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.

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.

The Bacillus subtilis crh gene encodes a HPr-like protein involved in carbon catabolite repression

Carbon catabolite repression (CCR) of several Bacillus subtilis catabolic genes is mediated by ATP-dependent phosphorylation of histidine-containing protein (HPr), a phosphocarrier protein of the phosphoenolpyruvate (PEP): sugar phosphotransferase system. In this study, we report the discovery of a new B. subtilis gene encoding a HPr-like protein, Crh (for catabolite repression HPr), composed of 85 amino acids. Crh exhibits 45% sequence identity with HPr, but the active site His-15 of HPr is replaced with a glutamine in Crh. Crh is therefore not phosphorylated by PEP and enzyme I, but is phosphorylated by ATP and the HPr kinase in the presence of fructose-1,6-bisphosphate. We determined Ser-46 as the site of phosphorylation in Crh by carrying out mass spectrometry with peptides obtained by tryptic digestion or CNBr cleavage. In a B. subtilis ptsH1 mutant strain, synthesis of beta-xylosidase, inositol dehydrogenase, and levanase was only partially relieved from CCR. Additional disruption of the crh gene caused almost complete relief from CCR. In a ptsH1 crh1 mutant, producing HPr and Crh in which Ser-46 is replaced with a nonphosphorylatable alanyl residue, expression of beta-xylosidase was also completely relieved from glucose repression. These results suggest that CCR of certain catabolic operons requires, in addition to CcpA, ATP-dependent phosphorylation of Crh, and HPr at Ser-46.

ATP-dependent protein kinase-catalyzed phosphorylation of a seryl residue in HPr, a phosphate carrier protein of the phosphotransferase system in Streptococcus pyogenes

HPr, a phosphate carrier protein of the streptococcal phosphotransferase system, is phosphorylated at the N-1 position of a single histidyl residue in a reaction requiring phosphoenolpyruvate (P-ePrv), Mg2+, and enzyme I (P-ePrv-HPr phosphotransferase, EC2.7.3.9). We demonstrate that in addition to this reaction, a seryl residue within HPr can be phosphorylated in an ATP-dependent process. This reaction is catalyzed by a protein kinase with an approximate Mr of 20,000. In whole cells the kinase activity is stimulated by glucose, whereas in crude extracts the activity is stimulated by glycolytic intermediates such as glucose 6-phosphate, fructose 1,6-diphosphate, and 2-phosphoglycerate. P-(Ser)-HPr cannot transfer its phosphate group via enzyme II to a sugar as does the P-(His)-HPr. Instead, a phosphatase (Mr = 70,000) was found to hydrolyze the phosphate group of P-(Ser)-HPr. The phosphatase reaction is strongly inhibited by the addition of P-ePrv and enzyme I. Protein kinase-catalyzed phosphorylation of the enzyme constituents of the phosphotransferase system in Escherichia coli has also been demonstrated. These observations lead us to suggest that phosphorylation of a seryl residue in HPr is involved in the regulation of sugar transport in the bacteria cell.

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.

Loss of protein kinase-catalyzed phosphorylation of HPr, a phosphocarrier protein of the phosphotransferase system, by mutation of the ptsH gene confers catabolite repression resistance to several catabolic genes of Bacillus subtilis

