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

Novel motif associated with carbon catabolite repression in two major Gram-positive pathogen virulence regulatory proteins

Carbon catabolite repression (CCR) is a widely conserved regulatory process that ensures enzymes and transporters of less-preferred carbohydrates are transcriptionally repressed in the presence of a preferred carbohydrate. This phenomenon can be regulated via a CcpA-dependent or CcpA-independent mechanism. The CcpA-independent mechanism typically requires a transcriptional regulator harboring a phosphotransferase regulatory domain (PRD) that interacts with phosphotransferase system (PTS) components. PRDs contain a conserved histidine residue that is phosphorylated by the PTS-associated HPr-His15~P protein. PRD-containing regulators often harbor additional domains that resemble PTS-associated EIIB protein domains with a conserved cysteine residue that can be phosphorylated by cognate PTS components. We noted that Mga, the PRD-containing central virulence regulator of Streptococcus pyogenes, has an EIIBGat domain containing a cysteine that, based on the presence of a similar motif in glycerol kinase, could be a target for phosphorylation. Using site-directed mutagenesis, we constructed phospho-ablative and phospho-mimetic substitutions of this cysteine and found that these substitutions modify the CCR of the Rgg2/3 quorum-sensing system. Moreover, we provide genetic evidence that the phospho-donor of this cysteine residue is likely to be ManL, the EIIA/B subunit of the mannose PTS system. Interestingly, a structurally distinct virulence gene regulator, PrfA of Listeria monocytogenes, harbors a similar cysteine-containing motif, and phospho-ablative and phospho-mimetic substitutions of the cysteine-altered CCR of PrfA-dependent virulence gene expression. Collectively, our data suggest that phosphorylation of a cysteine within the shared novel motif in Mga and PrfA may be a heretofore missing link between cellular metabolism and virulence.IMPORTANCEIn this study, we identified a novel cysteine-containing motif within the amino acid sequence of two structurally distinct transcriptional regulators of virulence in two Gram-positive pathogens that appears to link carbon metabolism with virulence gene expression. The results also highlight the potential post-translational modification of cysteine in bacterial species, a rare and understudied modification.

Carbon catabolite repression on the Rgg2/3 quorum sensing system in Streptococcus pyogenes is mediated by PTSMan and Mga

Streptococcus pyogenes, also known as group A Streptococcus or GAS, is a human-restricted pathogen causing a diverse array of infections. The ability to adapt to different niches requires GAS to adjust gene expression in response to environmental cues. We previously identified the abundance of biometals and carbohydrates led to natural induction of the Rgg2/3 cell-cell communication system (quorum sensing, QS). Here we determined the mechanism by which the Rgg2/3 QS system is stimulated exclusively by mannose and repressed by glucose, a phenomenon known as carbon catabolite repression (CCR). Instead of carbon catabolite protein A, the primary mediator of CCR in Gram-positive bacteria; CCR of Rgg2/3 requires the PTS regulatory domain (PRD)-containing transcriptional regulator Mga. Deletion of Mga led to carbohydrate-independent activation of Rgg2/3 by down-regulating rgg3, the QS repressor. Through phosphoablative and phosphomimetic substitutions within Mga PRDs, we demonstrated that selective phosphorylation of PRD1 conferred repression of the Rgg2/3 system. Moreover, given the carbohydrate specificity mediating Mga-dependent governance over Rgg2/3, we tested mannose-specific PTS components and found the EIIA/B subunit ManL was required for Mga-dependent repression. These findings provide newfound connections between PTS^Man , Mga, and QS, and further demonstrate that Mga is a central regulatory nexus for integrating nutritional status and virulence.

Carbon Catabolite Repression and the Related Genes of ccpA, ptsH and hprK in Thermoanaerobacterium aotearoense

