Bacterial phosphoenolpyruvate-dependent phosphotransferase system: P-Ser-HPr and its possible regulatory function

Intracellular Ser/Thr/Tyr phosphoproteome of the oral commensal Streptococcus gordonii DL1

Background

To respond and adapt to environmental challenges, prokaryotes regulate cellular processes rapidly and reversibly through protein phosphorylation and dephosphorylation. This study investigates the intracellular proteome and Ser/Thr/Tyr phosphoproteome of the oral commensal Streptococcus gordonii. Intracellular proteins from planktonic cells of S. gordonii DL1 were extracted and subjected to 2D-gel electrophoresis. Proteins in general were visualized using Coomassie Brilliant Blue and T-Rex staining. Phosphorylated proteins were visualized with Pro-Q Diamond Phosphoprotein Gel Stain. Proteins were identified by LC-MS/MS and sequence analysis.

Results

In total, sixty-one intracellular proteins were identified in S. gordonii DL1, many of which occurred at multiple isoelectric points. Nineteen of these proteins were present as one or more Ser/Thr/Tyr phosphorylated form. The identified phosphoproteins turned out to be involved in a variety of cellular processes.

Conclusion

Nineteen phosphoproteins involved in various cellular functions were identified in S. gordonii. This is the first time the global intracellular Ser/Thr/Tyr phosphorylation profile has been analysed in an oral streptococcus. Comparison with phosphoproteomes of other species from previous studies showed many similarities. Proteins that are consistently found in a phosphorylated state across several species and growth conditions may represent a core phosphoproteome profile shared by many bacteria.

Enterococcus faecalis Maltodextrin Gene Regulation by Combined Action of Maltose Gene Regulator MalR and Pleiotropic Regulator CcpA

Enterococci are Gram-positive bacteria present in the healthy human microbiota, but they are also a leading cause of nosocomial infections. Maltodextrin utilization by Enterococcus faecalis has been identified as an important factor for colonization of mammalians hosts. Here, we show that the LacI/GalR transcriptional regulator MalR, the maltose gene regulator, is also the main regulator of the operons encoding an ABC transporter (mdxEFG) and three metabolic enzymes (mmdH-gmdH-mmgT) required for the uptake and catabolism of maltotetraose and longer maltodextrins. The utilization of maltose and maltodextrins is consequently coordinated and induced by the disaccharide maltose, which binds to MalR. Carbon catabolite repression of the mdxEFG and mmdH-gmdH-mmgT operons is mediated by both P-Ser-HPr/MalR and P-Ser-HPr/CcpA. The latter complex exerts only moderate catabolite repression, which became visible when comparing maltodextrin operon expression levels of a malR ^- mutant (with a mutant allele for the malR gene) and a malR ^- ΔccpA double mutant grown in the presence of maltose, which is transported via a phosphotransferase system and, thus, favors the formation of P-Ser-HPr. Moreover, maltodextrin transport via MdxEFG slows rapidly when glucose is added, suggesting an additional regulation via inducer exclusion. This complex regulation of metabolic operons likely allows E. faecalis to fine-tune gene expression in response to changing environmental conditions.IMPORTANCE Enterococcus faecalis represents a leading cause of hospital-acquired infections worldwide. Several studies highlighted the importance of carbohydrate metabolism in the infection process of this bacterium. The genes required for maltodextrin metabolism are particularly induced during mouse infection and, therefore, should play an important role for pathogenesis. Since no data were hitherto available concerning the regulation of expression of the maltodextrin operons, we have conducted experiments to study the underlying mechanisms.

Inducer exclusion in Firmicutes: insights into the regulation of a carbohydrate ATP binding cassette transporter from Lactobacillus casei BL23 by the signal transducing protein P-Ser46-HPr

Catabolite repression is a mechanism that enables bacteria to control carbon utilization. As part of this global regulatory network, components of the phosphoenolpyruvate:carbohydrate phosphotransferase system inhibit the uptake of less favorable sugars when a preferred carbon source such as glucose is available. This process is termed inducer exclusion. In bacteria belonging to the phylum Firmicutes, HPr, phosphorylated at serine 46 (P-Ser46-HPr) is the key player but its mode of action is elusive. To address this question at the level of purified protein components, we have chosen a homolog of the Escherichia coli maltose/maltodextrin ATP-binding cassette transporter from Lactobacillus casei (MalE1-MalF1G1K1₂ ) as a model system. We show that the solute binding protein, MalE1, binds linear and cyclic maltodextrins but not maltose. Crystal structures of MalE1 complexed with these sugars provide a clue why maltose is not a substrate. P-Ser46-HPr inhibited MalE1/maltotetraose-stimulated ATPase activity of the transporter incorporated in proteoliposomes. Furthermore, cross-linking experiments revealed that P-Ser46-HPr contacts the nucleotide-binding subunit, MalK1, in proximity to the Walker A motif. However, P-Ser46-HPr did not block binding of ATP to MalK1. Together, our findings provide first biochemical evidence that P-Ser-HPr arrests the transport cycle by preventing ATP hydrolysis at the MalK1 subunits of the transporter.

The bacterial phosphoenolpyruvate:carbohydrate phosphotransferase system: regulation by protein phosphorylation and phosphorylation-dependent protein-protein interactions

The bacterial phosphoenolpyruvate (PEP):carbohydrate phosphotransferase system (PTS) carries out both catalytic and regulatory functions. It catalyzes the transport and phosphorylation of a variety of sugars and sugar derivatives but also carries out numerous regulatory functions related to carbon, nitrogen, and phosphate metabolism, to chemotaxis, to potassium transport, and to the virulence of certain pathogens. For these different regulatory processes, the signal is provided by the phosphorylation state of the PTS components, which varies according to the availability of PTS substrates and the metabolic state of the cell. PEP acts as phosphoryl donor for enzyme I (EI), which, together with HPr and one of several EIIA and EIIB pairs, forms a phosphorylation cascade which allows phosphorylation of the cognate carbohydrate bound to the membrane-spanning EIIC. HPr of firmicutes and numerous proteobacteria is also phosphorylated in an ATP-dependent reaction catalyzed by the bifunctional HPr kinase/phosphorylase. PTS-mediated regulatory mechanisms are based either on direct phosphorylation of the target protein or on phosphorylation-dependent interactions. For regulation by PTS-mediated phosphorylation, the target proteins either acquired a PTS domain by fusing it to their N or C termini or integrated a specific, conserved PTS regulation domain (PRD) or, alternatively, developed their own specific sites for PTS-mediated phosphorylation. Protein-protein interactions can occur with either phosphorylated or unphosphorylated PTS components and can either stimulate or inhibit the function of the target proteins. This large variety of signal transduction mechanisms allows the PTS to regulate numerous proteins and to form a vast regulatory network responding to the phosphorylation state of various PTS components.

Transcription regulators controlled by interaction with enzyme IIB components of the phosphoenolpyruvate: sugar phosphotransferase system

Numerous bacteria possess transcription activators and antiterminators composed of regulatory domains phosphorylated by components of the phosphoenolpyruvate:sugar phosphotransferase system (PTS). These domains, called PTS regulation domains (PRDs), usually contain two conserved histidines as potential phosphorylation sites. While antiterminators possess two PRDs with four phosphorylation sites, transcription activators contain two PRDs plus two regulatory domains resembling PTS components (EIIA and EIIB). The activity of these transcription regulators is controlled by up to five phosphorylations catalyzed by PTS proteins. Phosphorylation by the general PTS components EI and HPr is usually essential for the activity of PRD-containing transcription regulators, whereas phosphorylation by the sugar-specific components EIIA or EIIB lowers their activity. For a specific regulator, for example the Bacillus subtilis mtl operon activator MtlR, the functional phosphorylation sites can be different in other bacteria and consequently the detailed mode of regulation varies. Some of these transcription regulators are also controlled by an interaction with a sugar-specific EIIB PTS component. The EIIBs are frequently fused to the membrane-spanning EIIC and EIIB-mediated membrane sequestration is sometimes crucial for the control of a transcription regulator. This is also true for the Escherichia coli repressor Mlc, which does not contain a PRD but nevertheless interacts with the EIIB domain of the glucose-specific PTS. In addition, some PRD-containing transcription activators interact with a distinct EIIB protein located in the cytoplasm. The phosphorylation state of the EIIB components, which changes in response to the presence or absence of the corresponding carbon source, affects their interaction with transcription regulators. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases (2012).