In gram-positive bacteria, HPr, a phosphocarrier protein of the phosphoenolpyruvate:sugar phosphotransferase system (PTS), is phosphorylated by an ATP-dependent, metabolite-activated protein kinase on seryl residue 46. In a Bacillus subtilis mutant strain in which Ser-46 of HPr was replaced with a nonphosphorylatable alanyl residue (ptsH1 mutation), synthesis of gluconate kinase, glucitol dehydrogenase, mannitol-1-P dehydrogenase and the mannitol-specific PTS permease was completely relieved from repression by glucose, fructose, or mannitol, whereas synthesis of inositol dehydrogenase was partially relieved from catabolite repression and synthesis of alpha-glucosidase and glycerol kinase was still subject to catabolite repression. When the S46A mutation in HPr was reverted to give S46 wild-type HPr, expression of gluconate kinase and glucitol dehydrogenase regained full sensitivity to repression by PTS sugars. These results suggest that phosphorylation of HPr at Ser-46 is directly or indirectly involved in catabolite repression. A strain deleted for the ptsGHI genes was transformed with plasmids expressing either the wild-type ptsH gene or various S46 mutant ptsH genes (S46A or S46D). Expression of the gene encoding S46D HPr, having a structure similar to that of P-ser-HPr according to nuclear magnetic resonance data, caused significant reduction of gluconate kinase activity, whereas expression of the genes encoding wild-type or S46A HPr had no effect on this enzyme activity. When the promoterless lacZ gene was put under the control of the gnt promoter and was subsequently incorporated into the amyE gene on the B. subtilis chromosome, expression of beta-galactosidase was inducible by gluconate and repressed by glucose. However, we observed no repression of beta-galactosidase activity in a strain carrying the ptsH1 mutation. Additionally, we investigated a ccpA mutant strain and observed that all of the enzymes which we found to be relieved from carbon catabolite repression in the ptsH1 mutant strain were also insensitive to catabolite repression in the ccpA mutant. Enzymes that were repressed in the ptsH1 mutant were also repressed in the ccpA mutant.

Specific recognition of the Bacillus subtilis gnt cis-acting catabolite-responsive element by a protein complex formed between CcpA and seryl-phosphorylated HPr

Catabolite repression of various Bacillus subtilis catabolic operons which carry a cis-acting catabolite-responsive element (CRE), such as the gnt operon, is mediated by CcpA, a protein belonging to the GalR-Lacl family of bacterial transcriptional repressors/activators, and the seryl-phosphorylated form of HPr, a phosphocarrier protein of the phosphoenolpyruvate:sugar phosphotransferase system. Footprinting experiments revealed that the purified CcpA protein interacted with P-ser-HPr to cause specific protection of the gnt CRE against DNase I digestion. The specific recognition of the gnt CRE was confirmed by the results of footprinting experiments using mutant gnt CREs carrying one of the following base substitutions within the CRE consensus sequence: G to T at position +149 or C to T at position +154 (+1 is the gnt transcription initiation nucleotide). The two mutant CREs causing a partial relief from catabolite repression were not protected by the CcpA/P-ser-HPr complex in footprinting experiments. Based on these and previous findings, we propose a molecular mechanism underlying catabolite repression in B. subtilis mediated by CcpA and P-ser-HPr.

The phosphoenolpyruvate:sugar phosphotransferase system in gram-positive bacteria: properties, mechanism, and regulation

This review consists of three major sections. The first and largest section reviews the protein constituents and known properties of the phosphotransferase systems present in well-studied Gram-positive bacteria. These bacteria include species of the following genera: (1) Staphylococcus, (2) Streptococcus, (3) Bacillus, (4) Lactobacillus, (5) Clostridium, (6) Arthrobacter, and (7) Brochothrix. The properties of the different systems are compared. The second major section deals with the regulation of carbohydrate uptake. There are four parts: (1) inhibition by intracellular sugar phosphates in Staphylococcus aureus, (2) PTS-mediated regulation of glycerol uptake in Bacillus subtilis, (3) competition for phospho-HPr in Streptococcus mutans, and (4) the possible involvement of protein kinases in the regulation of sugar uptake via the phosphotransferase system. The third section deals with the phenomenon of inducer expulsion. The first part is concerned with the physiological characterization of the phenomenon; then the consequences of unregulated uptake and expulsion, a futile cycle of energy expenditure, are considered. Finally, the biochemistry of the protein kinase and the protein phosphate phosphatase system, which appears to regulate sugar transport via the phosphotransferase system, is defined. The review, therefore, concentrates on the phosphotransferase system, its functions in carbohydrate transport and phosphorylation, the mechanisms of its regulation, and the mechanism by which it participates in the regulation of other physiological processes in the bacterial cell.

