Regulatory functions of serine-46-phosphorylated HPr in Lactococcus lactis

Co-utilization of carbon sources in microorganisms for the bioproduction of chemicals

Carbon source is crucial for the cell growth and metabolism in microorganisms, and its utilization significantly affects the synthesis efficiency of target products in microbial cell factories. Compared with a single carbon source, co-utilizing carbon sources provide an alternative approach to optimize the utilization of different carbon sources for efficient biosynthesis of many chemicals with higher titer/yield/productivity. However, the efficiency of bioproduction is significantly limited by the sequential utilization of a preferred carbon source and secondary carbon sources, attributed to carbon catabolite repression (CCR). This review aimed to introduce the mechanisms of CCR and further focus on the summary of the strategies for co-utilization of carbon sources, including alleviation of CCR, engineering of the transport and metabolism of secondary carbon sources, compulsive co-utilization in single culture, co-utilization of carbon sources via co-culture, and evolutionary approaches. The findings of representative studies with a significant improvement in the bioproduction of chemicals via the co-utilization of carbon sources were discussed in this review. It suggested that by combining rational metabolic engineering and irrational evolutionary approaches, co-utilizing carbon sources can significantly contribute to the bioproduction of chemicals.

Deciphering the Regulation of the Mannitol Operon Paves the Way for Efficient Production of Mannitol in Lactococcus lactis

Lactococcus lactis has great potential for high-yield production of mannitol, which has not yet been fully realized. In this study, we characterize how the mannitol genes in L. lactis are organized and regulated and use this information to establish efficient mannitol production. Although the organization of the mannitol genes in L. lactis was similar to that in other Gram-positive bacteria, mtlF and mtlD, encoding the enzyme IIA component (EIIA^mtl) of the mannitol phosphotransferase system (PTS) and the mannitol-1-phosphate dehydrogenase, respectively, were separated by a transcriptional terminator, and the mannitol genes were found to be organized in two transcriptional units: an operon comprising mtlA, encoding the enzyme IIBC component (EIIBC^mtl) of the mannitol PTS, mtlR, encoding a transcriptional activator, and mtlF, as well as a separately expressed mtlD gene. The promoters driving expression of the two transcriptional units were somewhat similar, and both contained predicted catabolite responsive element (cre) genes. The presence of carbon catabolite repression was demonstrated and was shown to be relieved in stationary-phase cells. The transcriptional activator MtlR (mtlR), in some Gram-positive bacteria, is repressed by phosphorylation by EIIA^mtl, and when we knocked out mtlF, we indeed observed enhanced expression from the two promoters, which indicated that this mechanism was in place. Finally, by overexpressing the mtlD gene and using stationary-phase cells as biocatalysts, we attained 10.1 g/liter mannitol with a 55% yield, which, to the best of our knowledge, is the highest titer ever reported for L. lactis. Summing up, the results of our study should be useful for improving the mannitol-producing capacity of this important industrial organism. IMPORTANCE Lactococcus lactis is the most studied species of the lactic acid bacteria, and it is widely used in various food fermentations. To date, there have been several attempts to persuade L. lactis to produce mannitol, a sugar alcohol with important therapeutic and food applications. Until now, to achieve mannitol production in L. lactis with significant titer and yield, it has been necessary to introduce and express foreign genes, which precludes the use of such strains in foods, due to their recombinant status. In this study, we systematically characterize how the mannitol genes in L. lactis are regulated and demonstrate how this impacts mannitol production capability. We harnessed this information and managed to establish efficient mannitol production without introducing foreign genes.

HprKXcc is a serine kinase that regulates virulence in the Gram-negative phytopathogen Xanthomonas campestris

The HprK serine kinase is a component of the phosphoenolpyruvate phosphotransferase system (PTS) of bacteria that generally regulates catabolite repression through phosphorylation/dephosphorylation of the PTS protein PtsH at a conserved serine residue. However, many bacteria do not encode a complete PTS or even have an HprK homologue. Xanthomonas campestris pv. campestris (Xcc) is a pathogen that cause black rot disease in crucifer plants and one of the few Gram-negative bacteria that encodes a homologue of HprK protein (herein HprK_Xcc ). To gain insight into the role of HprK_Xcc and other PTS-related components in Xcc we individually mutated and phenotypically assessed the resulting strains. Deletion of hprK _Xcc demonstrated its requirement for virulence and other diverse cellular processes associated including extracellular enzyme activity, extracellular-polysaccharide production and cell motility. Global transcriptome analyses revealed the HprK_Xcc had a broad regulatory role in Xcc. Additionally, through overexpression, double gene deletion and transcriptome analysis we demonstrated that hprK _Xcc shares an epistatic relationship with ptsH. Furthermore, we demonstrate that HprK_Xcc is a functional serine kinase, which has the ability to phosphorylate PtsH. Taken together, the data illustrates the previously unappreciated global regulatory role of HprK_Xcc and previously uncharacterized PTS components that control virulence in this pathogen.