The strictly anaerobic, Gram-positive bacterium, Thermoanaerobacterium aotearoense SCUT27, is capable of producing ethanol, hydrogen and lactic acid by directly fermenting glucan, xylan and various lignocellulosically derived sugars. By using non-metabolizable and metabolizable sugars as substrates, we found that cellobiose, galactose, arabinose and starch utilization was strongly inhibited by the existence of 2-deoxyglucose (2-DG). However, the xylose and mannose consumptions were not markedly affected by 2-DG at the concentration of one-tenth of the metabolizable sugar. Accordingly, T. aotearoense SCUT27 could consume xylose and mannose in the presence of glucose. The carbon catabolite repression (CCR) related genes, ccpA, ptsH and hprK were confirmed to exist in T. aotearoense SCUT27 through gene cloning and protein characterization. The highly purified Histidine-containing Protein (HPr) could be specifically phosphorylated at Serine 46 by HPr kinase/phosphatase (HPrK/P) with no need to add fructose-1,6-bisphosphate (FBP) or glucose-6-phosphate (Glc-6-P) in the reaction mixture. The specific protein-interaction of catabolite control protein A (CcpA) and phosphorylated HPr was proved via affinity chromatography in the absence of formaldehyde. The equilibrium binding constant (K_D) of CcpA and HPrSerP was determined as 2.22 ± 0.36 nM by surface plasmon resonance (SPR) analysis, indicating the high affinity between these two proteins.

Fueling the caries process: carbohydrate metabolism and gene regulation by Streptococcus mutans

The nature of the oral cavity and host behaviors has mandated that the oral microbiota evolve mechanisms for coping with environmental fluctuations, especially changes in the type and availability of carbohydrates. In the case of human dental caries, the presence of excess carbohydrates is often responsible for altering the local environment to be more favorable for species associated with the initiation and progression of disease, including Streptococcus mutans. Some of the earliest endeavors to understand how cariogenic species respond to environmental perturbations were carried out using chemostat cultivation, which provides fine control over culture conditions and bacterial behaviors. The development of genome-scale methodologies has allowed for the combination of sophisticated cultivation technologies with genome-level analysis to more thoroughly probe how bacterial pathogens respond to environmental stimuli. Recent investigations in S. mutans and other closely related streptococci have begun to reveal that carbohydrate metabolism can drastically impact pathogenic potential and highlight the important influence that nutrient acquisition has on the success of pathogens; inside and outside of the oral cavity. Collectively, research into pathogenic streptococci, which have evolved in close association with the human host, has begun to unveil the essential nature of careful orchestration of carbohydrate acquisition and catabolism to allow the organisms to persist and, when conditions allow, initiate or worsen disease.

Modification of gene expression and virulence traits in Streptococcus mutans in response to carbohydrate availability

The genetic and phenotypic responses of Streptococcus mutans, an organism that is strongly associated with the development of dental caries, to changes in carbohydrate availability were investigated. S. mutans UA159 or a derivative of UA159 lacking ManL, which is the EIIAB component (EIIAB(Man)) of a glucose/mannose permease of the phosphoenolpyruvate:sugar phosphotransferase system (PTS) and a dominant effector of catabolite repression, was grown in continuous culture to steady state under conditions of excess (100 mM) or limiting (10 mM) glucose. Microarrays using RNA from S. mutans UA159 revealed that 174 genes were differentially expressed in response to changes in carbohydrate availability (P < 0.001). Glucose-limited cells possessed higher PTS activity, could acidify the environment more rapidly and to a greater extent, and produced more ManL protein than cultures grown with excess glucose. Loss of ManL adversely affected carbohydrate transport and acid tolerance. Comparison of the histidine protein (HPr) in S. mutans UA159 and the manL deletion strain indicated that the differences in the behaviors of the strains were not due to major differences in HPr pools or HPr phosphorylation status. Therefore, carbohydrate availability alone can dramatically influence the expression of physiologic and biochemical pathways that contribute directly to the virulence of S. mutans, and ManL has a profound influence on this behavior.

Surface export of GAPDH/SDH, a glycolytic enzyme, is essential for Streptococcus pyogenes virulence

Streptococcal surface dehydrogenase (SDH) (glyceraldehyde-3-phosphate dehydrogenase [GAPDH]) is an anchorless major multifunctional surface protein in group A Streptococcus (GAS) with the ability to bind important mammalian proteins, including plasmin(ogen). Although several biological properties of SDH are suggestive of its possible role in GAS virulence, its direct role in GAS pathogenesis has not been ascertained because it is essential for GAS survival. Thus, it has remained enigmatic as to “how and why” SDH/GAPDH is exported onto the bacterial surface. The present investigation highlights “why” SDH is exported onto the GAS surface. Differential microarray-based genome-wide transcript abundance analysis was carried out using a specific mutant, which was created by inserting a hydrophobic tail at the C-terminal end of SDH (M1-SDH_HBtail) and thus preventing its exportation onto the GAS surface. This analysis revealed downregulation of the majority of genes involved in GAS virulence and genes belonging to carbohydrate and amino acid metabolism and upregulation of those related to lipid metabolism. The complete attenuation of this mutant for virulence in the mouse model and the decreased and increased virulence of the wild-type and mutant strains postcomplementation with SDH_HBtail and SDH, respectively, indicated that the SDH surface export indeed regulates GAS virulence. M1-SDH_HBtail also displayed unaltered growth patterns, increased intracellular ATP concentration and Hpr double phosphorylation, and significantly reduced pH tolerance, streptolysin S, and SpeB activities. These phenotypic and physiological changes observed in the mutant despite the unaltered expression levels of established transcriptional regulators further highlight the fact that SDH interfaces with many regulators and its surface exportation is essential for GAS virulence.