Carbon catabolite repression in Thermoanaerobacterium saccharolyticum

BACKGROUND

The thermophilic anaerobe Thermoanaerobacterium saccharolyticum is capable of directly fermenting xylan and the biomass-derived sugars glucose, cellobiose, xylose, mannose, galactose and arabinose. It has been metabolically engineered and developed as a biocatalyst for the production of ethanol.

RESULTS

We report the initial characterization of the carbon catabolite repression system in this organism. We find that sugar metabolism in T. saccharolyticum is regulated by histidine-containing protein HPr. We describe a mutation in HPr, His15Asp, that leads to derepression of less-favored carbon source utilization.

CONCLUSION

Co-utilization of sugars can be achieved by mutation of HPr in T. saccharolyticum. Further manipulation of CCR in this organism will be instrumental in achieving complete and rapid conversion of all available sugars to ethanol.

Toxicogenomic mechanisms of 6-HO-BDE-47, 6-MeO-BDE-47, and BDE-47 in E. coli

Cytotoxicity of 6-HO-BDE-47 and its two analogues, BDE-47 and 6-MeO-BDE-47, and the associated molecular mechanisms were assessed by use of a live cell reporter assay system which contains a library of 1820 modified green fluorescent protein (GFP) expressing promoter reporter vectors constructed from E. coli K12 strains. 6-HO-BDE-47 inhibited growth of E. coli with a 4 h median effect concentration (EC50) of 22.52 ± 2.20 mg/L, but neither BDE-47 nor 6-MeO-BDE-47 were cytotoxic. Thus, 6-HO-BDE-47 might serve as an antibiotic in some living organisms. Exposure to 6-HO-BDE-47 resulted in 65 (fold change >2) or 129 (fold change >1.5) genes being differentially expressed. The no observed transcriptional effect concentration (NOTEC) and median transcriptional effect concentration (TEC50) based on transcriptional end points, of 6-HO-BDE-47 were 0.0438 and 0.580 mg/L, respectively. The transcriptional responses were 514- and 39-fold more sensitive than the acute EC50 to inhibit cell growth. Most of the genes that were differentially expressed in response to 6-HO-BDE-47 were not modulated by BDE-47 or 6-MeO-BDE-47. These results suggest that cytotoxicity of 6-HO-BDE-47 to E. coli was via a mechanism that was different from that of either BDE-47 or 6-MeO-BDE-47. Gene expression associated with metabolic pathways was more responsive to 6-HO-BDE-47, which suggests that this pathway might be the primary target of this compound.

Malate-mediated carbon catabolite repression in Bacillus subtilis involves the HPrK/CcpA pathway

Most organisms can choose their preferred carbon source from a mixture of nutrients. This process is called carbon catabolite repression. The Gram-positive bacterium Bacillus subtilis uses glucose as the preferred source of carbon and energy. Glucose-mediated catabolite repression is caused by binding of the CcpA transcription factor to the promoter regions of catabolic operons. CcpA binds DNA upon interaction with its cofactors HPr(Ser-P) and Crh(Ser-P). The formation of the cofactors is catalyzed by the metabolite-activated HPr kinase/phosphorylase. Recently, it has been shown that malate is a second preferred carbon source for B. subtilis that also causes catabolite repression. In this work, we addressed the mechanism by which malate causes catabolite repression. Genetic analyses revealed that malate-dependent catabolite repression requires CcpA and its cofactors. Moreover, we demonstrate that HPr(Ser-P) is present in malate-grown cells and that CcpA and HPr interact in vivo in the presence of glucose or malate but not in the absence of a repressing carbon source. The formation of the cofactor HPr(Ser-P) could be attributed to the concentrations of ATP and fructose 1,6-bisphosphate in cells growing with malate. Both metabolites are available at concentrations that are sufficient to stimulate HPr kinase activity. The adaptation of cells to environmental changes requires dynamic metabolic and regulatory adjustments. The repression strength of target promoters was similar to that observed in steady-state growth conditions, although it took somewhat longer to reach the second steady-state of expression when cells were shifted to malate.

Seryl-phosphorylated HPr regulates CcpA-independent carbon catabolite repression in conjunction with PTS permeases in Streptococcus mutans

Carbohydrate catabolite repression (CCR) in Streptococcus mutans can be independent of catabolite control protein A (CcpA) and requires specific components of phosphoenolpyruvate-dependent sugar:phosphotransferase system (PTS) permeases. Here, the effects of various ptsH (HPr) and hprK (HPr kinase/phosphatase) mutations on growth and CCR were evaluated. An hprKV265F mutation, which enhanced Ser46 phosphorylation of HPr, inhibited growth on multiple PTS sugars. A ptsHS46A mutation reversed the effects of hprKV265F in most cases. A strain carrying a ptsHS46D mutation, which mimics HPr(Ser-P), presented with more severe growth defects than the hprKV265F mutant. The hprKV265F mutant enhanced CCR of the fruA and levD operons, a phenotype reversible by the ptsHS46A mutation. The effects of the hprKV265F mutation on fruA and levD expression were independent of CcpA, but dependent on ManL (IIAB(Man)) and, to a lesser extent, on FruI (IIABC(Fru)), in a carbohydrate-specific fashion. Expression of the Bacillus subtilis ptsG gene in the manL mutant did not restore CCR of the lev or fru operons. The hprKV265F mutation inhibited growth on cellobiose and lactose, but only the transcription of the cel operon was decreased. Thus, in S. mutans, serine-phosphorylated HPr functions in concert with particular PTS permeases to prioritize carbohydrate utilization by modulating sugar transport and transcription of catabolic operons.

Streptococci and lactococci synthesize large amounts of HPr(Ser-P)(His~P)

HPr is a protein of the phosphoenolpyruvate:sugar phosphotransferase transport system (PTS). In gram-positive bacteria, HPr can be phosphorylated on Ser-46 by the kinase/phosphorylase HprK/P and on His-15 by phospho-enzyme I (EI~P) of the PTS. In vitro studies with purified HPrs from Bacillus subtilis , Enterococcus faecalis , and Streptococcus salivarius have indicated that the phosphorylation of one residue impedes the phosphorylation of the other. However, a recent study showed that while the rate of Streptococcus salivarius HPr phosphorylation by EI~P is reduced at acidic pH, the phosphorylation of HPr(Ser-P) by EI~P, generating HPr(Ser-P)(His~P), is stimulated. This suggests that HPr(Ser-P)(His~P) synthesis may occur in acidogenic bacteria unable to maintain their intracellular pH near neutrality. Consistent with this hypothesis, significant amounts of HPr(Ser-P)(His~P) have been detected in some streptococci. The present study was aimed at determining whether the capacity to synthesize HPr(Ser-P)(His~P) is common to streptococcal species, as well as to lactococci, which are also unable to maintain their intracellular pH near neutrality in response to a decrease in extracellular pH. Our results indicated that unlike Staphylococcus aureus, B. subtilis, and E. faecalis, all the streptococcal and lactococcal species tested were able to synthesize large amounts of HPr(Ser-P)(His~P) during growth. We also showed that Streptococcus salivarius IIABLMan, a protein involved in sugar transport by the PTS, could be efficiently phosphorylated by HPr(Ser-P)(His~P).