GlaR (YugA)-a novel RpiR-family transcription activator of the Leloir pathway of galactose utilization in Lactococcus lactis IL1403

Bacteria can utilize diverse sugars as carbon and energy source, but the regulatory mechanisms directing the choice of the preferred substrate are often poorly understood. Here, we analyzed the role of the YugA protein (now designated GlaR—Galactose–lactose operon Regulatory protein) of the RpiR family as a transcriptional activator of galactose (gal genes) and lactose (lac genes) utilization genes in Lactococcus lactis IL1403. In this bacterium, gal genes forming the Leloir operon are combined with lac genes in a single so‐called gal–lac operon. The first gene of this operon is the lacS gene encoding galactose permease. The glaR gene encoding GlaR lies directly upstream of the gal–lac gene cluster and is transcribed in the same direction. This genetic layout and the presence of glaR homologues in the closest neighborhood to the Leloir or gal–lac operons are highly conserved only among Lactococcus species. Deletion of glaR disabled galactose utilization and abrogated or decreased expression of the gal–lac genes. The GlaR‐dependent regulation of the gal–lac operon depends on its specific binding to a DNA region upstream of the lacS gene activating lacS expression and increasing the expression of the operon genes localized downstream. Notably, expression of lacS‐downstream genes, namely galMKTE, thgA and lacZ, is partially independent of the GlaR‐driven activation likely due to the presence of additional promoters. The glaR transcription itself is not subject to catabolite control protein A (CcpA) carbon catabolite repression (CRR) and is induced by galactose. Up to date, no similar mechanism has been reported in other lactic acid bacteria species. These results reveal a novel regulatory protein and shed new light on the regulation of carbohydrate catabolism in L. lactis IL1403, and by similarity, probably also in other lactococci.

Tyrosine 601 of Bacillus subtilis DnaK Undergoes Phosphorylation and Is Crucial for Chaperone Activity and Heat Shock Survival

In order to screen for cellular substrates of the Bacillus subtilis BY-kinase PtkA, and its cognate phosphotyrosine-protein phosphatase PtpZ, we performed a triple Stable Isotope Labeling by Amino acids in Cell culture-based quantitative phosphoproteome analysis. Detected tyrosine phosphorylation sites for which the phosphorylation level decreased in the ΔptkA strain and increased in the ΔptpZ strain, compared to the wild type (WT), were considered as potential substrates of PtkA/PtpZ. One of those sites was the residue tyrosine 601 of the molecular chaperone DnaK. We confirmed that DnaK is a substrate of PtkA and PtpZ by in vitro phosphorylation and dephosphorylation assays. In vitro, DnaK Y601F mutant exhibited impaired interaction with its co-chaperones DnaJ and GrpE, along with diminished capacity to hydrolyze ATP and assist the re-folding of denatured proteins. In vivo, loss of DnaK phosphorylation in the mutant strain dnaK Y601F, or in the strain overexpressing the phosphatase PtpZ, led to diminished survival upon heat shock, consistent with the in vitro results. The decreased survival of the mutant dnaK Y601F at an elevated temperature could be rescued by complementing with the WT dnaK allele expressed ectopically. We concluded that the residue tyrosine 601 of DnaK can be phosphorylated and dephosphorylated by PtkA and PtpZ, respectively. Furthermore, Y601 is important for DnaK chaperone activity and heat shock survival of B. subtilis.

The phosphocarrier protein HPr of Neisseria meningitidis interacts with the transcription regulator CrgA and its deletion affects capsule production, cell adhesion, and virulence

