Carbon catabolite repression by the catabolite control protein CcpA in Staphylococcus xylosus

Carbon catabolic repression (CR) by the catabolite control protein CcpA has been analyzed in Staphylococcus xylosus. Genes encoding components needed to utilize lactose, sucrose, and maltose were found to be repressed by CcpA. In addition, the ccpA gene is under negative autogenous control. Among several tested sugars, glucose caused strongest CcpA-dependent repression. Glucose can enter S. xylosus in nonphosphorylated form via the glucose uptake protein GlcU. Internal glucose is then phosphorylated by the glucose kinase GlkA. Alternatively, glucose can be transported and concomitantly phosphorylated by glucose-specific permease(s) of the phosphotransferase system (PTS). S. xylosus mutant strains deficient in GlcU or GlkA showed partial relief of glucose-specific, CcpA-dependent repression. Likewise, blocking PTS activity completely by inactivation of the gene encoding the general PTS protein enzyme I resulted in diminished glucose-mediated repression. Thus, both glucose entry routes contribute to glucose-specific CR in S. xylosus. The sugar transport activity of the PTS is not required to trigger glucose-specific repression. The phosphocarrier protein HPr however, is absolutely essential for CcpA activity. Inactivation of the HPr gene led to a complete loss of CR. Repression is also abolished upon inactivation of the HPr kinase gene or by replacing serine at position 46 of HPr by alanine. These results clearly show that HPr kinase provides the signal, seryl-phosphorylated HPr, to activate CcpA in S. xylosus.     

Enzymes II of the phosphotransferase system do not catalyze sugar transport in the absence of phosphorylation.

In Salmonella typhimurium, glucose, mannose, and fructose are normally transported and phosphorylated by the phosphoenolpyruvate:sugar phosphotransferase system. We have investigated the transport of these sugars and their non-metabolizable analogs in mutant strains lacking the phospho-carrier proteins of the phosphoenolpyruvate:sugar phosphotransferase system, the enzymes I and HPr, to determine whether the sugar-specific, membrane-bound components of the phosphonenolpyruvate: sugar phosphotransferase system, the enzymes II, can catalyze the uptake of these sugars in the absence of phosphorylation. This process does not occur. We have also isolated mutant strains which lack enzyme I and HPr, but have regained the ability to grow on mannose or fructose. These mutants contained elevated levels of mannokinase (fructokinase). In addition, growth on mannose required constitutive synthesis of the galactose permease. When strains were constructed which lacked the galactose permease, they were unable to grow even on high concentrations of mannose, although elevated levels of mannokinase (fructokinase) were present. These results substantiate the conclusion that the enzymes II of the phosphoenolpyruvate:sugar phosphotransferase system are unable to carry out facilitated diffusion.     

The phosphotransferase system of Corynebacterium glutamicum: features of sugar transport and carbon regulation

In this review, we describe the phosphotransferase system (PTS) of Corynebacterium glutamicum and discuss genes for putative global carbon regulation associated with the PTS. C. glutamicum ATCC 13032 has PTS genes encoding the general phosphotransferases enzyme I, HPr and four enzyme II permeases, specific for glucose, fructose, sucrose and one yet unknown substrate. C. gluamicum has a peculiar sugar transport system involving fructose efflux after hydrolyzing sucrose transported via sucrose EII. Also, in addition to their primary PTS, fructose and glucose are each transported by a second transporter, glucose EII and a non-PTS permease, respectively. Interestingly, C. glutamicum does not show any preference for glucose, and thus co-metabolizes glucose with other sugars or organic acids. Studies on PTS-mediated sugar uptake and its related regulation in C. glutamicum are important because the production yield of lysine and cell growth are dependent on the PTS sugars used as substrates for fermentation. In many bacteria, the PTS is also involved in several regulatory processes. However, the detailed molecular mechanism of global carbon regulation associated with the PTS in this organism has not yet been revealed.     

Regulation of the glucose:H+ symporter by metabolite-activated ATP-dependent phosphorylation of HPr in Lactobacillus brevis.