Tyrosine phosphorylation: an emerging regulatory device of bacterial physiology

Tyrosine phosphorylation is a key device in numerous cellular functions in eukaryotes, but in bacteria this protein modification was largely ignored until the mid-1990s. The first conclusive evidence of bacterial tyrosine phosphorylation came only a decade ago. Since then, several tyrosine kinases exhibiting unexpected features have been identified in a variety of bacteria. These enzymes use homologues of Walker motifs of nucleotide-binding proteins for their catalytic mechanism, thus defining an idiosyncratic type of bacterial tyrosine kinases. Recently, bacterial tyrosine kinases have been found to phosphorylate an increasing list of endogenous protein substrates. This discovery contributes to the emerging picture that bacterial tyrosine phosphorylation is an important regulatory arsenal of bacterial physiology in addition to the classical serine/threonine kinases, and the 'two-component' and phosphotransferase systems.

New protein kinase and protein phosphatase families mediate signal transduction in bacterial catabolite repression

Carbon catabolite repression (CCR) is the prototype of a signal transduction mechanism. In enteric bacteria, cAMP was considered to be the second messenger in CCR by playing a role reminiscent of its actions in eukaryotic cells. However, recent results suggest that CCR in Escherichia coli is mediated mainly by an inducer exclusion mechanism. In many Gram-positive bacteria, CCR is triggered by fructose-1,6-bisphosphate, which activates HPr kinase, presumed to be one of the most ancient serine protein kinases. We here report cloning of the Bacillus subtilis hprK and hprP genes and characterization of the encoded HPr kinase and P-Ser-HPr phosphatase. P-Ser-HPr phosphatase forms a new family of phosphatases together with bacterial phosphoglycolate phosphatase, yeast glycerol-3-phosphatase, and 2-deoxyglucose-6-phosphate phosphatase whereas HPr kinase represents a new family of protein kinases on its own. It does not contain the domain structure typical for eukaryotic protein kinases. Although up to now the HPr modifying/demodifying enzymes were thought to exist only in Gram-positive bacteria, a sequence comparison revealed that they also are present in several Gram-negative pathogenic bacteria.

A functional genomics approach to establish the complement of carbohydrate transporters in Streptococcus pneumoniae

The aerotolerant anaerobe Streptococcus pneumoniae is part of the normal nasopharyngeal microbiota of humans and one of the most important invasive pathogens. A genomic survey allowed establishing the occurrence of twenty-one phosphotransferase systems, seven carbohydrate uptake ABC transporters, one sodium:solute symporter and a permease, underlining an exceptionally high capacity for uptake of carbohydrate substrates. Despite high genomic variability, combined phenotypic and genomic analysis of twenty sequenced strains did assign the substrate specificity only to two uptake systems. Systematic analysis of mutants for most carbohydrate transporters enabled us to assign a phenotype and substrate specificity to twenty-three transport systems. For five putative transporters for galactose, pentoses, ribonucleosides and sulphated glycans activity was inferred, but not experimentally confirmed and only one transport system remains with an unknown substrate and lack of any functional annotation. Using a metabolic approach, 80% of the thirty-two fermentable carbon substrates were assigned to the corresponding transporter. The complexity and robustness of sugar uptake is underlined by the finding that many transporters have multiple substrates, and many sugars are transported by more than one system. The present work permits to draw a functional map of the complete arsenal of carbohydrate utilisation proteins of pneumococci, allows re-annotation of genomic data and might serve as a reference for related species. These data provide tools for specific investigation of the roles of the different carbon substrates on pneumococcal physiology in the host during carriage and invasive infection.