Control of the phosphorylation state of the HPr protein of the phosphotransferase system in Bacillus subtilis: implication of the protein phosphatase PrpC

In the Gram-positive bacterium Bacillus subtilis as well as in other firmicutes, the HPr protein of the phosphotransferase system (PTS) has two distinct phosphorylation sites, His-15 and Ser-46. These sites are phosphorylated by the Enzyme I of the PTS and by the ATP-dependent HPr kinase/phosphorylase, respectively. As a result, the phosphorylation state of HPr reflects the nutrient supply of the cell and is in turn involved in several responses at the levels of transport activity and expression of catabolic genes. Most important, HPr(Ser-P) serves as a cofactor for the pleiotropic transcription regulator CcpA. In addition to the proteins that phosphorylate HPr, those that are involved in the dephosphorylation are important in controlling the overall HPr phosphorylation state and the resulting regulatory and physiological outputs. In this study, we found that in addition to the phosphorylase activity of the HPr kinase/phosphorylase, the serine/threonine protein phosphatase PrpC uses HPr(Ser-P) as a target.

Maltose transport in Lactobacillus casei and its regulation by inducer exclusion

Transport of maltose in Lactobacillus casei BL23 is subject to regulation by inducer exclusion. The presence of glucose or other rapidly metabolized carbon sources blocks maltose transport by a control mechanism that depends on the phosphorylation of the HPr protein at serine residue 46. We have identified the L. casei gene cluster for maltose/maltodextrin utilization by sequence analysis and mutagenesis. It is composed of genes coding for a transcriptional regulator, oligosaccharide hydrolytic enzymes, an ABC transporter (MalEFGK2) and the enzymes for the metabolism of maltose or the degradation products of maltodextrins: maltose phosphorylase and beta-phospho-glucomutase. These genes are induced by maltose and repressed by the presence of glucose via the catabolite control protein A (CcpA). A mutant strain was constructed which expressed the hprKV267F allele and therefore formed large amounts of P-Ser-HPr even in the absence of a repressive carbon source. In this mutant, transport of maltose was severely impaired, whereas transport of sugars not subject to inducer exclusion was not changed. These results strengthen the idea that P-Ser-HPr controls inducer exclusion and make the maltose system of L. casei a suitable model for studying this process in Firmicutes.

How phosphotransferase system-related protein phosphorylation regulates carbohydrate metabolism in bacteria

The phosphoenolpyruvate(PEP):carbohydrate phosphotransferase system (PTS) is found only in bacteria, where it catalyzes the transport and phosphorylation of numerous monosaccharides, disaccharides, amino sugars, polyols, and other sugar derivatives. To carry out its catalytic function in sugar transport and phosphorylation, the PTS uses PEP as an energy source and phosphoryl donor. The phosphoryl group of PEP is usually transferred via four distinct proteins (domains) to the transported sugar bound to the respective membrane component(s) (EIIC and EIID) of the PTS. The organization of the PTS as a four-step phosphoryl transfer system, in which all P derivatives exhibit similar energy (phosphorylation occurs at histidyl or cysteyl residues), is surprising, as a single protein (or domain) coupling energy transfer and sugar phosphorylation would be sufficient for PTS function. A possible explanation for the complexity of the PTS was provided by the discovery that the PTS also carries out numerous regulatory functions. Depending on their phosphorylation state, the four proteins (domains) forming the PTS phosphorylation cascade (EI, HPr, EIIA, and EIIB) can phosphorylate or interact with numerous non-PTS proteins and thereby regulate their activity. In addition, in certain bacteria, one of the PTS components (HPr) is phosphorylated by ATP at a seryl residue, which increases the complexity of PTS-mediated regulation. In this review, we try to summarize the known protein phosphorylation-related regulatory functions of the PTS. As we shall see, the PTS regulation network not only controls carbohydrate uptake and metabolism but also interferes with the utilization of nitrogen and phosphorus and the virulence of certain pathogens.

Regulation of Carbon Metabolism in the Mollicutes and Its Relation to Virulence

The mollicutes are cell wall-less bacteria that live in close association with their eukaryotic hosts. Their genomes are strongly reduced and so are their metabolic capabilities. A survey of the available genome sequences reveals that the mollicutes are capable of utilizing sugars as source of carbon and energy via glycolysis. The pentose phosphate pathway is incomplete in these bacteria, and genes encoding enzymes of the tricarboxylic acid cycle are absent from the genomes. Sugars are transported by the phosphotransferase system. As in related bacteria, the phosphotransferase system does also seem to play a regulatory role in the mollicutes as can be concluded from the functionality of the regulatory HPr kinase/phosphorylase. In Mycoplasma pneumoniae, the activity of HPr kinase is triggered in the presence of glycerol. This carbon source may be important for the mollicutes since it is available in epithelial tissues and its metabolism results in the formation of hydrogen peroxide, the major virulence factor of several mollicutes. In plant-pathogenic mollicutes such as Spiroplasma citri, the regulation of carbon metabolism is crucial in the adaptation to life in plant tissues or the insect vectors. Thus, carbon metabolism seems to be intimately linked to pathogenicity in the mollicutes.

Molecular analysis of the glucose-specific phosphoenolpyruvate : sugar phosphotransferase system from Lactobacillus casei and its links with the control of sugar metabolism

Lactobacillus caseitransports glucose preferentially by a mannose-class phosphoenolpyruvate : sugar phosphotransferase system (PTS). The genomic analysis ofL. caseiallowed the authors to find a gene cluster (manLMNO) encoding the IIAB (manL), IIC (manM) and IID (manN) proteins of a mannose-class PTS, and a putative 121 aa protein of unknown function (encoded bymanO), homologues of which are also present inmanclusters that encode glucose/mannose transporters in other Gram-positive bacteria. TheL. casei manoperon is constitutively expressed into amanLMNOmessenger, but an additionalmanOtranscript was also detected. Upstream of themanoperon, two genes (upsRandupsA) were found which encode proteins resembling a transcriptional regulator and a membrane protein, respectively. Disruption of eitherupsRorupsAdid not affectmanLMNOtranscription, and had no effect on glucose uptake. Cells carrying amanOdeletion transported glucose at a rate similar to that of the wild-type strain. By contrast, amanMdisruption resulted in cells unable to transport glucose by the PTS, thus confirming the functional role of themangenes. In addition, themanMmutant exhibited neither inducer exclusion of maltose nor glucose repression. This result confirms the need for glucose transport through the PTS to trigger these regulatory processes inL. casei.

Dual phosphorylation ofMycoplasma pneumoniaeHPr by Enzyme I and HPr kinase suggests an extended phosphoryl group susceptibility of HPr

In Gram-positive bacteria, the HPr protein of the phosphoenolpyruvate:sugar phosphotransferase system can be phosphorylated at two distinct sites, His-15 and Ser-46. While the former phosphorylation is implicated in phosphoryl transfer to the incoming sugars, the latter serves regulatory purposes. In Bacillus subtilis, the two phosphorylation events are mutually exclusive. In contrast, doubly phosphorylated HPr is present in cell extracts of Mycoplasma pneumoniae. In this work, we studied the ability of the two single phosphorylated HPr species to accept a second phosphoryl group. Indeed, both Enzyme I and the HPr kinase/phosphorylase from M. pneumoniae are able to use phosphorylated HPr as a substrate. The formation of doubly phosphorylated HPr is substantially slower as compared to the phosphorylation of free HPr. However, the rate of formation of doubly phosphorylated HPr is sufficient to account for the amount of HPr(His approximately P)(Ser-P) detected in M. pneumoniae cells.