The bacterial phosphotransferase system (PTS) transports and phosphorylates sugars, but also carries out numerous regulatory functions. The β-proteobacterium Neisseria meningitidis possesses an incomplete PTS unable to transport carbon sources because it lacks a membrane component. Nevertheless, the residual phosphorylation cascade is functional and the meningococcal PTS was therefore expected to carry out regulatory roles. Interestingly, a ΔptsH mutant (lacks the PTS protein HPr) exhibited reduced virulence in mice and after intraperitoneal challenge it was rapidly cleared from the bloodstream of BALB/c mice. The rapid clearance correlates with lower capsular polysaccharide production by the ΔptsH mutant, which is probably also responsible for its increased adhesion to Hec-1-B epithelial cells. In addition, compared to the wild-type strain more apoptotic cells were detected when Hec-1-B cells were infected with the ΔptsH strain. Coimmunoprecipitation revealed an interaction of HPr and P-Ser-HPr with the LysR type transcription regulator CrgA, which among others controls its own expression. Moreover, ptsH deletion caused increased expression of a ΦcrgA-lacZ fusion. Finally, the presence of HPr or phospho-HPr's during electrophoretic mobility shift assays enhanced the affinity of CrgA for its target sites preceding crgA and pilE, but HPr did not promote CrgA binding to the sia and pilC1 promoter regions.

The bacterial phosphoenolpyruvate:carbohydrate phosphotransferase system: regulation by protein phosphorylation and phosphorylation-dependent protein-protein interactions

The bacterial phosphoenolpyruvate (PEP):carbohydrate phosphotransferase system (PTS) carries out both catalytic and regulatory functions. It catalyzes the transport and phosphorylation of a variety of sugars and sugar derivatives but also carries out numerous regulatory functions related to carbon, nitrogen, and phosphate metabolism, to chemotaxis, to potassium transport, and to the virulence of certain pathogens. For these different regulatory processes, the signal is provided by the phosphorylation state of the PTS components, which varies according to the availability of PTS substrates and the metabolic state of the cell. PEP acts as phosphoryl donor for enzyme I (EI), which, together with HPr and one of several EIIA and EIIB pairs, forms a phosphorylation cascade which allows phosphorylation of the cognate carbohydrate bound to the membrane-spanning EIIC. HPr of firmicutes and numerous proteobacteria is also phosphorylated in an ATP-dependent reaction catalyzed by the bifunctional HPr kinase/phosphorylase. PTS-mediated regulatory mechanisms are based either on direct phosphorylation of the target protein or on phosphorylation-dependent interactions. For regulation by PTS-mediated phosphorylation, the target proteins either acquired a PTS domain by fusing it to their N or C termini or integrated a specific, conserved PTS regulation domain (PRD) or, alternatively, developed their own specific sites for PTS-mediated phosphorylation. Protein-protein interactions can occur with either phosphorylated or unphosphorylated PTS components and can either stimulate or inhibit the function of the target proteins. This large variety of signal transduction mechanisms allows the PTS to regulate numerous proteins and to form a vast regulatory network responding to the phosphorylation state of various PTS components.

ClaR--a novel key regulator of cellobiose and lactose metabolism in Lactococcus lactis IL1403

In a number of previous studies, our group has discovered an alternative pathway for lactose utilization in Lactococcus lactis that, in addition to a sugar-hydrolyzing enzyme with both P-β-glucosidase and P-β-galactosidase activity (BglS), engages chromosomally encoded components of cellobiose-specific PTS (PTS^Cel-Lac), including PtcA, PtcB, and CelB. In this report, we show that this system undergoes regulation via ClaR, a novel activator protein from the RpiR family of transcriptional regulators. Although RpiR proteins are widely distributed among lactic acid bacteria, their roles have yet to be confirmed by functional assays. Here, we show that ClaR activity depends on intracellular cellobiose-6-phosphate availability, while other sugars such as glucose or galactose have no influence on it. We also show that ClaR is crucial for activation of the bglS and celB expression in the presence of cellobiose, with some limited effects on ptcA and ptcB activation. Among 190 of carbon sources tested, the deletion of claR reduces L. lactis growth only in lactose- and/or cellobiose-containing media, suggesting a narrow specificity of this regulator within the context of sugar metabolism.

Quantitative phosphoproteome analysis of Bacillus subtilis reveals novel substrates of the kinase PrkC and phosphatase PrpC