Streptococcal surface dehydrogenase (SDH), a classical anchorless cytoplasmically localized glycolytic enzyme, is exported onto the group A Streptococcus (GAS) surface through a hitherto unknown mechanism(s). It has not been known why GAS or other prokaryotes should export this protein onto the surface. By genetic manipulations, we created a novel GAS mutant strain expressing SDH with a 12-amino-acid hydrophobic tail at its C-terminal end and thus were able to prevent its surface exportation without altering its enzymatic activity or growth pattern. Interestingly, the mutant was completely attenuated for virulence in a mouse peritonitis model. The global gene expression profiles of this mutant reveal that the surface exportation of SDH is mandatory to maintain GAS virulence. The ability of GAS as a successful pathogen to localize SDH in the cytoplasm as well as on the surface is physiologically relevant and dynamically obligatory to fine-tune the functions of many transcriptional regulators and also to exploit its virulence properties for infection.

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).

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.

Synthesis of HPr(Ser-P)(His-P) by enzyme I of the phosphoenolpyruvate: sugar phosphotransferase system of Streptococcus salivarius

HPr is a protein of the bacterial phosphoenolpyruvate:sugar phosphotransferase transport system (PTS). In Gram-positive bacteria, HPr can be phosphorylated on Ser(46) by HPr(Ser) kinase/phosphorylase (HPrK/P) and on His(15) by enzyme I (EI) of the PTS. In vitro studies have shown that phosphorylation on one residue greatly inhibits the second phosphorylation. However, streptococci contain significant amounts of HPr(Ser-P)(His approximately P) during exponential growth, and recent studies suggest that phosphorylation of HPr(Ser-P) by EI is involved in the recycling of HPr(Ser-P)(His approximately P). We report in this paper a study on the phosphorylation of Streptococcus salivarius HPr, HPr(Ser-P), and HPr(S46D) by EI. Our results indicate that (i) the specificity constant (k(cat)/K(m)) of EI for HPr(Ser-P) at pH 7.9 was approximately 5000-fold smaller than that observed for HPr, (ii) no metabolic intermediates were able to stimulate HPr(Ser-P) phosphorylation, (iii) the rate of HPr phosphorylation decreased at pHs below 6.5, while that of HPr(Ser-P) increased and was almost 10-fold higher at pH 6.1 than at pH 7.9, (iv) HPr(S46D), a mutated HPr alleged to mimic HPr(Ser-P), was also phosphorylated more efficiently under acidic conditions, and, lastly, (v) phosphorylation of Bacillus subtilis HPr(Ser-P) by B. subtilis EI was also stimulated at acidic pH. Our results suggest that the high levels of HPr(Ser-P)(His approximately P) in streptococci result from the combination of two factors, a high physiological concentration of HPr(Ser-P) and stimulation of HPr(Ser-P) phosphorylation by EI at acidic pH, an intracellular condition that occurs in response to the acidification of the external medium during growth of the culture.

Two-dimensional fluorescence difference gel electrophoretic analysis of Streptococcus mutans biofilms

Compared with traditional two-dimensional (2D) proteome analysis of Streptococcus mutans grown as a biofilm from a planktonic culture at steady state (Rathsam et al., Microbiol. 2005, 151, 1823-1837), the use of 2D fluorescence difference gel electrophoresis (DIGE) led to a 3-fold increase in the number of identified protein spots that were significantly altered in their level of expression (P < 0.050). Of the 73 identified proteins, only nine were up-regulated in biofilm grown cells. The results supported the previously surmised hypothesis that general metabolic functions were down-regulated in response to a reduction in growth rate in mature S. mutans biofilms. Up-regulation of competence proteins without any concomitant increase in stress-responsive proteins was confirmed, while the levels of glucosyltransferase C (GtfC), involved in glucan formation, O-acetylserine sulfhyrylase (cysteine synthetase A; CsyK), implicated in the formation of [Fe-S] clusters, and a hypothetical protein encoded by the open reading frame, SMu0188, were also up-regulated.