The doubly phosphorylated form of HPr, HPr(Ser~P)(His-P), is abundant in exponentially growing cells of Streptococcus thermophilus and phosphorylates the lactose transporter LacS as efficiently as HPr(His~P)

In Streptococcus thermophilus, lactose is taken up by LacS, a transporter that comprises a membrane translocator domain and a hydrophilic regulatory domain homologous to the IIA proteins and protein domains of the phosphoenolpyruvate:sugar phosphotransferase system (PTS). The IIA domain of LacS (IIALacS) possesses a histidine residue that can be phosphorylated by HPr(His~P), a protein component of the PTS. However, determination of the cellular levels of the different forms of HPr, namely, HPr, HPr(His~P), HPr(Ser-P), and HPr(Ser-P)(His~P), in exponentially lactose-growing cells revealed that the doubly phosphorylated form of HPr represented 75% and 25% of the total HPr in S. thermophilus ATCC 19258 and S. thermophilus SMQ-301, respectively. Experiments conducted with [32P]PEP and purified recombinant S. thermophilus ATCC 19258 proteins (EI, HPr, and IIALacS) showed that IIALacS was reversibly phosphorylated by HPr(Ser-P)(His~P) at a rate similar to that measured with HPr(His~P). Sequence analysis of the IIALacS protein domains from several S. thermophilus strains indicated that they can be divided into two groups on the basis of their amino acid sequences. The amino acid sequence of IIALacS from group I, to which strain 19258 belongs, differed from that of group II at 11 to 12 positions. To ascertain whether IIALacS from group II could also be phosphorylated by HPr(His~P) and HPr(Ser-P)(His~P), in vitro phosphorylation experiments were conducted with purified proteins from Streptococcus salivarius ATCC 25975, which possesses a IIALacS very similar to group II S. thermophilus IIALacS. The results indicated that S. salivarius IIALacS was phosphorylated by HPr(Ser-P)(His~P) at a higher rate than that observed with HPr(His~P). Our results suggest that the reversible phosphorylation of IIALacS in S. thermophilus is accomplished by HPr(Ser-P)(His~P) as well as by HPr(His~P).

HPr kinase/phosphorylase, a Walker motif A-containing bifunctional sensor enzyme controlling catabolite repression in Gram-positive bacteria

Carbon catabolite repression (CCR) in Gram-positive bacteria is regulated by the bifunctional enzyme HPr kinase/phosphorylase (HprK/P). This enzyme catalyses the ATP- as well as the pyrophosphate-dependent phosphorylation of Ser-46 in HPr, a phosphocarrier protein of a sugar transport and phosphorylation system. HprK/P also catalyses the pyrophosphate-producing, inorganic phosphate-dependent dephosphorylation (phosphorolysis) of seryl-phosphorylated HPr (P-Ser-HPr). P-Ser-HPr functions as catabolite co-repressor by interacting with the LacI/GalR-type repressor, catabolite control protein A (CcpA), and allowing it to bind to operator sites preceding catabolite-regulated transcription units. HprK/P thus indirectly controls the expression of about 10% of the genes of Gram-positive bacteria. The two antagonistic activities of HprK/P are regulated by intracellular metabolites, which change their concentration in response to the absence or presence of rapidly metabolisable carbon sources (glucose, fructose, etc.) in the growth medium. Biochemical and structural studies revealed that HprK/P exhibits no similarity to eukaryotic protein kinases and that it contains a Walker motif A (or P-loop) as nucleotide binding site. Interestingly, HprK/P has a structural fold resembling that in kinases phosphorylating certain low molecular weight substrates such as nucleosides, nucleotides or oxaloacetate. The structures of the complexes of HprK/P with HPr and P-Ser-HPr have also been determined, which allowed proposing a detailed mechanism for the kinase and phosphorylase functions of HprK/P.

Mycoplasma pneumoniae HPr kinase/phosphorylase

HPr kinase/phosphorylase (HPrK/P) is the key regulator of carbon metabolism in many Gram-positive bacteria. It phosphorylates/dephosphorylates the HPr protein of the bacterial phosphotransferase system on a regulatory serine residue in response to the nutrient status of the cell. In Mycoplasma pneumoniae, HPrK/P is one of the very few regulatory proteins encoded in the genome. The regulation of this enzyme by metabolites is unique among HPrK/P proteins studied so far: it is active as a kinase at low ATP concentrations, whereas the proteins from other bacteria need high ATP concentrations as an indicator of a good nutrient supply for kinase activity. We studied the interaction of M. pneumoniae HPrK/P with ATP, Fru1,6P2 and Pi by fluorescence spectroscopy. In agreement with the previously observed unique regulation, we found a very high affinity for ATP (K(d)=5.4 microM) compared with the HPrK/P proteins from other bacteria. The Kd for Fru1,6P2 was three orders of magnitude higher, which explains why Fru1,6P2 has only a weak regulatory effect on M. pneumoniae HPrK/P. Mutations of two important regions in the active site of HPrK/P, the nucleotide binding P-loop and the HPrK/P family signature sequence, had different effects. P-loop region mutations strongly affect ATP binding and thus all enzymatic functions, whereas the signature sequence motif seems to be important for the catalytic mechanism rather than for nucleotide binding.

Molecular characterization of HPr and related enzymes, and regulation of HPr phosphorylation in the ruminal bacterium Streptococcus bovis

Molecular properties of HPr, enzyme I, and HPr kinase in Streptococcus bovis, and the regulation of HPr phosphorylation were examined. The genes encoding HPr (ptsH) and enzyme I (ptsI) were found to be cotranscribed. Two transcriptional start sites were detected in a region upstream of the HPr kinase gene (hprK). HPr kinase had both HPr-phosphorylating and HPr-dephosphorylating activities. The importance of phosphorylation of Ser-46 in HPr was shown by using a mutant HPr in which Ser-46 was replaced by Ala. When S. bovis was grown in glucose-limited medium, the amount of seryl-phosphorylated HPr (HPr-[Ser-P]) decreased drastically as the growth rate decreased. In contrast, the amount of histidyl-phosphorylated HPr (HPr-[His-P]) increased gradually as the growth rate decreased. The amount of HPr kinase did not greatly change with the growth phase, whereas the intracellular P(i) concentration increased as the growth rate decreased. HPr-[Ser-P] decreased as the intracellular P(i) increased as a consequence of inhibition of HPr kinase activity by P(i) and simultaneous enhancement of HPr-[Ser-P] phosphatase activity by P(i). Thus, it is conceivable that the ratio of HPr-[Ser-P] to HPr-[His-P] is regulated by the bifunctional activity of HPr kinase in response to intracellular P(i) concentration.

Pyrophosphate-producing protein dephosphorylation by HPr kinase/phosphorylase: a relic of early life?

In most Gram-positive bacteria, serine-46-phosphorylated HPr (P-Ser-HPr) controls the expression of numerous catabolic genes ( approximately 10% of their genome) by acting as catabolite corepressor. HPr kinase/phosphorylase (HprK/P), the bifunctional sensor enzyme for catabolite repression, phosphorylates HPr, a phosphocarrier protein of the sugar-transporting phosphoenolpyruvate/glycose phosphotransferase system, in the presence of ATP and fructose-1,6-bisphosphate but dephosphorylates P-Ser-HPr when phosphate prevails over ATP and fructose-1,6-bisphosphate. We demonstrate here that P-Ser-HPr dephosphorylation leads to the formation of HPr and pyrophosphate. HprK/P, which binds phosphate at the same site as the beta phosphate of ATP, probably uses the inorganic phosphate to carry out a nucleophilic attack on the phosphoryl bond in P-Ser-HPr. HprK/P is the first enzyme known to catalyze P-protein dephosphorylation via this phospho-phosphorolysis mechanism. This reaction is reversible, and at elevated pyrophosphate concentrations, HprK/P can use pyrophosphate to phosphorylate HPr. Growth of Bacillus subtilis on glucose increased intracellular pyrophosphate to concentrations ( approximately 6 mM), which in in vitro tests allowed efficient pyrophosphate-dependent HPr phosphorylation. To effectively dephosphorylate P-Ser-HPr when glucose is exhausted, the pyrophosphate concentration in the cells is lowered to 1 mM. In B. subtilis, this might be achieved by YvoE. This protein exhibits pyrophosphatase activity, and its gene is organized in an operon with hprK.