Reversible protein phosphorylation on serine, threonine, and tyrosine (Ser/Thr/Tyr) residues plays a critical role in regulation of vital processes in the cell. Despite of considerable progress in our understanding of the role of this modification in bacterial physiology, the dynamics of protein phosphorylation during bacterial growth has rarely been systematically addressed. In addition, little is known about in vivo substrates of bacterial Ser/Thr/Tyr kinases and phosphatases. An excellent candidate to study these questions is the Gram-positive bacterium Bacillus subtilis, one of the most intensively investigated bacterial model organism with both research and industrial applications. Here we employed gel-free phosphoproteomics combined with SILAC labeling and high resolution mass spectrometry to study the proteome and phosphoproteome dynamics during the batch growth of B. subtilis. We measured the dynamics of 1666 proteins and 64 phosphorylation sites in five distinct phases of growth. Enzymes of the central carbon metabolism and components of the translation machinery appear to be highly phosphorylated in the stationary phase, coinciding with stronger expression of Ser/Thr kinases. We further used the SILAC workflow to identify novel putative substrates of the Ser/Thr kinase PrkC and the phosphatase PrpC during stationary phase. The overall number of putative substrates was low, pointing to a high kinase and phosphatase specificity. One of the phosphorylation sites affected by both, PrkC and PrpC, was the Ser281 on the oxidoreductase YkwC. We showed that PrkC phosphorylates and PrpC dephosphorylates YkwC in vitro and that phosphorylation at Ser281 abolishes the oxidoreductase activity of YkwC in vitro and in vivo. Our results present the most detailed phosphoproteomic analysis of B. subtilis growth to date and provide the first global in vivo screen of PrkC and PrpC substrates.

A LacI-family regulator activates maltodextrin metabolism of Enterococcus faecium

Enterococcus faecium is a gut commensal of humans and animals. In the intestinal tract, E. faecium will have access to a wide variety of carbohydrates, including maltodextrins and maltose, which are the sugars that result from the enzymatic digestion of starch by host-derived and microbial amylases. In this study, we identified the genetic determinants for maltodextrin utilization of E. faecium E1162. We generated a deletion mutant of the mdxABCD-pulA gene cluster that is homologous to maltodextrin uptake genes in other Gram-positive bacteria, and a deletion mutant of the mdxR gene, which is predicted to encode a LacI family regulator of mdxABCD-pulA. Both mutations impaired growth on maltodextrins but had no effect on the growth on maltose and glucose. Comparative transcriptome analysis showed that eight genes (including mdxABCD-pulA) were expressed at significantly lower levels in the isogenic ΔmdxR mutant strain compared to the parental strain when grown on maltose. Quantitative real-time RT-PCR confirmed the results of transcriptome analysis and showed that the transcription of a putative maltose utilization gene cluster is induced in a semi-defined medium supplemented with maltose but is not regulated by MdxR. Understanding the maltodextrin metabolism of E. faecium could yield novel insights into the underlying mechanisms that contribute to the gut commensal lifestyle of E. faecium.

Transcription regulators controlled by interaction with enzyme IIB components of the phosphoenolpyruvate: sugar phosphotransferase system

Numerous bacteria possess transcription activators and antiterminators composed of regulatory domains phosphorylated by components of the phosphoenolpyruvate:sugar phosphotransferase system (PTS). These domains, called PTS regulation domains (PRDs), usually contain two conserved histidines as potential phosphorylation sites. While antiterminators possess two PRDs with four phosphorylation sites, transcription activators contain two PRDs plus two regulatory domains resembling PTS components (EIIA and EIIB). The activity of these transcription regulators is controlled by up to five phosphorylations catalyzed by PTS proteins. Phosphorylation by the general PTS components EI and HPr is usually essential for the activity of PRD-containing transcription regulators, whereas phosphorylation by the sugar-specific components EIIA or EIIB lowers their activity. For a specific regulator, for example the Bacillus subtilis mtl operon activator MtlR, the functional phosphorylation sites can be different in other bacteria and consequently the detailed mode of regulation varies. Some of these transcription regulators are also controlled by an interaction with a sugar-specific EIIB PTS component. The EIIBs are frequently fused to the membrane-spanning EIIC and EIIB-mediated membrane sequestration is sometimes crucial for the control of a transcription regulator. This is also true for the Escherichia coli repressor Mlc, which does not contain a PRD but nevertheless interacts with the EIIB domain of the glucose-specific PTS. In addition, some PRD-containing transcription activators interact with a distinct EIIB protein located in the cytoplasm. The phosphorylation state of the EIIB components, which changes in response to the presence or absence of the corresponding carbon source, affects their interaction with transcription regulators. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases (2012).

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

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

The maltose ABC transporter in Lactococcus lactis facilitates high-level sensitivity to the circular bacteriocin garvicin ML