Lactobacillus brevis takes up glucose and the nonmetabolizable glucose analog 2-deoxyglucose (2DG), as well as lactose and the nonmetabolizable lactose analoge thiomethyl beta-galactoside (TMG), via proton symport. Our earlier studies showed that TMG, previously accumulated in L. brevis cells via the lactose:H+ symporter, rapidly effluxes from L. brevis cells or vesicles upon addition of glucose and that glucose inhibits further accumulation of TMG. This regulation was shown to be mediated by a metabolite-activated protein kinase that phosphorylase serine 46 in the HPr protein. We have now analyzed the regulation of 2DG uptake and efflux and compared it with that of TMG. Uptake of 2DG was dependent on an energy source, effectively provided by intravesicular ATP or by extravesicular arginine which provides ATP via an ATP-generating system involving the arginine deiminase pathway. 2DG uptake into these vesicles was not inhibited, and preaccumulated 2DG did not efflux from them upon electroporation of fructose 1,6-diphosphate or gluconate 6-phosphate into the vesicles. Intravesicular but not extravesicular wild-type or H15A mutant HPr of Bacillus subtilis promoted inhibition (53 and 46%, respectively) of the permease in the presence of these metabolites. Counterflow experiments indicated that inhibition of 2DG uptake is due to the partial uncoupling of proton symport from sugar transport. Intravesicular S46A mutant HPr could not promote regulation of glucose permease activity when electroporated into the vesicles with or without the phosphorylated metabolites, but the S46D mutant protein promoted regulation, even in the absence of a metabolite. The Vmax but not the Km values for both TMG and 2DG uptake were affected. Uptake of the natural, metabolizable substrates of the lactose, glucose, mannose, and ribose permeases was inhibited by wild-type HPr in the presence of fructose 1,6-diphosphate or by S46D mutant HPr. These results establish that HPr serine phosphorylation by the ATP-dependent, metabolite-activated HPr kinase regulates glucose and lactose permease activities in L. brevis and suggest that other permeases may also be subject to this mode of regulation.     

Three-dimensional structures of protein-protein complexes in the E. coli PTS

The bacterial phosphoenolpyruvate:sugar phosphotransferase system (PTS) includes a collection of proteins that accomplish phosphoryl transfer from phosphoenolpyruvate (PEP) to a sugar in the course of transport. The soluble proteins of the glucose transport pathway also function as regulators of diverse systems. The mechanism of interaction of the phosphoryl carrier proteins with each other as well as with their regulation targets has been amenable to study by nuclear magnetic resonance (NMR) spectroscopy. The three-dimensional solution structures of the complexes between the N-terminal domain of enzyme I and HPr and between HPr and enzyme IIA(Glc) have been elucidated. An analysis of the binding interfaces of HPr with enzyme I, IIA(Glc) and glycogen phosphorylase revealed that a common surface on HPr is involved in all these interactions. Similarly, a common surface on IIA(Glc) interacts with HPr, IIB(Glc) and glycerol kinase. Thus, there is a common motif for the protein-protein interactions characteristic of the PTS.     

Analysis of mutations that uncouple transport from phosphorylation in enzyme IIGlc of the Escherichia coli phosphoenolpyruvate-dependent phosphotransferase system.

Mutations that uncouple glucose transport from phosphorylation were isolated in plasmid-encoded Escherichia coli enzyme IIGlc of the phosphoenolpyruvate-dependent phosphotransferase system (PTS). The uncoupled enzymes IIGlc were able to transport glucose in the absence of the general phosphoryl-carrying proteins of the PTS, enzyme I and HPr, although with relatively low affinity. Km values of the uncoupled enzymes IIGlc for glucose ranged from 0.5 to 2.5 mM, 2 orders of magnitude higher than the value of normal IIGlc. Most of the mutant proteins were still able to phosphorylate glucose and methyl alpha-glucoside (a non-metabolizable glucose analog specific for IIGlc), indicating that transport and phosphorylation are separable functions of the enzyme. Some of the uncoupled enzymes IIGlc transported glucose with a higher rate and lower apparent Km in a pts+ strain than in a delta ptsHI strain lacking the general proteins enzyme I and HPr. Since the properties of these uncoupled enzymes IIGlc in the presence of PTS-mediated phosphoryl transfer resembled those of wild-type IIGlc, these mutants appeared to be conditionally uncoupled. Sequencing of the mutated ptsG genes revealed that all amino acid substitutions occurred in a hydrophilic segment within the hydrophobic N-terminal part of IIGlc. These results suggest that this hydrophilic loop is involved in binding and translocation of the sugar substrate.     

