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.

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.

Analysis of a ptsH homologue from Streptomyces coelicolor A3(2)

A ptsH homologue of Streptomyces coelicolor A3(2) was identified in the emerging genome sequence, cloned in Escherichia coli and the S. coelicolor HPr over-produced and purified. The protein was phosphorylated in vitro in a phosphoenolpyruvate (PEP)-dependent manner by purified enzyme I (EI) from Bacillus subtilis, and much less efficiently in an ATP-dependent manner by purified HPr kinase, also from B. subtilis. There was no indication of ATP-dependent phosphorylation of the purified protein by cell extracts of either S. coelicolor or Streptomyces lividans. Deletion of the ptsH homologue from the S. coelicolor and S. lividans chromosomes had no effect on growth when fructose was supplied as sole carbon source, and in S. coelicolor it had no effect on glucose repression of agarase and galactokinase synthesis, suggesting that the HPr encoded by this gene does not play an essential role in fructose transport nor a general role in carbon catabolite repression.

Phosphorylation of HPr and Crh by HprK, early steps in the catabolite repression signalling pathway for the Bacillus subtilis levanase operon

Carbon catabolite repression (CCR) of Bacillus subtilis catabolic genes is mediated by CcpA and in part by P-Ser-HPr. For certain operons, Crh, an HPr-like protein, is also implicated in CCR. In this study we demonstrated that in ptsH1 crh1 and hprK mutants, expression of the lev operon was completely relieved from CCR and that both P-Ser-HPr and P-Ser-Crh stimulated the binding of CcpA to the cre sequence of the lev operon.

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.

Regulation of the activity of the Bacillus subtilis antiterminator LicT by multiple PEP-dependent, enzyme I- and HPr-catalysed phosphorylation

The transcriptional antiterminator LicT regulates the induction and carbon catabolite repression of the Bacillus subtilis bglPH operon. LicT is inactive in mutants affected in one of the two general components of the phosphoenolpyruvate (PEP):glycose phosphotransferase system, enzyme I or histidine-containing protein (HPr). We demonstrate that LicT becomes phosphorylated in the presence of PEP, enzyme I and HPr. The phosphoryl group transfer between HPr and LicT is reversible. Phosphorylation of LicT with PEP, enzyme I and HPr led to the appearance of three additional LicT bands on polyacrylamide-urea gels. These bands probably correspond to one-, two- and threefold phosphorylated LicT. After phosphorylation of LicT with [32P]-PEP, enzyme I and HPr, proteolytic digestion of [32P]-P-LicT, separation of the peptides by reverse-phase chromatography, mass spectrometry and N-terminal sequencing of radiolabelled peptides, three histidyl residues were found to be phosphorylated in LicT. These three histidyl residues (His-159, His-207 and His-269) are conserved in most members of the BglG/SacY family of transcriptional antiterminators. Phosphorylation of LicT in the presence of serylphosphorylated HPr (P-Ser-HPr) was much slower compared with its phosphorylation in the presence of HPr. The slower phosphorylation in the presence of P-Ser-HPr leading to reduced LicT activity is presumed to play a role in a recently described LicT-mediated CcpA-independent carbon catabolite repression mechanism operative for the bglPH operon.

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.

Practical aspects of managing preschool children with type 1 diabetes

Day-to-day variations in diet and physical exercise, large variations in the glucose response to small changes in insulin doses, and high insulin sensitivity are characteristic of preschool children with diabetes. Hence, difficulties in achieving adequate metabolic control and stable glycaemia in preschool children are common. In addition, hypoglycaemic episodes tend to be frequent and severe in this age group. Problems identifying and treating hypoglycaemia present an additional challenge for the diabetes team and for the family caring for the young child with diabetes. Specific glucose targets are provided for this age group: premeal levels of 6-12 mmoll(-1)(110-220 mg dl(-1)) with bedtime levels above 8 mmoll(-1)(140 mg dl(-1)). It is important to note that children who suffer severe hypoglycaemic events at a young age show evidence of subtle cognitive deficits when tested during adolescence. The question of whether or not the years before pubertal onset contribute less towards the development of diabetes-related microvascular complications than do the years starting with the onset of puberty remains controversial. Twice-daily or multiple insulin injections, dietary adjustments and considerations, home blood-glucose monitoring, family education, support groups and 24-h hotline information facilities can help to achieve good metabolic control without severe hypoglycaemia in the preschool child. In general, good metabolic control without severe hypoglycaemia can be achieved using frequent counselling and a caring team approach.