Clinical aspects of obesity in childhood and adolescence

The level of fatness of a child at which morbidity acutely and/or later in life increases is determined on an acturial basis. Direct measurements of body fat content, e.g. hydrodensitometry, bioimpedance, or DEXA, are useful tools in scientific studies. However, body mass index (BMI) is easy to calculate and is generally accepted now to be used to define obesity in children and adolescents clinically. An increased risk of death from cardiovascular disease in adults has been found in subjects whose BMI had been greater than the 75th percentile as adolescents. Childhood obesity seems to substantially increase the risk of subsequent morbidity whether or not obesity persists into adulthood. The genetic basis of childhood obesity has been elucidated to some extent through the discovery of leptin, the ob gene product, and the increasing knowledge on the role of neuropeptides such as POMC, neuropeptide Y (NPY) and the melanocyte concentrating hormone receptors (for example, MC4R). Environmental/exogenous factors largely contribute to the development of a high degree of body fatness early in life. Twin studies suggest that approximately 50% of the tendency toward obesity is inherited. There are numerous disorders including a number of endocrine disorders (Cushing's syndrome, hypothyroidism, etc.) and genetic syndromes (Prader-Labhard-Willi syndrome, Bardet Biedl syndrome, etc.) that can present with obesity. A simple diagnostic algorithm allows for the differentiation between primary or secondary obesity. Among the most common sequelae of primary childhood obesity are hypertension, dyslipidemia, back pain and psychosocial problems. Therapeutic strategies include psychological and family therapy, lifestyle/behaviour modification and nutrition education. The role of regular exercise and exercise programmes is emphasized. Surgical procedures and drugs used in adult obesity are still not generally recommended in children and adolescents with obesity. As obesity is the most common chronic disorder in industrialized societies, its impact on individual lives as well as on health economics has to be recognized more widely. This review is aimed towards defining the clinical problem of childhood obesity on the basis of current knowledge and towards outlining future research areas in the field of energy homoesostasis and food intake in relation to child health. Finally, one should aim to increase public awareness of the ever increasing health burden and economic dimension of the childhood obesity epidemic that is present around the globe.

Transmembrane modulator-dependent bacterial tyrosine kinase activates UDP-glucose dehydrogenases

Protein-tyrosine kinases regulating bacterial exopolysaccharide synthesis autophosphorylate on tyrosines located in a conserved C-terminal region. So far no other substrates have been identified for these kinases. Here we demonstrate that Bacillus subtilis YwqD not only autophosphorylates at Tyr-228, but that it also phosphorylates the two UDP-glucose dehydrogenases (UDP-glucose DHs) YwqF and TuaD at a tyrosine residue. However, phosphorylation of YwqF and TuaD occurs only in the presence of the transmembrane protein YwqC. The presumed intracellular C-terminal part of YwqC (last 50 amino acids) seems to interact with the tyrosine-kinase and to allow YwqD-catalysed phosphorylation of the two UDP-glucose DHs, which are key enzymes for the synthesis of acidic polysaccharides. However, only when phosphorylated by YwqD do the two enzymes exhibit detectable UDP-glucose DH activity. Dephosphorylation of P-Tyr-YwqF and P-Tyr-TuaD by the P-Tyr-protein phosphatase YwqE switched off their UDP-glucose DH activity. YwqE, which is encoded by the fourth gene of the B.subtilis ywqCDEF operon, also dephosphorylates P-Tyr-YwqD.

Protein phosphorylation and regulation of carbon metabolism in gram-negative versus gram-positive bacteria

Bacteria impose regulatory mechanisms on metabolic processes to ensure that the needs of the cell are met but not exceeded. Here, we discuss the basic features of a mechanism by which carbohydrate catabolism in Gram-positive bacteria is regulated. Although the physiological consequences of this regulation are the same as in Gram-negative bacteria, the mechanism is entirely different. These regulatory processes evidently evolved late, after the divergence of Gram-negative bacteria, even though the targets of regulation are universal.