Topography of the surface of the Escherichia coli phosphotransferase system protein enzyme IIAglc that interacts with lactose permease

The unphosphorylated form of enzyme IIAglc of the Escherichia coli phosphoenolpyruvate:sugar phosphotransferase system inhibits transport catalyzed by lactose permease. We (Seok et al. (1997) Proc. Natl. Acad. Sci. U.S.A. 94, 13515-13519) previously characterized the area on the cytoplasmic face of lactose permease that interacts with enzyme IIAglc, using radioactive enzyme IIAglc. Subsequent studies (Sondej et al. (1999) Proc. Natl. Acad. Sci. U.S.A. 96, 3525-3530) suggested consensus binding sequences on proteins that interact with enzyme IIAglc. The present study characterizes a region on the surface of enzyme IIAglc that interfaces with lactose permease. Acetylation of lysine residues by sulfosuccinimidyl acetate treatment of enzyme IIAglc, but not lactose permease, reduced the degree of interaction between the two proteins. To localize the lysine residue(s) on enzyme IIAglc that is(are) involved in the regulatory interaction, selected lysine residues were mutagenized. Conversion of nine separate lysines to glutamic acid resulted in proteins that were still capable of phosphoryl acceptance from HPr. Except for Lys69, all the modified proteins were as effective as the wild-type enzyme IIAglc in a test for binding to lactose permease. The Lys69 mutant was also defective in phosphoryl transfer to glucose permease. To derive further information concerning the contact surface, additional selected residues in the vicinity of Lys69 were mutagenized and tested for binding to lactose permease. On the basis of these studies, a model for the region of the surface of enzyme IIAglc that interacts with lactose permease is proposed.     

Fine control of adenylate cyclase by the phosphoenolpyruvate:sugar phosphotransferase systems in Escherichia coli and Salmonella typhimurium.

Inhibition of cellular adenylate cyclase activity by sugar substrates of the phosphoenolpyruvate-dependent phosphotransferase system was reliant on the activities of the protein components of this enzyme system and on a gene designated crrA. In bacterial strains containing very low enzyme I activity, inhibition could be elicited by nanomolar concentrations of sugar. An antagonistic effect between methyl alpha-glucoside and phosphoenolpyruvate was observed in permeabilized Escherichia coli cells containing normal activities of the phosphotransferase system enzymes. In contrast, phosphoenolpyruvate could not overcome the inhibitory effect of this sugar in strains deficient for enzyme I or HPr. Although the in vivo sensitivity of adenylate cyclase to inhibition correlated with sensitivity of carbohydrate permease function to inhibition in most strains studied, a few mutant strains were isolated in which sensitivity of carbohydrate uptake to inhibition was lost and sensitivity of adenylate cyclase to regulation was retained. These results are consistent with the conclusions that adenylate cyclase and the carbohydrate permeases were regulated by a common mechanism involving phosphorylation of a cellular constituent by the phosphotransferase system, but that bacterial cells possess mechanisms for selectively uncoupling carbohydrate transport from regulation.     

The phosphotransferase system of Streptomyces coelicolor.

We have investigated the crr gene of Streptomyces coelicolor that encodes a homologue of enzyme IIAGlucose of Escherichia coli, which, as a component of the phosphoenolpyruvate-dependent sugar phosphotransferase system (PTS) plays a key role in carbon regulation by triggering glucose transport, carbon catabolite repression, and inducer exclusion. As in E. coli, the crr gene of S. coelicolor is genetically associated with the ptsI gene that encodes the general phosphotransferase enzyme I. The gene product IIACrr was overproduced, purified, and polyclonal antibodies were obtained. Western blot analysis revealed that IIACrr is expressed in vivo. The functionality of IIACrr was demonstrated by phosphoenolpyruvate-dependent phosphorylation via enzyme I and the histidine-containing phosphoryl carrier protein HPr. Phosphorylation was abolished when His72, which corresponds to the catalytic histidine of E. coli IIAGlucose, was mutated. The capacity of IIACrr to operate in sugar transport was shown by complementation of the E. coli glucose-PTS. The striking functional resemblance between IIACrr and IIAGlucose was further demonstrated by its ability to confer inducer exclusion of maltose to E. coli. A specific interaction of IIACrr with the maltose permease subunit MalK from Salmonella typhimurium was uncovered by surface plasmon resonance. These data suggest that this IIAGlucose-like protein may be involved in carbon metabolism in S. coelicolor.     