An ectoprotein kinase of group C streptococci binds hyaluronan and regulates capsule formation

A 56-kDa protein had been isolated and cloned from protoplast membranes of group C streptococci that had erroneously been identified as hyaluronan synthase. The function of this protein was reexamined. When streptococcal membranes were separated on an SDS-polyacrylamide gel and renatured, a 56-kDa protein was detected that had kinase activity for a casein substrate. When this recombinant protein was expressed in Escherichia coli and incubated in the presence of [32P]ATP, it was responsible for phosphorylation of two proteins with 30 and 56 kDa that were not present in the control lysate. The 56-kDa protein was specifically phosphorylated in an immunoprecipitate of a detergent extract of the recombinant E. coli lysate with antibodies against the 56-kDa protein, indicating that it was autophosphorylated. The E. coli lysate containing the recombinant protein could bind hyaluronan, and hyaluronan binding was abolished by the addition of ATP. Kinetic analysis of hyaluronan synthesis and release from isolated protoplast membranes indicated that phosphorylation by ATP stimulated hyaluronan release and synthesis. Incubation of membranes with antibodies to the 56-kDa protein increased hyaluronan release. The addition of [32P]ATP to intact streptococci led to rapid phosphorylation of two proteins, 56 and 75 kDa each at threonine residues. This phosphorylation was neither observed with [32P]phosphate nor in the presence of trypsin, indicating that the kinase was localized extracellularly. The addition of ATP to growing group C streptococci led to increased hyaluronan synthesis and release. However marked differences were found between group A and group C streptococci. Antibodies against the 56-kDa protein from group C streptococci did not recognize proteins from group A strains, and a homologous DNA sequence could not be detected by polymerase chain reaction or Southern blotting. In addition, Group A streptococci did not retain a large hyaluronan capsule like group C strains. These results indicated that the 56-kDa protein is an ectoprotein kinase specific for group C streptococci that regulates hyaluronan capsule shedding by phosphorylation.

Leptin as a metabolic regulator during fetal and neonatal life and in childhood and adolescence

Body weight is regulated by a feedback loop in which peripheral signals report nutritional information to an integratory center in the brain. The cloning of the ob gene is consistent with this concept and suggests that body fat content in adult rodents is regulated by a negative feedback loop centered in the hypothalamus/1-8/. In a recent report, two severely obese children with congenital leptin deficiency due to a homozygous frame-shift mutation involving the deletion of a single guanine nucleotide in codon 133 of the ob gene have been described. This discovery provides the first genetic evidence that leptin is an important regulator of energy balance in humans. However, it has become increasingly clear that apart from leptin's function in the central nervous system and in regulation of energy balance, leptin also acts in the periphery and might be important as a hormone modulating processes in regard to reproduction, glucose metabolism and insulin resistance, as well as growth and development of many tissues and organs either directly or indirectly. This report reviews some of the topics of leptin research that are of particular importance and relevance for pediatric and adolescent medicine and for pediatric endocrinology in particular.

Antagonistic effects of dual PTS-catalysed phosphorylation on the Bacillus subtilis transcriptional activator LevR

LevR, which controls the expression of the levoperon of Bacillus subtilis, is a regulatory protein containing an N-terminal domain similar to the NifA/NtrC transcriptional activator family and a C-terminal domain similar to the regulatory part of bacterial anti-terminators, such as BgIG and LicT. Here, we demonstrate that the activity of LevR is regulated by two phosphoenolpyruvate (PEP)-dependent phosphorylation reactions catalysed by the phosphotransferase system (PTS), a transport system for sugars, polyols and other sugar derivatives. The two general components of the PTS, enzyme I and HPr, and the two soluble, sugar-specific proteins of the lev-PTS, LevD and LevE, form a signal transduction chain allowing the PEP-dependent phosphorylation of LevR, presumably at His-869. This phosphorylation seems to inhibit LevR activity and probably regulates the induction of the lev operon. Mutants in which His-869 of LevR has been replaced with a non-phosphorylatable alanine residue exhibited constitutive expression from the lev promoter, as do levD or levE mutants. In contrast, PEP-dependent phosphorylation of LevR in the presence of only the general components of the PTS, enzyme I and HPr, regulates LevR activity positively. This phosphorylation most probably occurs at His-585. Mutants in which His-585 has been replaced with an alanine had lost stimulation of LevR activity and PEP-dependent phosphorylation by enzyme I and HPr. This second phosphorylation of LevR at His-585 is presumed to play a role in carbon catabolite repression.