We generated and characterized a series of spontaneous mutants of Lactococcus lactis IL1403 with average 6- to 11-fold-lowered sensitivities to the circular bacteriocin garvicin ML (GarML). Carbohydrate fermentation assays highlighted changes in carbohydrate metabolism, specifically loss of the ability to metabolize starch and maltose, in these mutants. PCR and sequencing showed that a 13.5-kb chromosomal deletion encompassing 12 open reading frames, mainly involved in starch and maltose utilization, had spontaneously occurred in the GarML-resistant mutants. Growth experiments revealed a correlation between sensitivity to GarML and carbon catabolite repression (CCR); i.e., sensitivity to GarML increased significantly when wild-type cells were grown on maltose and galactose as sole carbohydrates, an effect which was alleviated by the presence of glucose. Among the genes deleted in the mutants were malEFG, which encode a CCR-regulated membrane-bound maltose ABC transporter. The complementation of mutants with these three genes recovered normal sensitivity to the bacteriocin, suggesting an essential role of the maltose ABC transporter in the antimicrobial activity of GarML. This notion was supported by the fact that the level of sensitivity to GarML was dose dependent, increasing with higher expression levels of malEFG over a 50-fold range. To our knowledge, this is the first time a specific protein complex has been demonstrated to be involved in sensitivity to a circular bacteriocin.

From meadows to milk to mucosa - adaptation of Streptococcus and Lactococcus species to their nutritional environments

Lactic acid bacteria (LAB) are indigenous to food-related habitats as well as associated with the mucosal surfaces of animals. The LAB family Streptococcaceae consists of the genera Lactococcus and Streptococcus. Members of the family include the industrially important species Lactococcus lactis, which has a long history safe use in the fermentative food industry, and the disease-causing streptococci Streptococcus pneumoniae and Streptococcus pyogenes. The central metabolic pathways of the Streptococcaceae family have been extensively studied because of their relevance in the industrial use of some species, as well as their influence on virulence of others. Recent developments in high-throughput proteomic and DNA-microarray techniques, in in vivo NMR studies, and importantly in whole-genome sequencing have resulted in new insights into the metabolism of the Streptococcaceae family. The development of cost-effective high-throughput sequencing has resulted in the publication of numerous whole-genome sequences of lactococcal and streptococcal species. Comparative genomic analysis of these closely related but environmentally diverse species provides insight into the evolution of this family of LAB and shows that the relatively small genomes of members of the Streptococcaceae family have been largely shaped by the nutritionally rich environments they inhabit.

Maltose and maltodextrin utilization by Listeria monocytogenes depend on an inducible ABC transporter which is repressed by glucose

Background

In the environment as well as in the vertebrate intestine, Listeriae have access to complex carbohydrates like maltodextrins. Bacterial exploitation of such compounds requires specific uptake and utilization systems.

Methodology/Principal Findings

We could show that Listeria monocytogenes and other Listeria species contain genes/gene products with high homology to the maltodextrin ABC transporter and utilization system of B. subtilis. Mutant construction and growth tests revealed that the L. monocytogenes gene cluster was required for the efficient utilization of maltodextrins as well as maltose. The gene for the ATP binding protein of the transporter was located distant from the cluster. Transcription analyses demonstrated that the system was induced by maltose/maltodextrins and repressed by glucose. Its induction was dependent on a LacI type transcriptional regulator. Repression by glucose was independent of the catabolite control protein CcpA, but was relieved in a mutant defective for Hpr kinase/phosphorylase.

Conclusions/Significance

The data obtained show that in L. monocytogenes the uptake of maltodextrin and, in contrast to B. subtilis, also maltose is exclusively mediated by an ABC transporter. Furthermore, the results suggest that glucose repression of the uptake system possibly is by inducer exclusion, a mechanism not described so far in this organism.

Multiple control of the acetate pathway in Lactococcus lactis under aeration by catabolite repression and metabolites

To explore the factors controlling metabolite formation under aeration in Lactococcus lactis, metabolic patterns, enzymatic activities, and transcriptional profiles of genes involved in the aerobic pathway for acetate anabolism were compared between a parental L. lactis strain and its NADH-oxidase-overproducer derivative. Deregulated catabolite repression mutans in the ccpA or pstH genes, encoding CcpA or its co-activator HPr, respectively, were compared with a parental strain, as well. Although the NADH-oxidase activity was derepressed in ccpA, but not in the pstH background, a mixed fermentation was displayed by either mutant, with a higher acetate production in the pstH variant. Moreover, transcription of genes encoding phosphotransacetylase and acetate kinase were derepressed, and the corresponding enzymatic activities increased, in both catabolite repression mutants. These results and the dependence on carbon source for acetate production in the NADH-oxidase-overproducer support the conclusion that catabolite repression, rather than NADH oxidation, plays a critical role to control acetate production. Furthermore, fructose 1,6-bisphosphate influenced the in vitro phosphotransacetylase and acetate kinase activities, while the former was sensitive to diacetyl. Our study strongly supports the model that, under aerobic conditions, the homolactic fermentation in L. lactis MG1363 is maintained by CcpA-mediated repression of mixed acid fermentation.