Corynebacterium diphtheriae: a PTS view to the genome

We have surveyed the publicly available genome sequence of Corynebacterium diphtheriae (www.sanger.ac.uk) to identify components of the phosphotransferase system (PTS), which plays a central role in carbon metabolism in many bacteria. Three gene loci were found to contain putative pts genes. These comprise: (i) the genes of the general phosphotransferases enzyme I (ptsI) and HPr (ptsH), a fructose-specific enzyme IIABC permease (fruA), and a fructose 1-phosphate kinase (fruK); (ii) a gene that encodes an enzyme IIAB of the fructose/mannitol family, and a novel HPr-like gene, ptsF, that encodes an HPr domain fused to a domain of unknown function; (iii) and a gene for a glucose-specific enzyme IIBCA (ptsG). A search for genes that may be putative PTS-targets or that may operate in general carbon regulation revealed a possible regulatory gene encoding an antiterminator protein downstream from ptsG. Furthermore, genes were detected encoding glycerol kinase, glucose kinase, and a homologue of the global activator of carbon catabolite repression in Escherichia coli, CAP. The possible significance of these observations in carbon metabolism and the novel features of the detected genes are discussed.     

CcpA-independent carbon catabolite repression in Bacillus subtilis

The past decade has witnessed an exiting unveiling of numerous molecular mechanisms that characterize signal transduction by protein-protein interaction. The recent findings encouraged an increasing effort to understand the sequential metabolism of different sugars available as energy sources at the same time. It seems probable that at least three principle mechanisms which act together or separately, mediate carbon catabolite repression (CCR) depending on the system which is under metabolic control: i) by the main signal transducing chain via the ATP-dependent HPr-kinase, HPr(Ser46-P) or alternatively Crh via the central component CcpA and its interaction with cre, ii) by signals sensed from the specific regulators directly or via phosphorylation by HPr, iii) by inducer exclusion based on the concurrence of the enzyme IIA(Glc) domain of the glucose permease, and other PTS-dependent permeases composed only of the B and C domains and lacking the enzyme IIA domain.     

Identification of catalytic residues in the beta-glucoside permease of Escherichia coli by site-specific mutagenesis and demonstration of interdomain cross-reactivity between the beta-glucoside and glucose systems

beta-Glucoside Enzyme II (IIBgl) of the Escherichia coli phosphotransferase system transports and phosphorylates beta-glucosides, whereas the glucose Enzyme II-III pair (IIGlc-IIIGlc) transports and phosphorylates glucose as well as certain aliphatic alpha- and beta-glucosides. Comparisons of their respective amino acid sequences previously revealed that both systems are homologous and must be evolutionarily related. To gain more insight into the details of the transport mechanism, we made use of the observed homologies among phosphotransferase system permeases to design a suitable set of site-specific mutants within the gene encoding IIBgl. This set was used to study in vivo fermentation and to analyze in vitro P-enolpyruvate-dependent sugar phosphorylation as well as sugar phosphate-dependent sugar transphosphorylation. The following results were obtained. (i) IIBgl transports and phosphorylates glucose as well as aryl- and alkyl-beta-glucosides; (ii) histidyl 547 is essential for the phosphorylation of IIBgl by the histidine-containing phosphoryl carrier protein of the phosphotransferase system (HPr) (first phosphorylation site); (iii) both cysteyl 24 and histidyl 306 are essential for the transfer of the phosphoryl group to the sugar; (iv) replacement of Cys-24 by serine leads to uncoupling of sugar transport from phosphorylation; and (v) histidyl 183 is important for substrate specificity. Our studies also revealed heterologous phosphoryl transfer between the beta-glucoside and glucose permease components which probably occurs as follows: 1) HPr-P----IIBgl (His-547)----IIGlc----alkyl-alpha- or -beta-glucosides or glucose (but not aryl-beta-glucosides) and 2) HPr-P----IIIGlc----IIBgl (Cys-24 or His-306)----alkyl- or aryl-beta-glucosides or glucose (but not methyl-alpha-glucoside). In addition to the essential residues noted above, several residues in IIBgl were identified which when mutated reduced the in vitro catalytic efficiency of the enzyme more than 10-fold. Thus, aspartyl 551 and arginyl 625 appeared to function together with histidyl 547 in phosphoryl transfer involving the first phosphorylation site in the permease, whereas histidyl 183 appeared to function together with cysteyl 24 and histidyl 306 in phosphoryl transfer involving the second phosphorylation site in the permease.     