Control of the glycolytic gapA operon by the catabolite control protein A in Bacillus subtilis: a novel mechanism of CcpA-mediated regulation

Glycolysis is one of the main pathways of carbon catabolism in Bacillus subtilis. Expression of the gapA gene encoding glyceraldehyde-3-phosphate dehydrogenase, the key enzyme of glycolysis from an energetic point of view, is induced by glucose and other sugars. Two regulators are involved in induction of the gapA operon, the product of the first gene of the operon, the CggR repressor, and catabolite control protein A (CcpA). CcpA is required for induction of the gapA operon by glucose. Genetic evidence has demonstrated that CcpA does not control the expression of the gapA operon by binding directly to a target in the promoter region. Here, we demonstrate by physiological analysis of the inducer spectrum that CcpA is required only for induction by sugars transported by the phosphotransferase system (PTS). A functional CcpA is needed for efficient transport of these sugars. This interference of CcpA with PTS sugar transport results from an altered phosphorylation pattern of HPr, a phosphotransferase of the PTS. In a ccpA mutant strain, HPr is nearly completely phosphorylated on a regulatory site, Ser-46, and is trapped in this state, resulting in its inactivity in PTS phosphotransfer. A mutation in HPr affecting the regulatory phosphorylation site suppresses both the defect in PTS sugar transport and the induction of the gapA operon. We conclude that a low-molecular effector derived from glucose that acts as an inducer for the repressor CggR is limiting in the ccpA mutant.

Mutations lowering the phosphatase activity of HPr kinase/phosphatase switch off carbon metabolism

The oligomeric bifunctional HPr kinase/P-Ser-HPr phosphatase (HprK/P) regulates many metabolic functions in Gram-positive bacteria by phosphorylating the phosphocarrier protein HPr at Ser46. We isolated Lactobacillus casei hprK alleles encoding mutant HprK/Ps exhibiting strongly reduced phosphatase, but almost normal kinase activity. Two mutations affected the Walker motif A of HprK/P and four a conserved C-terminal region in contact with the ATP-binding site of an adjacent subunit in the hexamer. Kinase and phosphatase activity appeared to be closely associated and linked to the Walker motif A, but dephosphorylation of seryl-phosphorylated HPr (P-Ser-HPr) is not simply a reversal of the kinase reaction. When the hprKV267F allele was expressed in Bacillus subtilis, the strongly reduced phosphatase activity of the mutant enzyme led to increased amounts of P-Ser-HPr. The hprKV267F mutant was unable to grow on carbohydrates transported by the phosphoenolpyruvate:glycose phosphotransferase system (PTS) and on most non-PTS carbohydrates. Disrupting ccpA relieved the growth defect only on non-PTS sugars, whereas replacing Ser46 in HPr with alanine also restored growth on PTS substrates.

Regulation of carbon catabolism in Bacillus species

The gram-positive bacterium Bacillus subtilisis capable of using numerous carbohydrates as single sources of carbon and energy. In this review, we discuss the mechanisms of carbon catabolism and its regulation. Like many other bacteria, B. subtilis uses glucose as the most preferred source of carbon and energy. Expression of genes involved in catabolism of many other substrates depends on their presence (induction) and the absence of carbon sources that can be well metabolized (catabolite repression). Induction is achieved by different mechanisms, with antitermination apparently more common in B. subtilis than in other bacteria. Catabolite repression is regulated in a completely different way than in enteric bacteria. The components mediating carbon catabolite repression in B. subtilis are also found in many other gram-positive bacteria of low GC content.

The 1.9 A resolution structure of phospho-serine 46 HPr from Enterococcus faecalis

The histidine-containing phosphocarrier protein HPr is a central component of the phosphoenolpyruvate:sugar phosphotransferase system (PTS), which transfers metabolic carbohydrates across the cell membrane in many bacterial species. In Gram-positive bacteria, phosphorylation of HPr at conserved serine 46 (P-Ser-HPr) plays several regulatory roles within the cell; the major regulatory effect of P-Ser-HPr is its inability to act as a phosphocarrier substrate in the enzyme I reaction of the PTS. In order to investigate the structural nature of HPr regulation by phosphorylation at Ser46, the structure of the P-Ser-HPr from the Gram- positive bacterium Enterococcus faecalis has been determined. X-ray diffraction analysis of P-Ser-HPr crystals provided 10,043 unique reflections, with a 95.1 % completeness of data to 1.9 A resolution. The structure was solved using molecular replacement, with two P-Ser-HPr molecules present in the asymmetric unit. The final R-value and R(Free) are 0.178 and 0.239, respectively. The overall tertiary structure of P-Ser-HPr is that of other HPr structures. However the active site in both P-Ser-HPr molecules was found to be in the "open" conformation. Ala16 of both molecules were observed to be in a state of torsional strain, similar to that seen in the structure of the native HPr from E. faecalis. Regulatory phosphorylation at Ser46 does not induce large structural changes to the HPr molecule. The B-helix was observed to be slightly lengthened as a result of Ser46 phosphorylation. Also, the water mediated Met51-His15 interaction is maintained, again similar to that of the native E. faecalis HPr. The major structural, and thus regulatory, effect of phosphorylation at Ser46 is disruption of the hydrophobic interactions between EI and HPr, in particular the electrostatic repulsion between the phosphoryl group on Ser46 and Glu84 of EI and the prevention of a potential interaction of Met48 with a hydrophobic pocket of EI.

Regulation of the Bacillus subtilis GlcT antiterminator protein by components of the phosphotransferase system

Bacillus subtilis utilizes glucose as the preferred source of carbon and energy. The sugar is transported into the cell by a specific permease of the phosphoenolpyruvate:sugar phosphotransferase system (PTS) encoded by the ptsGHI operon. Expression of this operon is induced by glucose and requires the action of a positive transcription factor, the GlcT antiterminator protein. Glucose availability is sensed by glucose-specific enzyme II (EIIGlc), the product of ptsG. In the absence of inducer, the glucose permease negatively controls the activity of the antiterminator. The GlcT antiterminator has a modular structure. The isolated N-terminal part contains the RNA-binding protein and acts as a constitutively acting antiterminator. GlcT contains two PTS regulation domains (PRDs) at the C terminus. One (PRD-I) is the target of negative control exerted by EIIGlc. A conserved His residue (His-104 in GlcT) is involved in inactivation of GlcT in the absence of glucose. It was previously proposed that PRD-containing transcriptional antiterminators are phosphorylated and concomitantly inactivated in the absence of the substrate by their corresponding PTS permeases. The results obtained with B. subtilis glucose permease with site-specific mutations suggest, however, that the permease might modulate the phosphorylation reaction without being the phosphate donor.

Regulation of carbon metabolism in gram-positive bacteria by protein phosphorylation

The main function of the bacterial phosphotransferase system is to transport and to phosphorylate mono- and disaccharides as well as sugar alcohols. However, the phosphotransferase system is also involved in regulation of carbon metabolism. In Gram-positive bacteria, it is implicated in carbon catabolite repression and regulation of expression of catabolic genes by controlling either catabolic enzyme activities, transcriptional activators or antiterminators. All these different regulations follow a protein phosphorylation mechanism.