The Ser/Thr/Tyr phosphoproteome of Lactococcus lactis IL1403 reveals multiply phosphorylated proteins

Recent phosphoproteomics studies of several bacterial species have firmly established protein phosphorylation on Ser/Thr/Tyr residues as a PTM in bacteria. In particular, our recent reports on the Ser/Thr/Tyr phosphoproteomes of bacterial model organisms Bacillus subtilis and Escherichia coli detected over 100 phosphorylation events in each of the bacterial species. Here we extend our analyses to Lactococcus lactis, a lactic acid bacterium widely employed by the food industry, in which protein phosphorylation at Ser/Thr/Tyr residues was barely studied at all. Despite the lack of almost any prior evidence of Ser/Thr/Tyr protein phosphorylation in L. lactis, we identified a phosphoproteome of a size comparable to that of E. coli and B. subtilis, with 73 phosphorylation sites distributed over 63 different proteins. The presence of several multiply phosphorylated proteins, as well as over-representation of phosphothreonines seems to be the distinguishing features of the L. lactis phosphoproteome. Evolutionary comparison and the conservation of phosphorylation sites in different bacterial organisms indicate that a majority of the detected phosphorylation sites are species-specific, and therefore have probably co-evolved with the adaptation of the bacterial species to their present-day ecological niches.

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).

HPrK regulates succinate-mediated catabolite repression in the gram-negative symbiont Sinorhizobium meliloti

The HPrK kinase/phosphatase is a common component of the phosphotransferase system (PTS) of gram-positive bacteria and regulates catabolite repression through phosphorylation/dephosphorylation of its substrate, the PTS protein HPr, at a conserved serine residue. Phosphorylation of HPr by HPrK also affects additional phosphorylation of HPr by the PTS enzyme EI at a conserved histidine residue. Sinorhizobium meliloti can live as symbionts inside legume root nodules or as free-living organisms and is one of the relatively rare gram-negative bacteria known to have a gene encoding HPrK. We have constructed S. meliloti mutants that lack HPrK or that lack key amino acids in HPr that are likely phosphorylated by HPrK and EI. Deletion of hprK in S. meliloti enhanced catabolite repression caused by succinate, as did an S53A substitution in HPr. Introduction of an H22A substitution into HPr alleviated the strong catabolite repression phenotypes of strains carrying Delta hprK or hpr(S53A) mutations, demonstrating that HPr-His22-P is needed for strong catabolite repression. Furthermore, strains with a hpr(H22A) allele exhibited relaxed catabolite repression. These results suggest that HPrK phosphorylates HPr at the serine-53 residue, that HPr-Ser53-P inhibits phosphorylation at the histidine-22 residue, and that HPr-His22-P enhances catabolite repression in the presence of succinate. Additional experiments show that Delta hprK mutants overproduce exopolysaccharides and form nodules that do not fix nitrogen.

Sinorhizobium meliloti mutants lacking phosphotransferase system enzyme HPr or EIIA are altered in diverse processes, including carbon metabolism, cobalt requirements, and succinoglycan production

Sinorhizobium meliloti is a member of the Alphaproteobacteria that fixes nitrogen when it is in a symbiotic relationship. Genes for an incomplete phosphotransferase system (PTS) have been found in the genome of S. meliloti. The genes present code for Hpr and ManX (an EIIA(Man)-type enzyme). HPr and EIIA regulate carbon utilization in other bacteria. hpr and manX in-frame deletion mutants exhibited altered carbon metabolism and other phenotypes. Loss of HPr resulted in partial relief of succinate-mediated catabolite repression, extreme sensitivity to cobalt limitation, rapid die-off during stationary phase, and altered succinoglycan production. Loss of ManX decreased expression of melA-agp and lac, the operons needed for utilization of alpha- and beta-galactosides, slowed growth on diverse carbon sources, and enhanced accumulation of high-molecular-weight succinoglycan. A strain with both hpr and manX deletions exhibited phenotypes similar to those of the strain with a single hpr deletion. Despite these strong phenotypes, deletion mutants exhibited wild-type nodulation and nitrogen fixation when they were inoculated onto Medicago sativa. The results show that HPr and ManX (EIIA(Man)) are involved in more than carbon regulation in S. meliloti and suggest that the phenotypes observed occur due to activity of HPr or one of its phosphorylated forms.

