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

The CcpA regulon of Streptococcus suis reveals novel insights into the regulation of the streptococcal central carbon metabolism by binding of CcpA to two distinct binding motifs

Streptococcus suis (S. suis) is a neglected zoonotic streptococcus causing fatal diseases in humans and in pigs. The transcriptional regulator CcpA (catabolite control protein A) is involved in the metabolic adaptation to different carbohydrate sources and virulence of S. suis and other pathogenic streptococci. In this study, we determined the DNA binding characteristics of CcpA and identified the CcpA regulon during growth of S. suis. Electrophoretic mobility shift analyses showed promiscuous DNA binding of CcpA to cognate cre sites in vitro. In contrast, sequencing of immunoprecipitated chromatin revealed two specific consensus motifs, a pseudo-palindromic cre motif (WWGAAARCGYTTTCWW) and a novel cre2 motif (TTTTYHWDHHWWTTTY), within the regulatory elements of the genes directly controlled by CcpA. Via these elements CcpA regulates expression of genes involved in carbohydrate uptake and conversion, and in addition in important metabolic pathways of the central carbon metabolism, like glycolysis, mixed-acid fermentation, and the fragmentary TCA cycle. Furthermore, our analyses provide evidence that CcpA regulates the genes of the central carbon metabolism by binding either the pseudo-palindromic cre motif or the cre2 motif in a HPr(Ser)∼P independent conformation.

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).

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.

Genetic and biochemical characterization of the phosphoenolpyruvate:glucose/mannose phosphotransferase system of Streptococcus thermophilus

In most streptococci, glucose is transported by the phosphoenolpyruvate (PEP):glucose/mannose phosphotransferase system (PTS) via HPr and IIAB(Man), two proteins involved in regulatory mechanisms. While most strains of Streptococcus thermophilus do not or poorly metabolize glucose, compelling evidence suggests that S. thermophilus possesses the genes that encode the glucose/mannose general and specific PTS proteins. The purposes of this study were to determine (i) whether these PTS genes are expressed, (ii) whether the PTS proteins encoded by these genes are able to transfer a phosphate group from PEP to glucose/mannose PTS substrates, and (iii) whether these proteins catalyze sugar transport. The pts operon is made up of the genes encoding HPr (ptsH) and enzyme I (EI) (ptsI), which are transcribed into a 0.6-kb ptsH mRNA and a 2.3-kb ptsHI mRNA. The specific glucose/mannose PTS proteins, IIAB(Man), IIC(Man), IID(Man), and the ManO protein, are encoded by manL, manM, manN, and manO, respectively, which make up the man operon. The man operon is transcribed into a single 3.5-kb mRNA. To assess the phosphotransfer competence of these PTS proteins, in vitro PEP-dependent phosphorylation experiments were conducted with purified HPr, EI, and IIAB(Man) as well as membrane fragments containing IIC(Man) and IID(Man). These PTS components efficiently transferred a phosphate group from PEP to glucose, mannose, 2-deoxyglucose, and (to a lesser extent) fructose, which are common streptococcal glucose/mannose PTS substrates. Whole cells were unable to catalyze the uptake of mannose and 2-deoxyglucose, demonstrating the inability of the S. thermophilus PTS proteins to operate as a proficient transport system. This inability to transport mannose and 2-deoxyglucose may be due to a defective IIC domain. We propose that in S. thermophilus, the general and specific glucose/mannose PTS proteins are not involved in glucose transport but might have regulatory functions associated with the phosphotransfer properties of HPr and IIAB(Man).

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).

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.

Influence of the phosphorylation state on the biological activity of a low-molecular mitogen from group A streptococci

A low molecular weight mitogen (LMP) from Streptococcus pyogenes strain NY 5 was successively purified by adsorption on phenylsepharose, chromatography on Resource S and Superdex G 30 and finally by affinity chromatography on antiphosphothreonine agarose. The N-terminal protein sequence of the mitogen was determined. The occurrence of phosphoamino acids was investigated by immunoassay using monoclonal antibodies. The LMP is a threonine-phosphorylated protein different of HPR protein of PTS-system, its mitogenic activity was lost after treatment with streptococcal protein phosphatase or alkaline phosphatase. The inactivated LMP was activated by phosphorylation with phosphokinase and ATP. The active LMP was also inactivated in streptococcal cultures secreting acid protein phosphatase during the phase of phosphate limitation.