Corynebacterium glutamicum: a dissection of the PTS

The high-GC Gram-positive actinomycete Corynebacterium glutamicum is commercially exploited as a producer of amino acids that are used as animal feed additives and flavor enhancers. Despite its beneficial role, carbon metabolism and its possible influence on amino acid metabolism is poorly understood. We have addressed this issue by analyzing the phosphotransferase system (PTS), which in many bacteria controls the flux of nutrients and therefore regulates carbon metabolism. The general PTS phosphotransferases enzyme I (EI) and HPr were characterized by demonstration of PEP-dependent phosphotransferase activity. An EI mutant exhibited a pleiotropic negative phenotype in carbon utilization. The role of the PTS as a major sugar uptake system was further demonstrated by the finding that glucose and fructose negative mutants were deficient in the respective enzyme II PTS permease activities. These carbon sources also caused repression of glutamate uptake, which suggests an involvement of the PTS in carbon regulation. The observation that no HPr kinase/phosphatase could be detected suggests that the mechanism of carbon regulation in C. glutamicum is different to the one found in low-GC Gram-positive bacteria.     

Energetics of glucose uptake in a Salmonella typhimurium mutant containing uncoupled enzyme IIGlc

Uncoupled enzyme II^Glc of the phosphoenolpyruvate (PEP): glucose phosphotransferase system (PTS) in Salmonella typhimurium is able to catalyze glucose transport in the absence of PEP-dependent phosphorylation. We have studied the energetics of glucose uptake catalyzed by this uncoupled enzyme II^Glc. The molar growth yields on glucose of two strains cultured anaerobically in glucose-limited chemostat-and batch cultures were compared. Strain PP 799 transported and phosphorylated glucose via an intact PTS, while strain PP 952 took up glucose exclusively via uncoupled enzyme II^Glc, followed by ATP-dependent phosphorylation by glucokinase. Thus the strains were isogenic except for the mode of uptake and phosphorylation of the growth substrate. PP 799 and PP 952 exhibited similar Y _Glc values. Assuming equal Y _ATP values for both strains this result indicated that there were no energetic demands for glucose uptake via uncoupled enzyme II^Glc.Abbreviations PTS phosphoenolpyruvate: carbohydrate phosphotransferase system - HPr histidine-containing phosphocarrier protein - GalP galactose permease     

Promoter-like mutation affecting HPr and enzyme I of the phosphoenolpyruvate: sugar phosphotransferase system in Salmonella typhimurium

A promoter-like mutation, ptsP160, has been identified which drastically reduces expression of the genes specifying two proteins, HPr and enzyme I, of the phosphoenolpyruvate:sugar phosphotransferase system (PTS) in Salmonella typhimurium. This mutation lies between trzA, a gene specifying susceptibility to 1,2,4-triazole, and ptsH, the structural gene for HPr. It leads to a loss of active transport of those sugars that require the PTS for entry into the cell. Pseudorevertants of strains carrying this promoter-like mutation have additional lesions very closely linked to ptsP160 by transduction analysis and are noninducible for HPr and enzyme I above a basal level. Presumably, strains carrying ptsP160 are defective in the normal induction mechanism for HPr and enzyme I, and the pseudorevertants derived from them result from second-site initiation signals within or near this promoter-like element. The induction of HPr and enzyme I above their noninduced levels apparently is not required for transport of at least one PTS sugar, methyl alpha-d-glucopyranoside, since this sugar is taken up by the pseudorevertants at the same rate as by the wild type. The existence of a promoter-like element governing the coordinate inducibility of both HPr and enzyme I suggests that ptsH and ptsI constitute an operon. Wild-type levels of a sugar-specific PTS protein, factor III, are synthesized in response to the crr(+) gene in both a ptsP160 strain and its pseudorevertants; this suggests that the crr(+) gene has its own promoter distinct from ptsP.     

Properties of a Streptococcus salivarius spontaneous mutant in which the methionine at position 48 in the protein HPr has been replaced by a valine.