Catabolite control protein A (CcpA) contributes to virulence and regulation of sugar metabolism in Streptococcus pneumoniae

We characterized the role of catabolite control protein A (ccpA) in the physiology and virulence of Streptococcus pneumoniae. S. pneumoniae has a large percentage of its genome devoted to sugar uptake and metabolism, and therefore, regulation of these processes is likely to be crucial for fitness in the nasopharynx and may play a role during invasive disease. In many bacteria, carbon catabolite repression (CCR) is central to such regulation, influencing hierarchical sugar utilization and growth rates. CcpA is the major transcriptional regulator in CCR in several gram-positive bacteria. We show that CcpA functions in CCR of lactose-inducible beta-galactosidase activity in S. pneumoniae. CCR of maltose-inducible alpha-glucosidase, raffinose-inducible alpha-galactosidase, and cellobiose-inducible beta-glucosidase is unaffected in the ccpA strain, suggesting that other regulators, possibly redundant with CcpA, control these systems. The ccpA strain is severely attenuated for nasopharyngeal colonization and lung infection in the mouse, establishing its role in fitness on these mucosal surfaces. Comparison of the cell wall fraction of the ccpA and wild-type strains shows that CcpA regulates many proteins in this compartment that are involved in central and intermediary metabolism, a subset of which are required for survival and multiplication in vivo. Both in vitro and in vivo defects were complemented by providing ccpA in trans. Our results demonstrate that CcpA, though not a global regulator of CCR in S. pneumoniae, is required for colonization of the nasopharynx and survival and multiplication in the lung.

Quantitative interdependence of coeffectors, CcpA and cre in carbon catabolite regulation of Bacillus subtilis

The phosphoproteins HPrSerP and CrhP are the main effectors for CcpA-mediated carbon catabolite regulation (CCR) in Bacillus subtilis. Complexes of CcpA with HPrSerP or CrhP regulate genes by binding to the catabolite responsive elements (cre). We present a quantitative analysis of HPrSerP and CrhP interaction with CcpA by surface plasmon resonance (SPR) revealing small and similar equilibrium constants of 4.8 +/- 0.4 microm for HPrSerP-CcpA and 19.1 +/- 2.5 microm for CrhP-CcpA complex dissociation. Forty millimolar fructose-1,6-bisphosphate (FBP) or glucose-6-phosphate (Glc6-P) increases the affinity of HPrSerP to CcpA at least twofold, but have no effect on CrhP-CcpA binding. Saturation of binding of CcpA to cre as studied by fluorescence and SPR is dependent on 50 microm of HPrSerP or > 200 microm CrhP. The rate constants of HPrSerP-CcpA-cre complex formation are k(a) = 3 +/- 1 x 10(6) m(-1).s(-1) and k(d) = 2.0 +/- 0.4 x 10(-3).s(-1), resulting in a K(D) of 0.6 +/- 0.3 nm. FBP and Glc6-P stimulate CcpA-HPrSerP but not CcpA-CrhP binding to cre. Maximal HPrSerP-CcpA-cre complex formation in the presence of 10 mm FBP requires about 10-fold less HPrSerP. These data suggest a specific role for FBP and Glc6-P in enhancing only HPrSerP-mediated CCR.

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).

Acidic proteome of growing and restingLactococcus lactismetabolizing maltose

The acidic proteome of Lactococcus lactis grown anaerobically was compared for three different growth conditions: cells growing on maltose, resting cells metabolizing maltose, and cells growing on glucose. In maltose metabolizing cells several proteins were up-regulated compared with glucose metabolizing cells, however only some of the up-regulated proteins had apparent relation to maltose metabolism. Cells growing on maltose produced formate, acetate and ethanol in addition to lactate, whereas resting cells metabolizing maltose and cells growing on glucose produced only lactate. Increased levels of alcohol-acetaldehyde dehydrogenase (ADH) and phosphate acetyltransferase (PTA) in maltose-growing cells compared with glucose-growing cells coincided with formation of mixed acids in maltose-growing cells. The resting cells did not grow due to lack of an amino acid source and fermented maltose with lactate as the sole product, although ADH and PTA were present at high levels. The maltose consumption rate was approximately three times lower in resting cells than in exponentially growing cells. However, the enzyme levels in resting and growing cells metabolizing maltose were similar, which indicates that the difference in product formation in this case is due to regulation at the enzyme level. The levels of 30S ribosomal proteins S1 and S2 increased with increasing growth rate for resting cells metabolizing maltose, maltose-growing cells and glucose-growing cells. A modified form of HPr was synthesized under amino acid starvation. This is suggested to be due to alanine misincorporation for valine, which L. lactis is auxotrophic for. L. lactis conserves the protein profile to a high extent, even after prolonged amino acid starvation, so that the protein expression profile of the bacterium remains almost invariant.

