelectric focusing, on sodium dodecyl sulfate gels, it moved as three subcomponents. The molecular weights of the three subunits, alpha, beta, and gamma, were 44,500, 62,000 and 64,500, respectively. The holoprotein moved as a single component, in the region of 220,000 daltons, in a Bio-Gel A 0.5-agarose column. The molar ratio of subunits was estimated to be 2alpha:1beta:1gamma. The elution characteristics on a diethylaminoethyl column at pH 7.4 and 6.8, acid precipitation data, and amino acid composition indicated that the protein is acidic. Isoelectric focusing occurred at pH 4.8. N-terminal amino acids determined by the dansyl chloride method indicated that glycine, alanine, and tyrosine are N-terminal amino acids of the three subunits. Although the protein was stable for at least 14 months at -20 degrees C, it was irreversibly inactivated by the thiol reagent N-ethyl-maleimide.