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.

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 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.

Cloning and sequencing of two enterococcal glpK genes and regulation of the encoded glycerol kinases by phosphoenolpyruvate-dependent, phosphotransferase system-catalyzed phosphorylation of a single histidyl residue

The glpK genes of Enterococcus casseliflavus and Enterococcus faecalis, encoding glycerol kinase, the key enzyme of glycerol uptake and metabolism in bacteria, have been cloned and sequenced. The translated amino acid sequences exhibit strong homology to the amino acid sequences of other bacterial glycerol kinases. After expression of the enterococcal glpK genes in Escherichia coli, both glycerol kinases were purified and were found to be phosphorylated by enzyme I and the histidine-containing protein of the phosphoenolpyruvate:glycose phosphotransferase system. Phosphoenolpyruvate-dependent phosphorylation caused a 9-fold increase in enzyme activity. The site of phosphorylation in glycerol kinase of E. casseliflavus was determined as His-232. Site-specific mutagenesis was used to replace His-232 in glycerol kinase of E. casseliflavus with an alanyl, glutamate, or arginyl residue. The mutant proteins could no longer be phosphorylated confirming that His-232 of E. casseliflavus glycerol kinase represents the site of phosphorylation. The His232 --> Arg glycerol kinase exhibited an about 3-fold elevated activity compared with wild-type glycerol kinase. Fructose 1,6-bisphosphate was found to inhibit E. casseliflavus glycerol kinase activity. However, neither EIIAGlc from E. coli nor the EIIAGlc domain of Bacillus subtilis had an inhibitory effect on glycerol kinase of E. casseliflavus.

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.

Catabolite repression of the Bacillus subtilis gnt operon exerted by two catabolite-responsive elements

Catabolite repression of Bacillus subtilis catabolic operons is supposed to occur via a negative regulatory mechanism involving the recognition of a cis-acting catabolite-responsive element (cre) by a complex of CcpA, which is a member of the GalR-Lacl family of bacterial regulatory proteins, and the seryl-phosphorylated form of HPr (P-ser-HPr), as verified by recent studies on catabolite repression of the gnt operon. Analysis of the gnt promoter region by deletions and point mutations revealed that in addition to the cre in the first gene (gntR) of the gnt operon (credown), this operon contains another cre located in the promoter region (creup). A translational gntR'-'lacZ fusion expressed under the control of various combinations of wild-type and mutant credown and creup was integrated into the chromosomal amyE locus, and then catabolite repression of beta-galactosidase synthesis in the resultant integrants was examined. The in vivo results implied that catabolite repression exerted by creup was probably independent of catabolite repression exerted by credown; both creup and credown catabolite repression involved CcpA. Catabolite repression exerted by creup was independent of P-ser-HPr, and catabolite repression exerted by credown was partially independent of P-ser-HPr. DNase I footprinting experiments indicated that a complex of CcpA and P-ser-HPr did not recognize creup, in contrast to its specific recognition of credown. However, CcpA complexed with glucose-6-phosphate specifically recognized creup as well as credown, but the physiological significance of this complexing is unknown.

Protein phosphorylation chain of a Bacillus subtilis fructose-specific phosphotransferase system and its participation in regulation of the expression of the lev operon