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.

Phosphorylation of Streptococcus salivarius lactose permease (LacS) by HPr(His ~ P) and HPr(Ser-P)(His ~ P) and effects on growth

The oral bacterium Streptococcus salivarius takes up lactose via a transporter called LacS that shares 95% identity with the LacS from Streptococcus thermophilus, a phylogenetically closely related organism. S. thermophilus releases galactose into the medium during growth on lactose. Expulsion of galactose is mediated via LacS and stimulated by phosphorylation of the transporter by HPr(His approximately P), a phosphocarrier of the phosphoenolpyruvate:sugar phosphotransferase transport system (PTS). Unlike S. thermophilus, S. salivarius grew on lactose without expelling galactose and took up galactose and lactose concomitantly when it is grown in a medium containing both sugars. Analysis of the C-terminal end of S. salivarius LacS revealed a IIA-like domain (IIA(LacS)) almost identical to the IIA domain of S. thermophilus LacS. Experiments performed with purified proteins showed that S. salivarius IIA(LacS) was reversibly phosphorylated on a histidine residue at position 552 not only by HPr(His approximately P) but also by HPr(Ser-P)(His approximately P), a doubly phosphorylated form of HPr present in large amounts in rapidly growing S. salivarius cells. Two other major S. salivarius PTS proteins, IIAB(L)(Man) and IIAB(H)(Man), were unable to phosphorylate IIA(LacS). The effect of LacS phosphorylation on growth was studied with strain G71, an S. salivarius enzyme I-negative mutant that cannot synthesize HPr(His approximately P) or HPr(Ser-P)(His approximately P). These results indicated that (i) the wild-type and mutant strains had identical generation times on lactose, (ii) neither strain expelled galactose during growth on lactose, (iii) both strains metabolized lactose and galactose concomitantly when grown in a medium containing both sugars, and (iv) the growth of the mutant was slightly reduced on galactose.

The HPr(Ser) kinase of Streptococcus salivarius: a hexameric bifunctional enzyme controlled by glycolytic intermediates and inorganic phosphate

Phosphorylation of HPr, the small phosphocarrier protein of the phosphoenolpyruvate:sugar phosphotransferase system, on Ser46 by the HPr(Ser) kinase (HPrK/P) is a vital step in catabolite repression in Gram-positive bacteria. Streptococcus salivarius HPrK/P is reported to be a multimeric protein not regulated by metabolic intermediates. We re-evaluated the molecular mass of S. salivarius HPrK/P using sedimentation equilibrium ultracentrifugation, demonstrated that S. salivarius HPrK/P dephosphorylated HPr(Ser-P) and further characterised the effect of fructose 1,6-bisphosphate and other metabolic intermediates on enzyme activities. The molecular mass of S. salivarius HPrK/P was 201305 Da, suggesting that streptococcal HPrK/P was a hexameric protein. Fructose 1,6-bisphosphate poorly activated streptococcal HPrK/P but protected kinase activity against inhibition by inorganic phosphate and inhibited dephosphorylation of HPr(Ser-P). Phosphoenolpyruvate and 2-phosphoglycerate, but not fructose 1-P, fructose 6-P, and ribulose 1,5-bisphosphate, also protected kinase activity against inhibition by inorganic phosphate. Thus, unlike previous reports, we show that fructose 1,6-bisphosphate and other key glycolytic intermediates played a pivotal role as a modulator of streptococcal HPrK/P activities.

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.