Maltose transport in Lactobacillus casei and its regulation by inducer exclusion

Transport of maltose in Lactobacillus casei BL23 is subject to regulation by inducer exclusion. The presence of glucose or other rapidly metabolized carbon sources blocks maltose transport by a control mechanism that depends on the phosphorylation of the HPr protein at serine residue 46. We have identified the L. casei gene cluster for maltose/maltodextrin utilization by sequence analysis and mutagenesis. It is composed of genes coding for a transcriptional regulator, oligosaccharide hydrolytic enzymes, an ABC transporter (MalEFGK2) and the enzymes for the metabolism of maltose or the degradation products of maltodextrins: maltose phosphorylase and beta-phospho-glucomutase. These genes are induced by maltose and repressed by the presence of glucose via the catabolite control protein A (CcpA). A mutant strain was constructed which expressed the hprKV267F allele and therefore formed large amounts of P-Ser-HPr even in the absence of a repressive carbon source. In this mutant, transport of maltose was severely impaired, whereas transport of sugars not subject to inducer exclusion was not changed. These results strengthen the idea that P-Ser-HPr controls inducer exclusion and make the maltose system of L. casei a suitable model for studying this process in Firmicutes.

Structural analysis of the bacterial HPr kinase/phosphorylase V267F mutant gives insights into the allosteric regulation mechanism of this bifunctional enzyme

The HPr kinase/phosphorylase (HPrK/P) is a bifunctional enzyme that controls the phosphorylation state of the phospho-carrier protein HPr, which regulates the utilization of carbon sources in Gram-positive bacteria. It uses ATP or pyrophosphate for the phosphorylation of serine 46 of HPr and inorganic phosphate for the dephosphorylation of Ser(P)-46-HPr via a phosphorolysis reaction. HPrK/P is a hexameric protein kinase of a new type with a catalytic core belonging to the family of nucleotide-binding protein with Walker A motif. It exhibits no structural similarity to eukaryotic protein kinases. So far, HPrK/P structures have shown the enzyme in its phosphorylase conformation. They permitted a detailed characterization of the phosphorolysis mechanism. In the absence of a structure with bound nucleotide, we used the V267F mutant enzyme to assess the kinase conformation. Indeed, the V267F replacement was found to cause an almost entire loss of the phosphorylase activity of Lactobacillus casei HPrK/P. In contrast, the kinase activity remained conserved. To elucidate the structural alterations leading to this drastic change of activity, the x-ray structure of the catalytic domain of L. casei HPrK/P-V267F was determined at 2.6A resolution. A comparison with the structure of the wild type enzyme showed that the mutation induces conformation changes compatible with the switch from phosphorylase to kinase function. Together with nucleotide binding fluorescence measurements, these results allowed us to decipher the cooperative behavior of the protein and to gain new insights into the allosteric regulation mechanism of HPrK/P.

Regulation of sugar catabolism in Lactococcus lactis

The increasing number of genomic and post-genomic studies on Gram-positive organisms and especially on lactic acid bacteria brings a lot of information on sugar catabolism in these bacteria. Like for many other bacteria, glucose is the most preferred source of carbon and energy for Lactococcus lactis. Other carbon sources can induce their own utilization in the absence of well-metabolized sugar. These processes engage numbers of genes and undergo complex mechanisms of regulation. In this review, we discuss various biochemical and genetic control mechanisms involved in sugar catabolism, like regulation by repressors, activators, antiterminators or carbon catabolite repression control.

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.

The Phosphotransferase System of Lactobacillus casei: Regulation of Carbon Metabolism and Connection to Cold Shock Response

Genome sequencing of two different Lactobacillus casei strains (ATCC334 and BL23) is presently going on and preliminary data revealed that this lactic acid bacterium possesses numerous carbohydrate transport systems probably reflecting its capacity to proliferate under varying environmental conditions. Many carbohydrate transporters belong to the phosphoenolpyruvate:sugar phosphotransferase system (PTS), but all different kinds of non-PTS transporters are present as well and their substrates are known in a few cases. In L. casei regulation of carbohydrate transport and carbon metabolism is mainly achieved by PTS proteins. Carbon catabolite repression (CCR) is mediated via several mechanisms, including the major P-Ser-HPr/catabolite control protein A (CcpA)-dependent mechanism. Catabolite response elements, the target sites for the P-Ser-HPr/CcpA complex, precede numerous genes and operons. PTS regulation domain-containing antiterminators and transcription activators are also present in both L. casei strains. Their activity is usually controlled by two PTS-mediated phosphorylation reactions exerting antagonistic effects on the transcription regulators: P~EIIB-dependent phosphorylation regulates induction of the corresponding genes and P~His-HPr-mediated phosphorylation plays a role in CCR. Carbohydrate transport of L. casei is also regulated via inducer exclusion and inducer expulsion. The presence of glucose, fructose, etc. leads to inhibition of the transport or metabolism of less favorable carbon sources (inducer exclusion) or to the export of accumulated non-metabolizable carbon sources (inducer expulsion). While P-Ser-HPr is essential for inducer exclusion of maltose, it is not necessary for the expulsion of accumulated thio-methyl-beta-D-galactopyranoside. Surprisingly, recent evidence suggests that the PTS of L. casei also plays a role in cold shock response.