Catabolite repression resistance of gnt operon expression in Bacillus subtilis conferred by mutation of His-15, the site of phosphoenolpyruvate-dependent phosphorylation of the phosphocarrier protein HPr

Carbon catabolite repression of the gnt operon of Bacillus subtilis is mediated by the catabolite control protein CcpA and by HPr, a phosphocarrier protein of the phosphotransferase system. ATP-dependent phosphorylation of HPr at Ser-46 is required for carbon catabolite repression as ptsH1 mutants in which Ser-46 of HPr is replaced with an unphosphorylatable alanyl residue are resistant to carbon catabolite repression. We here demonstrate that mutation of His-15 of HPr, the site of phosphoenolpyruvate-dependent phosphorylation, also prevents carbon catabolite repression of the gnt operon. A strain which expressed two mutant HPrs (one in which Ser-46 is replaced by Ala [S46A HPr] and one in which His-15 is replaced by Ala [H15A HPr]) on the chromosome was barely sensitive to carbon catabolite repression, although the H15A mutant HPr can be phosphorylated at Ser-46 by the ATP-dependent HPr kinase in vitro and in vivo. The S46D mutant HPr which structurally resembles seryl-phosphorylated HPr has a repressive effect on gnt expression even in the absence of a repressing sugar. By contrast, the doubly mutated H15E,S46D HPr, which resembles the doubly phosphorylated HPr because of the negative charges introduced by the mutations at both phosphorylation sites, had no such effect. In vitro assays substantiated these findings and demonstrated that in contrast to the wild-type seryl-phosphorylated HPr and the S46D mutant HPr, seryl-phosphorylated H15A mutant HPr and H15E,S46D doubly mutated HPr did not interact with CcpA. These results suggest that His-15 of HPr is important for carbon catabolite repression and that either mutation or phosphorylation at His-15 can prevent carbon catabolite repression.

Regulation of sugar uptake via the phosphoenolpyruvate-dependent phosphotransferase systems in Bacillus subtilis and Lactococcus lactis is mediated by ATP-dependent phosphorylation of seryl residue 46 in HPr

By using both metabolizable and nonmetabolizable sugar substrates of the phosphoenolpyruvate-dependent phosphotransferase system (PTS), we show that PTS sugar uptake into intact cells and membrane vesicles of Lactococcus lactis and Bacillus subtilis is strongly inhibited by high concentrations of any of several metabolizable PTS sugars. Inhibition requires phosphorylation of seryl residue 46 in the phosphocarrier protein of the PTS, HPr, by the metabolite-activated, ATP-dependent protein kinase. Inhibition does not occur when wild-type HPr is replaced by the S46A mutant form of this protein either in vesicles of L. lactis or B. subtilis or in intact cells of B. subtilis. Nonmetabolizable PTS sugar analogs such as 2-deoxyglucose inhibit PTS sugar uptake by a distinct mechanism that is independent of HPr(ser-P) and probably involves cellular phosphoenolpyruvate depletion.

Transcriptional analysis of bglPH expression in Bacillus subtilis: evidence for two distinct pathways mediating carbon catabolite repression

In Bacillus subtilis, aryl-beta-glucosides such as salicin and arbutin are catabolized by the gene products of bglP and bglH, encoding an enzyme II of the phosphoenolpyruvate sugar-phosphotransferase system and a phospho-beta-glucosidase, respectively. These two genes are transcribed from a single promoter. The presence of a transcript of about 4,000 nucleotides detected by Northern (RNA) blot analysis indicates that bglP and bglH are part of an operon. However, this transcript is only present when cells are grown in the presence of the inducing substrate, salicin. In the absence of the inducer, a transcript of about 110 nucleotides can be detected, suggesting that transcription terminates downstream of the promoter at a stable termination structure. Initiation of transcription is abolished in the presence of rapidly metabolized carbon sources. Catabolite repression of bglPH expression involves the trans-acting factors CcpA and HPr. In a ccpA mutant, transcription initiation is relieved from glucose repression. Furthermore, we report a catabolite responsive element-CcpA-independent form of catabolite repression requiring the ribonucleic antiterminator-terminator region, which is the target of antitermination, and the wild-type HPr protein of the phosphotransferase system. Evidence that the antitermination protein LicT is a crucial element for this type of regulation is provided.

Catabolite repression and inducer control in Gram-positive bacteria

Results currently available clearly indicate that the metabolite-activated protein kinase-mediated phosphorylation of Ser-46 in HPr plays a key role in catabolite repression and the control of inducer levels in low-GC Gram-positive bacteria. This protein kinase is not found in enteric bacteria such as E. coli and Salmonella typhimurium where an entirely different PTS-mediated regulatory mechanism is responsible for catabolite repression and inducer concentration control. In Table 2 these two mechanistically dissimilar but functionally related processes are compared (Saier et al., 1995b). In Gram-negative enteric bacteria, an external sugar is sensed by the sugar-recognition constituent of an Enzyme II complex of the PTS (IIC), and a dephosphorylating signal is transmitted via the Enzyme IIB/HPr proteins to the central regulatory protein, IIAGlc. Targets regulated include (1) permeases specific for lactose, maltose, melibiose and raffinose, (2) catabolic enzymes such as glycerol kinase that generate cytoplasmic inducers, and (3) the cAMP biosynthetic enzyme, adenylate cyclase that mediates catabolite repression (Saier, 1989, 1993). In low-GC Gram-positive bacteria, cytoplasmic phosphorylated sugar metabolites are sensed by the HPr kinase which is allostericlaly activated. HPr becomes phosphorylated on Ser-46, and this phosphorylated derivative regulates the activities of its target proteins. These targets include (1) the PTS, (2) non-PTS permeases (both of which are inhibited) and (3) a cytoplasmic sugar-P phosphatase which is activated to reduce cytoplasmic inducer levels. Other important targets of HPr(ser-P) action are (4) the CcpA protein and probably (5) the CepB transcription factor. These two proteins together are believed to determine the intensity of catabolite repression. Their relative importance depends on physiological conditions. Both proteins may respond to the cytoplasmic concentration of HPr(ser-P) and appropriate metabolites. CepA possibly binds sugar metabolites such as FBP as well as HPr(ser-P). Because HPr(his-P, ser-P) does not bind to CepA, the regulatory cascade is also sensitive to the external PTS sugar concentration. Mutational analyses (unpublished results) suggest that CepA may bind to a site that includes His-15. Interestingly, both the CepA protein in the Gram-positive bacterium, B. subtilis, and glycerol kinase in the Gram-negative bacterium, E. coli, sense both a PTS protein and a cytoplasmic metabolic intermediate. The same may be true of target permeases and enzymes in both types of organisms, but this possibility has not yet been tested. The parallels between the Gram-negative and Gram-positive bacterial regulatory systems are superficial at the mechanistic level but fundamental at the functional level. Thus, the PTS participates in regulation in both cases, and phosphorylation of its protein constituents plays key roles. However, the stimuli sensed, the transmission mechanisms, the central PTS regulatory proteins that effect allosteric regulation, and some of the target proteins are completely different. It seems clear that these two transmission mechanisms evolved independently. They provide a prime example of functional convergence.

The HPr protein of the phosphotransferase system links induction and catabolite repression of the Bacillus subtilis levanase operon

The LevR protein is the activator of expression of the levanase operon of Bacillus subtilis. The promoter of this operon is recognized by RNA polymerase containing the sigma 54-like factor sigma L. One domain of the LevR protein is homologous to activators of the NtrC family, and another resembles antiterminator proteins of the BglG family. It has been proposed that the domain which is similar to antiterminators is a target of phosphoenolpyruvate:sugar phosphotransferase system (PTS)-dependent regulation of LevR activity. We show that the LevR protein is not only negatively regulated by the fructose-specific enzyme IIA/B of the phosphotransferase system encoded by the levanase operon (lev-PTS) but also positively controlled by the histidine-containing phosphocarrier protein (HPr) of the PTS. This second type of control of LevR activity depends on phosphoenolpyruvate-dependent phosphorylation of HPr histidine 15, as demonstrated with point mutations in the ptsH gene encoding HPr. In vitro phosphorylation of partially purified LevR was obtained in the presence of phosphoenolpyruvate, enzyme I, and HPr. The dependence of truncated LevR polypeptides on stimulation by HPr indicated that the domain homologous to antiterminators is the target of HPr-dependent regulation of LevR activity. This domain appears to be duplicated in the LevR protein. The first antiterminator-like domain seems to be the target of enzyme I and HPr-dependent phosphorylation and the site of LevR activation, whereas the carboxy-terminal antiterminator-like domain could be the target for negative regulation by the lev-PTS.