Properties and regulation of the bifunctional enzyme HPr kinase/phosphatase in Bacillus subtilis

The bifunctional allosteric enzyme HPr kinase/phosphatase (HPrK/P) from Bacillus subtilis is a key enzyme in the main mechanism of carbon catabolite repression/activation (i.e. a means for the bacteria to adapt rapidly to environmental changes in carbon sources). In this regulation system, the enzyme can phosphorylate and dephosphorylate two proteins, HPr/HPr(Ser(P)) and Crh/Crh(Ser(P)), sensing the metabolic state of the cell. To acquire further insight into the properties of HPrK/P, electrospray ionization mass spectrometry, dynamic light scattering, and BIACORE were used to determine the oligomeric state of the protein under native conditions, revealing that the enzyme exists as a hexamer at pH 6.8 and as a monomer and dimer at pH 9.5. Using an in vitro radioactive assay, the influence of divalent cations, pH, temperature, and different glycolytic intermediates on the activity as well as kinetic parameters were investigated. The presence of divalent cations was found to be essential for both opposing activities of the enzyme. Furthermore, pH values equal to the internal pH of vegetative cells seem to favor the kinase activity, whereas lower pH values increased the phosphatase activity. Among the glycolytic intermediates evaluated, fructose 1,6-diphosphate and fructose 2,6-diphosphate were found to be allosteric activators in the kinase assay, whereas high concentrations inhibited the phosphatase activity, except for fructose 1,6-diphosphate in the case of HPr(Ser(P)). Phosphatase activity was induced by inorganic phosphate as well as acetyl phosphate and glyceraldehyde 3-phosphate. Kinetic parameters indicate a preference for binding of HPr compared with Crh to the enzyme and supported a strong positive cooperativity. This work suggests that the oligomeric state of the enzyme is influenced by several effectors and is correlated to the kinase or phosphatase activity. The phosphatase activity is mainly supported by the hexameric form.

Insights into the functioning of Bacillus subtilis HPr kinase/phosphatase: affinity for its protein substrates and role of cations and phosphate

In Bacillus subtilis, carbon catabolite repression is mediated by the HPr kinase/phosphatase (HprK/P) which catalyzes both an ATP-dependent phosphorylation and a dephosphorylation on Ser-46 of either HPr (histidine-containing protein) or Crh (catabolite repression HPr). By using a surface plasmon resonance approach, it was shown here that the presence of magnesium is a prerequisite for the interaction of HprK/P with either HPr or Crh. HprK/P binds both protein substrates with a similar affinity (K(D) of about 40 nM), and addition of nucleotides increases by about 10-fold its affinity for each substrate. In addition, the specificity and the concentration of the cation required for the binding of protein substrates are different from that exhibited by the cation-binding site involved in the nucleotide binding, suggesting the presence of two cation-binding sites on HprK/P. The effects of phosphate on enzymatic activities of HprK/P were also investigated. Phosphate was able to unmask the phosphatase activity, especially in the presence of ATP or both ATP and fructose 1,6-bisphosphate whereas it was shown to inhibit the kinase activity of HprK/P. An apparent competition between phosphate and a fluorescent analogue of nucleotide led to the suggestion that phosphate mediates its effect by binding directly to the ATP-binding site of the enzyme.

Diversity of Streptococcus salivarius ptsH mutants that can be isolated in the presence of 2-deoxyglucose and galactose and characterization of two mutants synthesizing reduced levels of HPr, a phosphocarrier of the phosphoenolpyruvate:sugar phosphotransferase system

In streptococci, HPr, a phosphocarrier of the phosphoenolpyruvate:sugar phosphotransferase transport system (PTS), undergoes multiple posttranslational chemical modifications resulting in the formation of HPr(His approximately P), HPr(Ser-P), and HPr(Ser-P)(His approximately P), whose cellular concentrations vary with growth conditions. Distinct physiological functions are associated with specific forms of HPr. We do not know, however, the cellular thresholds below which these forms become unable to fulfill their functions and to what extent modifications in the cellular concentrations of the different forms of HPr modify cellular physiology. In this study, we present a glimpse of the diversity of Streptococcus salivarius ptsH mutants that can be isolated by positive selection on a solid medium containing 2-deoxyglucose and galactose and identify 13 amino acids that are essential for HPr to properly accomplish its physiological functions. We also report the characterization of two S. salivarius mutants that produced approximately two- and threefoldless HPr and enzyme I (EI) respectively. The data indicated that (i) a reduction in the synthesis of HPr due to a mutation in the Shine-Dalgarno sequence of ptsH reduced ptsI expression; (ii) a threefold reduction in EI and HPr cellular levels did not affect PTS transport capacity; (iii) a twofold reduction in HPr synthesis was sufficient to reduce the rate at which cells metabolized PTS sugars, increase generation times on PTS sugars and to a lesser extent on non-PTS sugars, and impede the exclusion of non-PTS sugars by PTS sugars; (iv) a threefold reduction in HPr synthesis caused a strong derepression of the genes coding for alpha-galactosidase, beta-galactosidase, and galactokinase when the cells were grown at the expense of a PTS sugar but did not affect the synthesis of alpha-galactosidase when cells were grown at the expense of lactose, a noninducing non-PTS sugar; and (v) no correlation was found between the magnitude of enzyme derepression and the cellular levels of HPr(Ser-P).