The proteins encoded by the fructose-inducible lev operon of Bacillus subtilis are components of a phosphotransferase system. They transport fructose by a mechanism which couples sugar uptake and phosphoenolpyruvate-dependent sugar phosphorylation. The complex transport system consists of two integral membrane proteins (LevF and LevG) and two soluble, hydrophilic proteins (LevD and LevE). The two soluble proteins from together with the general proteins of the phosphotransferase system, enzyme I and HPr, a protein phosphorylation chain which serves to phosphorylate fructose transported by LevF and LevG. We have synthesized modified LevD and LevE by fusing a His-tag to the N-terminus of each protein allowing rapid and efficient purification of the proteins. We determined His-9 in LevD and His-15 in LevE as the sites of PEP-dependent phosphorylation by isolating single, labeled peptides derived from 32P-labeled LevD, LevD(His)6, and LevE(His)6. The labeled peptides were subsequently analyzed by amino acid sequencing and mass spectroscopy. Mutations replacing the phosphorylatable histidyl residue in LevD with an alanyl residue and in LevE with a glutamate or aspartate were introduced in the levD and levE genes. These mutations caused strongly reduced fructose uptake via the lev-PTS. The mutant proteins were synthesized with a N-terminal His-tag and purified. Mutant LevD(His)6 was very slowly phosphorylated, whereas mutant LevE(His)6 was not phosphorylated at all. The corresponding levD and levE alleles were incorporated into the chromosome of a B. subtilis strain expressing the lacZ gene under control of the lev promoter. The mutations affecting the site of phosphorylation in either LevD or LevE were found to cause constitutive expression from the lev promoter of B. subtilis.

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

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

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

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

Cloning and characterization of the Bacillus subtilis prkA gene encoding a novel serine protein kinase

We have cloned and sequenced a 3574-bp Bacillus subtilis (Bs) DNA fragment located between the nrdA and citB genes at about 169 degrees on the chromosome. An Escherichia coli strain, LBG1605, carrying a mutated ptsH gene (encoding HPr (His-containing protein) of the bacterial phosphotransferase system (PTS)) and complemented for PTS activity with the ptsH of Staphylococcus carnosus, exhibited reduced mannitol fermentation activity when transformed with a plasmid bearing this 3574-bp Bs fragment. This fragment contained an incomplete and two complete open reading frames (ORFs). The product of the first complete ORF, a protein composed of 235 amino acids (aa) (25038 Da), was found to be responsible for the observed reduced mannitol fermentation. The 3' part of this 705-bp second ORF and the 428-bp incomplete first ORF encode aa sequences exhibiting almost 40% sequence identify. However, the function of these two proteins remains unknown. The third ORF, the 1893-bp prkA gene, encodes a protein (PrkA) of 72889 Da. PrkA possesses the A-motif of nucleotide-binding proteins and exhibits distant homology to eukaryotic protein kinases. Several of the essential aa in the loops known to form the active site of cyclic adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase appeared to be conserved in PrkA. After expression of prkA and purification of PrkA, we could demonstrate that PrkA can indeed phosphorylate a Bs 60-kDa protein at a Ser residue.

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

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

Glucose kinase-dependent catabolite repression in Staphylococcus xylosus

By transposon Tn917 mutagenesis, 16 mutants of Staphylococcus xylosus were isolated that showed higher levels of beta-galactosidase activity in the presence of glucose than the wild-type strain. The transposons were found to reside in three adjacent locations in the genome of S. xylosus. The nucleotide sequence of the chromosomal fragment affected by the Tn917 insertions yielded an open reading frame encoding a protein with a size of 328 amino acids with a high level of similarity to glucose kinase from Streptomyces coelicolor. Weaker similarity was also found to bacterial fructokinases and xylose repressors of gram-positive bacteria. The gene was designated glkA. Immediately downstream of glkA, two open reading frames were present whose deduced gene products showed no obvious similarity to known proteins. Measurements of catabolic enzyme activities in the mutant strains grown in the presence or absence of sugars established the pleiotropic nature of the mutations. Besides beta-galactosidase activity, which had been used to detect the mutants, six other tested enzymes were partially relieved from repression by glucose. Reduction of fructose-mediated catabolite repression was observed for some of the enzyme activities. Glucose transport and ATP-dependent phosphorylation of HPr, the phosphocarrier of the phosphoenolpyruvate:carbohydrate phosphotransferase system involved in catabolite repression in gram-positive bacteria, were not affected. The cloned glkA gene fully restored catabolite repression in the mutant strains in trans. Loss of GlkA function is thus responsible for the partial relief from catabolite repression. Glucose kinase activity in the mutants reached about 75% of the wild-type level, indicating the presence of another enzyme in S. xylosus. However, the cloned gene complemented an Escherichia coli strain in glucose kinase. Therefore, the glkA gene encodes a glucose kinase that participates in catabolite repression in S. xylosus.

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.