HPr is a protein of the phosphoenolpyruvate:sugar phosphotransferase system (PTS) that participates in the concomitant transport and phosphorylation of sugars in bacteria. In gram-positive bacteria, HPr is also reversibly phosphorylated at a seryl residue at position 46 (Ser-46) by a metabolite-activated ATP-dependent kinase and a Pi-dependent HPr(Ser-P) phosphatase. We report in this article the isolation of a spontaneous mutant (mutant A66) from a streptococcus (Streptococcus salivarius) in which the methionine at position 48 (Met-48) in the protein HPr has been replaced by a valine (Val). The mutation inhibited the phosphorylation of HPr on Ser-46 by the ATP-dependent kinase but did not prevent phosphorylation of HPr by enzyme I or the phosphorylation of enzyme II complexes by HPr(His-P). The results, however, suggested that replacement of Met-48 by Val decreased the affinity of enzyme I for HPr or the affinity of enzyme II proteins for HPr(His-P) or both. Characterization of mutant A66 demonstrated that it has pleiotropic properties, including the lack of IIILman, a specific protein of the mannose PTS; decreased levels of HPr; derepression of some cytoplasmic proteins; reduced growth on PTS as well as on non-PTS sugars; and aberrant growth in medium containing a mixture of sugars.     

Defective enzyme II-BGlc of the phosphoenolpyruvate:sugar phosphotransferase system leading to uncoupling of transport and phosphorylation in Salmonella typhimurium.

Transport and phosphorylation of glucose via enzymes II-A/II-B and II-BGlc of the phosphoenolpyruvate:sugar phosphotransferase system are tightly coupled in Salmonella typhimurium. Mutant strains (pts) that lack the phosphorylating proteins of this system, enzyme I and HPr, are unable to transport or to grow on glucose. From ptsHI deletion strains of S. typhimurium, mutants were isolated that regained growth on and transport of glucose. Several lines of evidence suggest that these Glc+ mutants have an altered enzyme II-BGlc as follows. (i) Insertion of a ptsG::Tn10 mutation (resulting in a defective II-BGlc) abolished growth on and transport of glucose in these Glc+ strains. Introduction of a ptsM mutation, on the other hand, which abolishes II-A/II-B activity, had no effect. (ii) Methyl alpha-glucoside transport and phosphorylation (specific for II-BGlc) was lowered or absent in ptsH+,I+ transductants of these Glc+ strains. Transport and phosphorylation of other phosphoenolpyurate:sugar phosphotransferase system sugars were normal. (iii) Membranes isolated from these Glc+ mutants were unable to catalyze transphosphorylation of methyl alpha-glucoside by glucose 6-phosphate, but transphosphorylation of mannose by glucose 6-phosphate was normal. (iv) The mutation was in the ptsG gene or closely linked to it. We conclude that the altered enzyme II-BGlc has acquired the capacity to transport glucose in the absence of phosphoenolpyruvate:sugar phosphotransferase system-mediated phosphorylation. However, the affinity for glucose decreased at least 1,000-fold as compared to the wild-type strain. At the same time the mutated enzyme II-BGlc lost the ability to catalyze the phosphorylation of its substrates via IIIGlc.     

A common interface on histidine-containing phosphocarrier protein for interaction with its partner proteins

The bacterial phosphoenolpyruvate:sugar phosphotransferase system accomplishes both the transport and phosphorylation of sugars as well as the regulation of some cellular processes. An important component of this system is the histidine-containing phosphocarrier protein, HPr, which accepts a phosphoryl group from enzyme I, transfers a phosphoryl group to IIA proteins, and is an allosteric regulator of glycogen phosphorylase. Because the nature of the surface on HPr that interacts with this multiplicity of proteins from Escherichia coli was previously undefined, we investigated these interactions by nuclear magnetic resonance spectroscopy. The chemical shift changes of the backbone and side-chain amide (1)H and (15)N nuclei of uniformly (15)N-labeled HPr in the absence and presence of natural abundance glycogen phosphorylase, glucose-specific enzyme IIA, or the N-terminal domain of enzyme I have been determined. Mapping these chemical shift perturbations onto the three-dimensional structure of HPr permitted us to identify the binding surface(s) of HPr for interaction with these proteins. Here we show that the mapped interfaces on HPr are remarkably similar, indicating that HPr employs a similar surface in binding to its partners.