Phosphorylation state of HPr determines the level of expression and the extent of phosphorylation of the lactose transport protein of Streptococcus thermophilus

The lactose transport protein (LacS) of Streptococcus thermophilus is composed of a translocator domain and a regulatory domain that is phosphorylated by HPr(His approximately P), the general energy coupling protein of the phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS). Lactose transport is affected by the phosphorylation state of HPr through changes in the activity of the LacS protein as well as expression of the lacS gene. To address whether or not CcpA-HPr(Ser-P)-mediated catabolite control is involved, the levels of LacS were determined under conditions in which the cellular phosphorylation state of HPr greatly differed. It appears that HPr(Ser-P) is mainly present in the exponential phase of growth, whereas HPr(His approximately P) dominates in the stationary phase. The transition from HPr(Ser-P) to HPr(His approximately P) parallels an increase in LacS level, a drop in lactose and an increase in galactose concentration in the growth medium. Because the K(m)(out) for lactose is higher than that for galactose, the lactose transport capacity decreases as lactose concentration decreases and galactose accumulates in the medium. Our data indicate that S. thermophilus compensates for the diminished transport capacity by synthesizing more LacS and phosphorylating the protein, which results in increased transport activity. The link between transport capacity and lacS expression levels and LacS phosphorylation are discussed.

The HPr kinase from Bacillus subtilis is a homo-oligomeric enzyme which exhibits strong positive cooperativity for nucleotide and fructose 1,6-bisphosphate binding

Carbon catabolite repression allows bacteria to rapidly alter the expression of catabolic genes in response to the availability of metabolizable carbon sources. In Bacillus subtilis, this phenomenon is controlled by the HPr kinase (HprK) that catalyzes ATP-dependent phosphorylation of either HPr (histidine containing protein) or Crh (catabolite repression HPr) on residue Ser-46. We report here that B. subtilis HprK forms homo-oligomers constituted most likely of eight subunits. Related to this complex structure, the enzyme displays strong positive cooperativity for the binding of its allosteric activator, fructose 1,6-bisphosphate, as evidenced by either kinetics of its phosphorylation activity or the intrinsic fluorescence properties of its unique tryptophan residue, Trp-235. It is further shown that activation of HPr phosphorylation by fructose 1,6-bisphosphate essentially occurs at low ATP and enzyme concentrations. A positive cooperativity was also detected for the binding of natural nucleotides or their 2'(3')-N-methylanthraniloyl derivatives, in either phosphorylation or fluorescence experiments. Most interestingly, quenching of the HprK tryptophan fluorescence by using either iodide or acrylamide revealed a heterogeneity of tryptophan residues within the population of oligomers, suggesting that the enzyme exists in two different conformations. This result suggests a concerted-symmetry model for the catalytic mechanism of positive cooperativity displayed by HprK.

Phenotypic consequences resulting from a methionine-to-valine substitution at position 48 in the HPr protein of Streptococcus salivarius