Surface location of HPr, a phosphocarrier of the phosphoenolpyruvate: sugar phosphotransferase system in Streptococcus suis

HPr is a low-molecular-mass phosphocarrier protein of the bacterial phosphoenolpyruvate (PEP): sugar phosphotransferase system (PTS) found in the cytoplasm or associated with the inner surface of the cytoplasmic membrane. Treatment of Streptococcus suis cells with a Sorvall Omnimixer, a technique used to extract cell surface components, resulted in the extraction of a major protein with a molecular mass of 9 kDa. Several lines of evidence suggested that this protein was HPr: (i) the S. suis protein showed homology over the first 35 N-terminal amino acid residues with the HPrs of Streptococcus salivarius and Streptococcus mutans, including the signature sequence for the site of PEP-dependent phosphorylation; (ii) it cross-reacted with the S. salivarius anti-HPr antibody preparation; (iii) it could be phosphorylated by enzyme I at the expense of PEP, and by a membrane-associated kinase at the expense of ATP; and (iv) it possessed phosphocarrier activity when used as a source of HPr in an in vitro PTS assay. The data suggested that a portion of the cellular HPr is associated with the external cell surface in S. suis, a result that was confirmed by immunogold electron microscopy. The cellular HPr of S. suis consisted of two forms that could be distinguished by the presence or the absence of the N-terminal methionine. Amino acid sequence analysis indicated that the cell-surface-associated HPr of S. suis lacked the N-terminal methionine residue.

Distribution of proteins similar to IIIManH and IIIManL of the Streptococcus salivarius phosphoenolpyruvate:mannose-glucose phosphotransferase system among oral and nonoral bacteria

In Streptococcus salivarius, the phosphoenolpyruvate (PEP):mannose-glucose phosphotransferase system, which concomitantly transports and phosphorylates mannose, glucose, fructose, and 2-deoxyglucose, is composed of the general energy-coupling proteins EI and HPr, the specific membrane-bound IIIMan, and two forms of a protein called IIIMan, with molecular weights of 38,900 (IIIManH) and 35,200 (IIIManL), that are found in the cytoplasm as well as associated with the membrane. Several lines of evidence suggest that IIIManH and/or IIIManL are involved in the control of sugar metabolism. To determine whether other bacteria possess these proteins, we tested for their presence in 28 oral streptococcus strains, 3 nonoral streptococcus strains, 2 lactococcus strains, 2 enterococcus strains, 2 bacillus strains, 1 lactobacillus strain, Staphylococcus aureus, and Escherichia coli. Three approaches were used to determine whether the IIIMan proteins were present in these bacteria: (i) Western blot (immunoblot) analysis of cytoplasmic and membrane proteins, using anti-IIIManH and anti-IIIManH rabbit polyclonal antibodies; (ii) analysis of PEP-dependent phosphoproteins by polyacrylamide gel electrophoresis; and (iii) inhibition by anti-IIIMan antibodies of the PEP-dependent phosphorylation of 2-deoxyglucose (a mannose analog) by crude cellular extracts. Only the species S. salivarius and Streptococcus vestibularis possessed the two forms of IIIMan. Fifteen other streptococcal species possessed one protein with a molecular weight between 35,200 and 38,900 that cross-reacted with both antibodies. In the case of 9 species, a protein possessing the same electrophoretic mobility was phosphorylated at the expense of PEP. No such phosphoprotein, however, could be detected in the other six species. A III(Man)-like protein with a molecular weight of 35,500 was also detected in Lactobacillus casei by Western blot experiments as well as by PEP-dependent phosphoprotein analysis, and a protein with a molecular weight of 38,900 that cross-reacted with anti-III(Man) antibodies was detected in Lactococcus lactis. In several cases, the involvement of these putative III(Man) proteins in the PEP-dependent phosphorylation of 2-deoxyglucose was substantiated by the inhibition of phosphorylation activity of anti-III(Man) antibodies. No proteins cross-reacting with anti-III(Man) antibodies were detected in enterococci, bacilli, and E. coli. In S. aureus, a membrane protein with a molecular weight of 50,000 reacted strongly with the antibodies. This protein, however, was not phosphorylated at the expense of PEP.

Sequence and expression of the genes for HPr (ptsH) and enzyme I (ptsI) of the phosphoenolpyruvate-dependent phosphotransferase transport system from Streptococcus mutans

We report the sequencing of a 2,242-bp region of the Streptococcus mutants NG5 genome containing the genes for ptsH and ptsI, which encode HPr and enzyme I (EI), respectively, of the phosphoenolpyruvate-dependent phosphotransferase transport system. The sequence was obtained from two cloned overlapping genomic fragments; one expresses HPr and a truncated EI, while the other expresses a full-length EI in Escherichia coli, as determined by Western immunoblotting. The ptsI gene appeared to be expressed from a region located in the ptsH gene. The S. mutans NG5 pts operon does not appear to be linked to other phosphotransferase transport system proteins as has been found in other bacteria. A positive fermentation pattern on MacConkey-glucose plates by an E. coli ptsI mutant harboring the S. mutans NG5 ptsI gene on a plasmid indicated that the S. mutans NG5 EI can complement a defect in the E. coli gene. This was confirmed by protein phosphorylation experiments with 32P-labeled phosphoenolpyruvate indicating phosphotransfer from the S. mutans NG5 EI to the E. coli HPr. Two forms of the cloned EI, both truncated to varying degrees in the C-terminal region, were inefficiently phosphorylated and unable to complement fully the ptsI defect in the E. coli mutant. The deduced amino acid sequence of HPr shows a high degree of homology, particularly around the active site, to the same protein from other gram-positive bacteria, notably, S. salivarius, and to a lesser extent with those of gram-negative bacteria. The deduced amino acid sequence of S. mutans NG5 EI also shares several regions of homology with other sequenced EIs, notably, with the region around the active site, a region that contains the only conserved cystidyl residue among the various proteins and which may be involved in substrate binding.