Regulation of ATP-dependent P-(Ser)-HPr formation in Streptococcus mutans and Streptococcus salivarius

Sugar transport via the phosphoenolpyruvate (PEP) phosphotransferase system involves PEP-dependent phosphorylation of the general phosphotransferase system protein, HPr, at histidine 15. However, gram-positive bacteria can also carry out ATP-dependent phosphorylation of HPr at serine 46 by means of (Ser)HPr kinase. In this study, we demonstrate that (Ser)HPr kinase in crude preparations of Streptococcus mutans Ingbritt and Streptococcus salivarius ATCC 25975 is membrane associated, with pH optima of 7.0 and 7.5, respectively. The latter organism possessed 7- to 27-fold-higher activity than S. mutans NCTC 10449, GS-5, and Ingbritt strains. The enzyme in S. salivarius was activated by fructose-1,6-bisphosphate (FBP) twofold with 0.05 mM ATP, but this intermediate was slightly inhibitory with 1.0 mM ATP at FBP concentrations up to 10 mM. Similar inhibition was observed with the enzyme from S. mutans Ingbritt. A variety of other glycolytic intermediates had no effect on kinase activity under these conditions. The activity and regulation of (Ser)HPr kinase were assessed in vivo by monitoring P-(Ser)-HPr formation in steady-state cells of S. mutans Ingbritt grown in continuous culture with limiting glucose (10 and 50 mM) and with excess glucose (100 and 200 mM). All four forms of HPr [free HPr, P approximately (His)-HPr, P-(Ser)-HPr, and P approximately (His)-P-(Ser)-HPr] could be detected in the cells; however, significant differences in the intracellular levels of the forms were apparent during growth at different glucose concentrations. The total HPr pool increased with increasing concentrations of glucose in the medium, with significant increases in the P-(Ser)-HPr and P approximately HHis)-P-(Ser)-HPr concentrations. For example, while total PEP-dependent phosphorylation [P approximately(His)-HPr plus P approximately (His)-P-(Ser)-HPr] varied only from 21.5 to 52.5 microgram mg of cell protein (-1) in cells grown at the four glucose concentrations, the total ATP-dependent phosphorylation [P-(Ser)-HPr plus P approximately (His)-P-(Ser)-HPr] increased 12-fold from the 10 mM glucose-grown cells (9.1 microgram mg of cell protein (-1) to 106 and 105 microgram mg(-1) in the 100 and 200 mM glucose-grown cultures, respectively. (Ser)HPr kinase activity in membrane preparations of the cells varied little between the 10, 50, and 100 mM glucose-grown cells but increased threefold in the 200 mM glucose-grown cells. The intracellular levels of ATP, glucose-6-phosphate, and FBP increased with external glucose concentration, with the level of FBP being 3.8-fold higher for cells grown with 200 mM glucose than for those grown with 10 mM glucose. However, the variation in the intracellular levels of FBP, particularly between cells grown with 100 and 200 mM glucose, did not correlate with the extent of P-(Ser)-HPr formation, suggesting that the activity of (Ser)HPr kinase is not critically dependent on the availability of intracellular FBP.

Protein kinase-dependent HPr/CcpA interaction links glycolytic activity to carbon catabolite repression in gram-positive bacteria

CcpA, the repressor/activator mediating carbon catabolite repression and glucose activation in many Gram-positive bacteria, has been purified from Bacillus megaterium after fusing it to a His tag. CcpA-his immobilized on a Ni-NTA resin specifically interacted with HPr phosphorylated at seryl residue 46. HPr, a phospho-carrier protein of the phosphoenolpyruvate: glycose phosphotransferase system (PTS), can be phosphorylated at two different sites: (i) at His-15 in a PEP-dependent reaction catalysed by enzyme I of the PTS; and (ii) at Ser-46 in an ATP-dependent reaction catalysed by a metabolite-activated protein kinase. Neither unphosphorylated HPr nor HPr phosphorylated at His-15 nor the doubly phosphorylated HPr bound to CcpA. The interaction with seryl-phosphorylated HPr required the presence of fructose 1,6-bisphosphate. These findings suggest that carbon catabolite repression in Gram-positive bacteria is a protein kinase-triggered mechanism. Glycolytic intermediates, stimulating the corresponding protein kinase and the P-ser-HPr/CcpA complex formation, provide a link between glycolytic activity and carbon catabolite repression. The sensitivity of this complex formation to phosphorylation of HPr at His-15 also suggests a link between carbon catabolite repression and PTS transport activity.

Presence of protein constituents of the gram-positive bacterial phosphotransferase regulatory system in Acholeplasma laidlawii

Acholeplasma species have been reported to lack a functional phosphoenolpyruvate:sugar phosphotransferase system (PTS). We show here that Acholeplasma laidlawii possesses activities of enzyme I, HPr, HPr(ser) kinase, and HPr(ser-P) phosphatase but lacks detectable activities of enzymes II of the PTS. HPr from this organism was purified, and the regulatory properties of the kinase and phosphatase were characterized and shown to differ from those of previously studied bacteria. The results suggest the presence of an incomplete PTS in A. laidlawii which has the potential to function in a unique regulatory capacity.

Phosphoenolpyruvate:carbohydrate phosphotransferase systems of bacteria

Numerous gram-negative and gram-positive bacteria take up carbohydrates through the phosphoenolpyruvate (PEP):carbohydrate phosphotransferase system (PTS). This system transports and phosphorylates carbohydrates at the expense of PEP and is the subject of this review. The PTS consists of two general proteins, enzyme I and HPr, and a number of carbohydrate-specific enzymes, the enzymes II. PTS proteins are phosphoproteins in which the phospho group is attached to either a histidine residue or, in a number of cases, a cysteine residue. After phosphorylation of enzyme I by PEP, the phospho group is transferred to HPr. The enzymes II are required for the transport of the carbohydrates across the membrane and the transfer of the phospho group from phospho-HPr to the carbohydrates. Biochemical, structural, and molecular genetic studies have shown that the various enzymes II have the same basic structure. Each enzyme II consists of domains for specific functions, e.g., binding of the carbohydrate or phosphorylation. Each enzyme II complex can consist of one to four different polypeptides. The enzymes II can be placed into at least four classes on the basis of sequence similarity. The genetics of the PTS is complex, and the expression of PTS proteins is intricately regulated because of the central roles of these proteins in nutrient acquisition. In addition to classical induction-repression mechanisms involving repressor and activator proteins, other types of regulation, such as antitermination, have been observed in some PTSs. Apart from their role in carbohydrate transport, PTS proteins are involved in chemotaxis toward PTS carbohydrates. Furthermore, the IIAGlc protein, part of the glucose-specific PTS, is a central regulatory protein which in its nonphosphorylated form can bind to and inhibit several non-PTS uptake systems and thus prevent entry of inducers. In its phosphorylated form, P-IIAGlc is involved in the activation of adenylate cyclase and thus in the regulation of gene expression. By sensing the presence of PTS carbohydrates in the medium and adjusting the phosphorylation state of IIAGlc, cells can adapt quickly to changing conditions in the environment. In gram-positive bacteria, it has been demonstrated that HPr can be phosphorylated by ATP on a serine residue and this modification may perform a regulatory function.