Ser/Thr/Tyr protein phosphorylation in bacteria - for long time neglected, now well established

The first clearly established example of Ser/Thr/Tyr phosphorylation of a bacterial protein was isocitrate dehydrogenase. In 1979, 25 years after the discovery of protein phosphorylation in eukaryotes, this enzyme was reported to become phosphorylated on a serine residue. In subsequent years, numerous other bacterial proteins phosphorylated on Ser, Thr or Tyr were discovered and the corresponding protein kinases and P-protein phosphatases were identified. These protein modifications regulate all kinds of physiological processes. Ser/Thr/Tyr phosphorylation in bacteria therefore seems to play a similar important role as in eukaryotes. Surprisingly, many bacterial protein kinases do not exhibit any similarity to eukaryotic protein kinases, but rather resemble nucleotide-binding proteins or kinases phosphorylating diverse low-molecular-weight substrates.

Binding of the catabolite repressor protein CcpA to its DNA target is regulated by phosphorylation of its corepressor HPr

Catabolite repression of a number of catabolic operons in bacilli is mediated by the catabolite control protein CcpA, the phosphocarrier protein HPr from the phosphoenolpyruvate-dependent sugar transport system (PTS), and a cis-acting DNA sequence termed the catabolite-responsive element (cre). We present evidence that CcpA interacts with HPr that is phosphorylated at Ser46 (Ser(P) HPr) and that these proteins form a specific ternary complex with cre DNA. Titration experiments following the circular dichroism signal of the cre DNA indicate that this complex consists of two molecules of Ser(P) HPr, a CcpA dimer, and the cre sequence. Limited proteolysis experiments indicate that the domain structure of CcpA is similar to other members of the LacI/GalR family of helix-turn-helix proteins, comprised of a helix-turn-helix DNA domain and a C-terminal effector domain. NMR titration of Ser(P) HPr demonstrates that the isolated C-terminal domain of CcpA forms a specific complex with Ser(P) HPr but not with unphosphorylated HPr. Based upon perturbations to the NMR spectrum, we propose that the binding site of the C-terminal domain of CcpA on Ser(P) HPr forms a contiguous surface that encompasses both Ser(P)46 and His15, the site of phosphorylation by enzyme I of the PTS. This allows CcpA to recognize the phosphorylation state of HPr, effectively linking the process of sugar import via the PTS to catabolite repression in bacilli.

The hprK gene of Enterococcus faecalis encodes a novel bifunctional enzyme: the HPr kinase/phosphatase

The HPr kinase of Gram-positive bacteria is an ATP-dependent serine protein kinase, which phosphorylates the HPr protein of the bacterial phosphotransferase system (PTS) and is involved in the regulation of carbohydrate metabolism. The hprK gene from Enterococcus faecalis was cloned via polymerase chain reaction (PCR) and sequenced. The deduced amino acid sequence was confirmed by microscale Edman degradation and mass spectrometry combined with collision-induced dissociation of tryptic peptides derived from the HPr kinase of E. faecalis. The gene was overexpressed in Escherichia coli, which does not contain any ATP-dependent HPr kinase or phosphatase activity. The homogeneous recombinant protein exhibits the expected HPr kinase activity as well as a P-Ser-HPr phosphatase activity, which was assumed to be a separate enzyme activity. The bifunctional HPr kinase/phosphatase acts preferentially as a kinase at high ATP levels of 2 mM occurring in glucose-metabolizing Streptococci. At low ATP levels, the enzyme hydrolyses P-Ser-HPr. In addition, high concentrations of phosphate present under starvation conditions inhibit the HPr kinase activity. Thus, a putative function of the enzyme may be to adjust the ratio of HPr and P-Ser-HPr according to the metabolic state of the cell; P-Ser-HPr is involved in carbon catabolite repression and regulates sugar uptake via the phosphotransferase system (PTS). Reinvestigation of the previously described Bacillus subtilis HPr kinase revealed that it also possesses P-Ser-HPr phosphatase activity. However, contrary to the E. faecalis enzyme, ATP alone was not sufficient to switch the phosphatase activity of the B. subtilis enzyme to the kinase activity. A change in activity of the B. subtilis HPr kinase was only observed when fructose-1,6-bisphosphate was also present.