Phosphotransferase system of Streptococcus salivarius: characterization of the ptsH gene and its product

The Streptococcus salivarius ptsH gene encoding histidine-containing phosphocarrier protein (HPr) of the phosphotransferase system (PTS) has been cloned, sequenced, and found to be part of a ptsH, ptsI operon. Upstream from ptsH, putative -35 and -10 boxes and a Shine-Dalgarno sequence highly similar to the Escherichia coli consensus regulatory elements were identified. A second promoter, located in the ptsH coding sequence was also observed and is sufficient for the expression of the S. salivarius ptsI gene, encoding enzyme I of the PTS in E. coli [Gagnon et al., Gene 121 (1992) 71-78]. The amino acid sequence of S. salivarius HPr, inferred from the ptsH sequence, shared identity varying between 37 and 76% with known HPr from other bacteria. Moreover, the S. salivarius HPr shared 78% identity with an HPr-like protein of Aspergillus fumigatus, a eukaroytic mold that does not possess a functional PTS. Expression analysis of S. salivarius HPr in E. coli demonstrated that (i) S. salivarius ptsH is expressed in E. coli under the control of its own promoter, (ii) S. salivarius HPr synthesized by E. coli is completely processed by methionine aminopeptidase, and (iii) S. salivarius HPr is phosphorylated in vivo by E. coli enzyme I. It was also observed that, in E. coli, the copy number of pUC18 bearing S. salivarius ptsH was reduced more than 25-fold, as compared to pUC18 without an insertion.

Amino-terminal methionine processing of the protein HPr in Streptococcus salivarius grown in continuous culture

HPr is a protein of the phosphoenolpyruvate:sugar phosphotransferase system (PTS). Streptococci possess two forms of HPr which differ by the presence or the absence of the N-terminal methionine (Met). These forms are called HPr-1 (without Met) and HPr-2 (with Met). In order to determine whether the ratio of these two forms varies with growth conditions, we measured the amount of HPr-1 and HPr-2 present in Streptococcus salivarius grown in continuous culture at pH 7.5. The results indicated that the HPr-1/HPr-2 ratio: 1) was not related to the cellular amount of total HPr; 2) was highest (10.2 +/- 3.5) under glucose (a PTS sugar) limitation (10 mM) and low dilution rate (D = 0.1 h-1; g = 6.9 h); 3) was decreased 2.4- to 5.7-fold when the amount of glucose and/or D was increased; 4) was not influenced by D when cells were cultured on galactose (a non-PTS sugar) but was two-fold higher under conditions of galactose excess (200 mM). We suggest that the cleavage of the N-terminal HPr Met is not a stochastic phenomenon but is dictated by growth conditions.

Altered expression of several genes in IIIManL-defective mutants of Streptococcus salivarius demonstrated by two-dimensional gel electrophoresis of cytoplasmic proteins

Mannose, glucose and fructose are transported in Streptococcus salivarius by a phosphoenolpyruvate:mannose phosphotransferase system (PTS) which consists of a membrane-bound Enzyme II (EII) and two forms of IIIMan having molecular weights of 38,900 (IIIManH) and 35,200 (IIIManL), respectively. We have previously reported the isolation of spontaneous mutants lacking IIIManL and showed that they exhibit higher beta-galactosidase activity than the parental strain after growth on glucose, and that some of them constitutively express a fructose PTS which is induced by fructose in the parental strain. In an attempt to determine whether the expression of other genes is affected by the mutation and what the physiological link is between them, we examined three S. salivarius IIIManL-defective mutants (strains A37, B31 and G29) and the parental strain using two-dimensional gel electrophoresis after growth of the cells on a variety of sugars. After growth on glucose, five new proteins were detected in the cytoplasm of the three mutants. Two of these proteins were induced in the parental strain by galactose or oligosaccharides containing galactose, and one was specifically induced by melibiose. The other two proteins were not detected in the parental strain under any of the growth conditions tested. Two other proteins were only detected in glucose-grown cells of mutant A37, and a protein associated with the metabolism of fructose was constitutively expressed in mutants B31 and G29. Moreover, we have found that under identical growth conditions the amounts of several other proteins which were detected in the parental strain were either increased or decreased in the mutants. Globally, our results have indicated that (1) the expression of several genes was affected in the spontaneous IIIManL-defective mutants; (2) some of the proteins abnormally produced in the mutants were specifically induced in the parental strain by sugars; (3) the phenotypic modifications observed in the mutants were of two types: most were observed solely after growth of the cells on glucose whereas the others were glucose-independent; and (4) the mutants shared common phenotypic traits, but also exhibited idiosyncratic characteristics.