Two glucose transport systems in Bacillus licheniformis

Bacillus licheniformis NCIB 6346 showed active accumulation of glucose which was inhibited by agents which affect the transmembrane proton gradient. Phosphotransferase (PTS) activity, identified as phosphoenolpyruvate-dependent phosphorylation of glucose, was found in cell extracts but could not be demonstrated in cells permeabilized with toluene when assays were conducted at pH 6.6. The same was true for mannitol and fructose phosphotransferase activities. Cells grown on fructose accumulated glucose at a slower rate than glucose-grown cells, and extracts prepared from them did not contain glucose PTS activity. Examination of the effects of analogs on glucose uptake and phosphorylation showed that 2-deoxyglucose was not a PTS substrate, but did markedly inhibit glucose uptake, with stronger inhibition in cells grown on fructose. Glucose accumulation by whole cells grown on glucose became less sensitive to the uncoupler tetrachlorosalicylanilide (TCS) as the pH was raised from 6.6 to 8.0, while in fructose-grown cells TCS was equally effective across this pH range. PTS activity was exhibited by toluene-treated cells at pH 7.5 and above, although the system itself in extracts was not affected by pH in the range of 5.0 to 8.0. The results are consistent with the presence of two glucose transport systems, one a PTS and the other operating by an alternative mechanisms, and suggest that the PTS in B. licheniformis may be regulated in a pH-dependent manner.

ATP-dependent protein kinases in bacteria

Protein phosphorylation has been shown to occur in over fifty different bacterial species and, therefore, seems to be a universal device among prokaryotes. Most of the protein kinases responsible for this modification of proteins share the common property of using adenosine triphosphate as phosphoryl donor. However, they differ from one another in a number of structural and functional aspects. Namely, they exhibit a varying acceptor amino acid specificity and can be classified, on this basis, in three main groups: protein-histidine kinases, protein-serine/threonine kinases and protein-tyrosine kinases.

The role of phosphorylation of HPr, a phosphocarrier protein of the phosphotransferase system, in the regulation of carbon metabolism in gram-positive bacteria

HPr of the Gram-positive bacterial phosphotransferase system (PTS) can be phosphorylated by an ATP-dependent protein kinase on a serine residue or by PEP-dependent Enzyme 1 on a histidyl residue. Both phosphorylation events appear to influence the metabolism of non-PTS carbon sources. Catabolite repression of the gluconate (gnt) operon of B. subtilis appears to be regulated by the former phosphorylation event, while glycerol kinase appears to be regulated by the latter phosphorylation reaction. The extent of our understanding of these processes will be described.

Structure of the IIA domain of the glucose permease of Bacillus subtilis at 2.2-A resolution

The crystal structure of the IIA domain of the glucose permease of the phosphoenolpyruvate:sugar phosphotransferase system (PTS) from Bacillus subtilis has been determined at 2.2-A resolution. Refinement of the structure is in progress, and the current R-factor is 0.201 (R = sigma h parallel Fo magnitude of - Fc parallel/sigma h magnitude of Fo, where magnitude of Fo and magnitude of Fc are the observed and calculated structure factor amplitudes, respectively) for data between 6.0- and 2.2-A resolution for which F greater than or equal to 2 sigma (F). This is an antiparallel beta-barrel structure that incorporates "Greek key" and "jellyroll" topological motifs. A shallow depression is formed at the active site by part of the beta-sheet and an omega-loop flanking one side of the sheet. His83, the histidyl residue which is the phosphorylation target of HPr and which transfers the phosphoryl group to the IIB domain of the permease, is located at the C-terminus of a beta-strand. The N epsilon atom is partially solvated and also interacts with the N epsilon atom of a second histidyl residue, His68, located at the N-terminus of an adjacent beta-strand, suggesting they share a proton. The geometry of the hydrogen bond is imperfect, though. Electrostatic interactions with other polar groups and van der Waals contacts with the side chains of two flanking phenylalanine residues assure the precise orientation of the imidazole rings. The hydrophobic nature of the surface around the His83-His68 pair may be required for protein-protein recognition by HPr or/and by the IIB domain of the permease. The side chains of two aspartyl residues, Asp31 and Asp87, are oriented toward each other across a narrow groove, about 7 A from the active-site His83, suggesting they may play a role in protein-protein interaction. A model of the phosphorylated form of the molecule is proposed, in which oxygen atoms of the phosphoryl group interact with the side chain of His68 and with the main-chain nitrogen atom of a neighboring residue, Val89. The model, in conjunction with previously reported site-directed mutagenesis experiments, suggests that the phosphorylation of His83 may be accompanied by the protonation of His68. This may be important for the interaction with the IIB domain of the permease and/or play a catalytic role in the phosphoryl transfer from IIA to IIB.

Quantitative determination of the intracellular concentration of the various forms of HPr, a phosphocarrier protein of the phosphoenolpyruvate: sugar phosphotransferase system in growing cells of oral streptococci

A simple procedure for quantitative estimation of the different phosphorylated forms of the phosphocarrier protein HPr in growing cells of oral streptococci is described. The growth of the cells was rapidly stopped by acidification of the medium and concomitant addition of the ionophore Gramicidin D. This procedure inactivated Enzyme I, HPr(Ser) kinase, HPr(Ser-P) phosphatase, and the enzymes involved in the metabolism of the allosteric effectors as well as the substrates of HPr phosphorylation. The cellular concentrations of HPr (His approximately P), HPr (Ser-P), HPr (His approximately P) (Ser-P), and free HPr were then determined by crossed immunoelectrophoresis.

The presence of two forms of the phosphocarrier protein HPr of the phosphoenolpyruvate:sugar phosphotransferase system in streptococci

The protein, HPr, a necessary component of the phosphoenolpyruvate phosphotransferase system (PTS) in bacteria, was purified from Streptococcus salivarius by column chromatography. The purified preparation gave only one band when analyzed by sodium dodecylsulfate gel electrophoresis or by isoelectric focusing in polyacrylamide gel (pI = 4.85). However, electrophoresis in Tris-containing buffers under non-denaturing conditions revealed 2 bands that could be phosphorylated by PEP in the presence of enzyme I of the PTS or by ATP with the HPr kinase. Homogeneous preparations of these 2 forms could be obtained by preparative electrophoresis. Each preparation exhibited only 1 band when analyzed by electrophoresis under non-denaturing conditions, indicating that the doublet observed before preparative electrophoresis was not an electrophoretic artefact. The electrophoretic mobility of each protein was not modified following heat-treatment at 100 degrees C for 20 min or storage at -40 degrees C for several months. Both HPr proteins catalyzed in vitro the PEP-dependent phosphorylation of glucose, but at a rate slightly lower than that observed with a preparation of HPr containing both forms of the protein. Both forms were also able to transfer the phosphate group from PEP to the other specific PTS proteins known in S salivarius. Rabbit polyclonal antibodies directed against each form reacted with both proteins. The presence of the 2 forms of HPr was detected in fresh cellular extracts of S salivarius; however, their intracellular ratio varied according to growth conditions. A doublet was also found in many other streptococcal species tested (S mutans, S sobrinus, S sanguis, S thermophilus, S bovis, S rattus) and also in L lactis.(ABSTRACT TRUNCATED AT 250 WORDS)

Staphylococcal phosphoenolpyruvate-dependent phosphotransferase system. Purification and protein sequencing of the Staphylococcus carnosus histidine-containing protein, and cloning and DNA sequencing of the ptsH gene

The histidine-containing protein (HPr) of the bacterial phosphoenolpyruvate-dependent phosphotransferase system (PTS) was isolated from Staphylococcus carnosus and purified to homogeneity. The protein sequence was determined by Edman degradation of peptides obtained by proteolytic digestion with proteases V8, trypsin and chemical cleavage with BrCN. Furthermore, immunological screening of a chromosomal S. carnosus DNA gene library in pUC19 vector enabled us to isolate S. carnosus HPr-expressing colonies. The nucleotide sequence of this ptsH gene and its flanking regions was determined by the dideoxy-chain-termination technique. Upstream, the 264-bp open reading frame of the ptsH gene is flanked by a putative S. carnosus promoter structure and a putative ptsI gene downstream suggesting that ptsH gene is the first gene in the PTS operon of S. carnosus. Comparison of the amino acid sequence of S. carnosus HPr with the HPr sequence of Staphylococcus aureus (derived from peptide sequencing) showed a high degree of similarity.