Autophosphorylation of the Escherichia coli protein kinase Wzc regulates tyrosine phosphorylation of Ugd, a UDP-glucose dehydrogenase

Autophosphorylation of protein-tyrosine kinases (PTKs) involved in exopolysaccharide and capsular polysaccharide biosynthesis and transport has been observed in a number of Gram-negative and Gram-positive bacteria. However, besides their own phosphorylation, little is known about other substrates targeted by these protein-modifying enzymes. Here, we present evidence that the protein-tyrosine kinase Wzc of Escherichia coli is able to phosphorylate an endogenous enzyme, UDP-glucose dehydrogenase (Ugd), which participates in the synthesis of the exopolysaccharide colanic acid. The process of phosphorylation of Ugd by Wzc was shown to be stimulated by previous autophosphorylation of Wzc on tyrosine 569. The phosphorylation of Ugd was demonstrated to actually occur on tyrosine and result in a significant increase of its dehydrogenase activity. In addition, the phosphotyrosine-protein phosphatase Wzb, which is known to effectively dephosphorylate Wzc, exhibited only a low effect, if any, on the dephosphorylation of Ugd. These data were related to the recent observation that two other UDP-glucose dehydrogenases have been also shown to be phosphorylated by a PTK in the Gram-positive bacterium Bacillus subtilis. Comparative analysis of the activities of PTKs from Gram-negative and Gram-positive bacteria showed that they are regulated by different mechanisms that involve, respectively, either the autophosphorylation of kinases or their interaction with a membrane protein activator.

Catabolite regulation of the pta gene as part of carbon flow pathways in Bacillus subtilis

In Bacillus subtilis, the products of the pta and ackA genes, phosphotransacetylase and acetate kinase, play a crucial role in the production of acetate, one of the most abundant by-products of carbon metabolism in this gram-positive bacterium. Although these two enzymes are part of the same pathway, only mutants with inactivated ackA did not grow in the presence of glucose. Inactivation of pta had only a weak inhibitory effect on growth. In contrast to pta and ackA in Escherichia coli, the corresponding B. subtilis genes are not cotranscribed. Expression of the pta gene was increased in the presence of glucose, as has been reported for ackA. The effects of the predicted cis-acting catabolite response element (CRE) located upstream from the promoter and of the trans-acting proteins CcpA, HPr, Crh, and HPr kinase on the catabolite regulation of pta were investigated. As for ackA, glucose activation was abolished in ccpA and hprK mutants and in the ptsH1 crh double mutant. Footprinting experiments demonstrated an interaction between CcpA and the pta CRE sequence, which is almost identical to the proposed CRE consensus sequence. This interaction occurs only in the presence of Ser-46-phosphorylated HPr (HPrSer-P) or Ser-46-phosphorylated Crh (CrhSer-P) and fructose-1,6-bisphosphate (FBP). In addition to CcpA, carbon catabolite activation of the pta gene therefore requires at least two other cofactors, FBP and either HPr or Crh, phosphorylated at Ser-46 by the ATP-dependent Hpr kinase.

Cooperative and non-cooperative DNA binding modes of catabolite control protein CcpA from Bacillus megaterium result from sensing two different signals