In gram-positive bacteria, the HPr protein of the phosphoenolpyruvate:sugar phosphotransferase system (PTS) can be phosphorylated on a histidine residue at position 15 (His(15)) by enzyme I (EI) of the PTS and on a serine residue at position 46 (Ser(46)) by an ATP-dependent protein kinase (His approximately P and Ser-P, respectively). We have isolated from Streptococcus salivarius ATCC 25975, by independent selection from separate cultures, two spontaneous mutants (Ga3.78 and Ga3.14) that possess a missense mutation in ptsH (the gene encoding HPr) replacing the methionine at position 48 by a valine. The mutation did not prevent the phosphorylation of HPr at His(15) by EI nor the phosphorylation at Ser(46) by the ATP-dependent HPr kinase. The levels of HPr(Ser-P) in glucose-grown cells of the parental and mutant Ga3.78 were virtually the same. However, mutant cells growing on glucose produced two- to threefold less HPr(Ser-P)(His approximately P) than the wild-type strain, while the levels of free HPr and HPr(His approximately P) were increased 18- and 3-fold, respectively. The mutants grew as well as the wild-type strain on PTS sugars (glucose, fructose, and mannose) and on the non-PTS sugars lactose and melibiose. However, the growth rate of both mutants on galactose, also a non-PTS sugar, decreased rapidly with time. The M48V substitution had only a minor effect on the repression of alpha-galactosidase, beta-galactosidase, and galactokinase by glucose, but this mutation abolished diauxie by rendering cells unable to prevent the catabolism of a non-PTS sugar (lactose, galactose, and melibiose) when glucose was available. The results suggested that the capacity of the wild-type cells to preferentially metabolize glucose over non-PTS sugars resulted mainly from inhibition of the catabolism of these secondary energy sources via a HPr-dependent mechanism. This mechanism was activated following glucose but not lactose metabolism, and it did not involve HPr(Ser-P) as the only regulatory molecule.

The HPr(Ser) kinase of Streptococcus salivarius: purification, properties, and cloning of the hprK gene

In gram-positive bacteria, HPr, a protein of the phosphoenolpyruvate:sugar phosphotransferase system, is phosphorylated on a serine residue at position 46 by an ATP-dependent protein kinase. The HPr(Ser) kinase of Streptococcus salivarius ATCC 25975 was purified, and the encoding gene (hprK) was cloned by using a nucleotide probe designed from the N-terminal amino acid sequence. The predicted amino acid sequence of the S. salivarius enzyme showed 45% identity with the Bacillus subtilis enzyme, the conserved residues being located mainly in the C-terminal half of the protein. The predicted hprK gene product has a molecular mass of 34,440 Da and a pI of 5.6. These values agree well with those found experimentally by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, molecular sieve chromatography in the presence of guanidine hydrochloride, and chromatofocusing using the purified protein. The native protein migrates on a Superdex 200 HR column as a 330,000-Da protein, suggesting that the HPr(Ser) kinase is a decamer. The enzyme requires Mg2+ for activity and functions optimally at pH 7.5. Unlike the enzyme from other gram-positive bacteria, the HPr(Ser) kinase from S. salivarius is not stimulated by FDP or other glycolytic intermediates. The enzyme is inhibited by inorganic phosphate, and its Kms for HPr and ATP are 31 microM and 1 mM, respectively.

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.

Bacterial metabolism in dental biofilms

Dental biofilms could have a structure which, in sections, looks like tissue. The internal structure of the dental biofilm could be the result of interbacterial adhesion mechanisms in combination with nutritional conditions characterized by multiple nutrient starvation. The preservation of the structure of the biofilm over time may also involve the ability of the bacteria to withstand environmental stresses such as starvation, reactive oxygen products, and acid. The present review will describe, first, the regulation of the metabolic defense against environmental stresses and then focus mainly on the energy metabolism of dental biofilms.

The phosphoenolpyruvate:sugar phosphotransferase system of oral streptococci and its role in the control of sugar metabolism

Oral streptococci are sugar-fermentative bacteria comprising at least 19 distinct species and are a significant proportion of the normal microbial population of the mouth and upper respiratory tract of humans. These streptococci transport several sugars by the phosphoenolpyruvate:sugar phosphotransferase system (PTS) which concomitantly catalyzes the phosphorylation and translocation of mono- and disaccharides via a chain of enzymic reactions that transfer a phosphate group from phosphoenolpyruvate to the incoming sugar. A number of PTS components, including HPr, Enzyme I and some Enzymes II, have been studied at the biochemical and/or genetical level in Streptococcus salivarius, Streptococcus mutans and Streptococcus sobrinus. Moreover, compelling evidence indicates that the oral streptococcal PTS is involved in the regulation of sugar metabolism. Results are accumulating suggesting that a protein called IIABMan, as well as the phosphocarrier protein HPr, are key regulatory components that allow these bacteria to select rapidly metabolizable sugars, such as glucose or fructose, over less readily utilizable carbohydrates. Circumstantial evidence suggests that the molecular mechanisms by which oral streptococcal PTS exert their regulatory functions differ from mechanisms in other Gram-negative or Gram-positive bacteria.

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.