Carbon catabolite repression (CCR) of several operons in Bacillus subtilis and Bacillus megaterium is mediated by the cis-acting cre sequence and trans-acting catabolite control protein (CcpA). We describe purification of CcpA from B. megaterium and its interaction with regulatory sequences from the xyl operon. Specific interaction of CcpA with cre as scored by DNase I footprints at concentrations similar to the in vivo situation requires the presence of effectors. We have found two molecular effectors for CcpA activity, which lead to different recognition modes of DNA. The heat-stable phosphotransfer protein HPr from the PTS sugar uptake system triggers non-cooperative binding of CcpA to cre when phosphorylated at Ser46 (HPr-Ser46-P). Glucose 6-phosphate (Glc-6-P) triggers cooperative binding of CcpA to cre and two auxiliary cre* sites, one of which overlaps the -35 box of the xyl promoter. Binding to cre* depends on the presence of the functional cre sequence. A mutation in cre abolishes carbon catabolite repression in vivo and binding of CcpA to cre and cre* in vitro, indicating looping of the intervening DNA. The two triggers are not simultaneously active. The acidity of the buffer determines which of them activates CcpA when both are present in vitro. Glc-6-P is preferred at pH values below 5.4, and HPr-Ser46-P is preferred at neutral pH. The Ccpa dimers present at neutral pH form tetramers and higher oligomers at pH 4.6, explaining cooperativity of binding to DNA. CcpA is the first member of the LacI/GalR family of regulators, for which oligomerization without the leucine zipper at the C terminus is demonstrated.

A cell-cell communication system regulates protease production during sporulation in bacteria of the Bacillus cereus group

In sporulating Bacillus, major processes like virulence gene expression and sporulation are regulated by communication systems involving signalling peptides and regulators of the RNPP family. We investigated the role of one such regulator, NprR, in bacteria of the Bacillus cereus group. We show that NprR is a transcriptional regulator whose activity depends on the NprX signalling peptide. In association with NprX, NprR activates the transcription of an extracellular protease gene (nprA) during the first stage of the sporulation process. The transcription start site of the nprA gene has been identified and the minimal region necessary for full activation has been characterized by promoter mutagenesis. We demonstrate that the NprX peptide is secreted, processed and then reimported within the bacterial cell. Once inside the cell, the mature form of NprX, presumably the SKPDIVG heptapeptide, directly binds to NprR allowing nprA transcription. Alignment of available NprR sequences from different species of the B. cereus group defines seven NprR clusters associated with seven NprX heptapeptide classes. This cell-cell communication system was found to be strain-specific with a possible cross-talk between some pherotypes. The phylogenic relationship between NprR and NprX suggests a coevolution of the regulatory protein and its signalling peptide.

Phosphorylation of either crh or HPr mediates binding of CcpA to the bacillus subtilis xyn cre and catabolite repression of the xyn operon

Carbon catabolite repression (CCR) of several Bacillus subtilis catabolic genes is mediated by ATP-dependent phosphorylation of Ser46 of the histidine-containing protein (HPr), a phosphocarrier protein of the phosphoenolpyruvate (PEP): sugar phosphotransferase system. A recently discovered HPr-like protein of B. subtilis, Crh, cannot be phosphorylated by PEP and enzyme I but becomes phosphorylated at Ser46 by the ATP-dependent, metabolite-activated HPr kinase. Genetic data suggested that Crh is also implicated in CCR. We here demonstrate that in a ptsH1 crh1 mutant, in which Ser46 of both HPr and Crh is replaced with an alanyl residue, expression of the beta-xylosidase-encoding xynB gene was completely relieved from CCR. No effect on CCR could be observed in strains carrying the crh1 allele, suggesting that under the experimental conditions P-Ser-HPr can substitute for P-Ser-Crh in CCR. By contrast, a ptsH1 mutant was slightly relieved from CCR of xynB, indicating that P-Ser-Crh can substitute only partly for P-Ser-HPr. Mapping experiments allowed us to identify the xyn promoter and a catabolite responsive element (cre) located 229 bp downstream of the transcription start point. Using DNase I footprinting experiments, we could demonstrate that similar to P-Ser-HPr, P-Ser-Crh stimulates binding of CcpA to the xyn cre. Fructose 1,6-bisphosphate was found to strongly enhance binding of the P-Ser-HPr/CcpA and P-Ser-Crh/CcpA complexes to the xyn cre, but had no effect on binding of CcpA alone.

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.