How seryl-phosphorylated HPr inhibits PrfA, a transcription activator of Listeria monocytogenes virulence genes

Listeria monocytogenes PrfA, a transcription activator for several virulence genes, including the hemolysin-encoding hly, is inhibited by rapidly metabolizable carbon sources (glucose, fructose, etc.). This inhibition is not mediated via the major carbon catabolite repression mechanism of gram-positive bacteria, since inactivation of the catabolite control protein A (CcpA) did not prevent the repression of virulence genes by the above sugars. In order to test whether the catabolite co-repressor P-Ser-HPr might be involved in PrfA regulation, we used a Bacillus subtilis strain (BUG1199) containing L. monocytogenes prfA under control of pspac and the lacZ reporter gene fused to the PrfA-activated hly promoter. Formation of P-Ser-HPr requires the bifunctional HPr kinase/phosphorylase (HprK/P), which, depending on the concentration of certain metabolites, either phosphorylates HPr at Ser-46 or dephosphorylates P-Ser-HPr. The hprKV267F allele codes for an HprK/P leading to the accumulation of P-Ser-HPr, since it has normal kinase, but almost no phosphorylase activity. Interestingly, introducing hprKV267F into BUG1199 strongly inhibited transcription activation by PrfA. Preventing the accumulation of P-Ser-HPr in the hprKV267F mutant by replacing Ser-46 in HPr with an alanine restored PrfA activity, while ccpA inactivation had no effect. Interestingly, disruption of ccpA in the hprK wild-type strain BUG1199 also led to inhibition of PrfA. The lowered lacZ expression in the ccpA strain is probably also due to elevated amounts of P-Ser-HPr, since it disappeared when Ser-46 in HPr was replaced with an alanine. To carry out its catalytic function in sugar transport, HPr of the phosphotransferase system (PTS) is also phosphorylated by phosphoenolpyruvate and enzyme I at His-15. However, P-Ser-HPr is only very slowly phosphorylated by enzyme I, which probably accounts for PrfA inhibition. In agreement with this concept, disruption of the enzyme I- or HPr-encoding genes also strongly inhibited PrfA activity. PrfA activity therefore seems to depend on a fully functional PTS phosphorylation cascade.

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.

Molecular analysis of the glucose-specific phosphoenolpyruvate : sugar phosphotransferase system from Lactobacillus casei and its links with the control of sugar metabolism

Lactobacillus caseitransports glucose preferentially by a mannose-class phosphoenolpyruvate : sugar phosphotransferase system (PTS). The genomic analysis ofL. caseiallowed the authors to find a gene cluster (manLMNO) encoding the IIAB (manL), IIC (manM) and IID (manN) proteins of a mannose-class PTS, and a putative 121 aa protein of unknown function (encoded bymanO), homologues of which are also present inmanclusters that encode glucose/mannose transporters in other Gram-positive bacteria. TheL. casei manoperon is constitutively expressed into amanLMNOmessenger, but an additionalmanOtranscript was also detected. Upstream of themanoperon, two genes (upsRandupsA) were found which encode proteins resembling a transcriptional regulator and a membrane protein, respectively. Disruption of eitherupsRorupsAdid not affectmanLMNOtranscription, and had no effect on glucose uptake. Cells carrying amanOdeletion transported glucose at a rate similar to that of the wild-type strain. By contrast, amanMdisruption resulted in cells unable to transport glucose by the PTS, thus confirming the functional role of themangenes. In addition, themanMmutant exhibited neither inducer exclusion of maltose nor glucose repression. This result confirms the need for glucose transport through the PTS to trigger these regulatory processes inL. casei.

P-Ser-HPr—a link between carbon metabolism and the virulence of some pathogenic bacteria

HPr kinase/phosphorylase phosphorylates HPr, a phosphocarrier protein of the phosphoenolpyruvate:carbohydrate phosphotransferase system, at serine-46. P-Ser-HPr is the central regulator of carbon metabolism in Gram-positive bacteria, but also plays a role in virulence development of certain pathogens. In Listeria monocytogenes, several virulence genes, which depend on the transcription activator PrfA, are repressed by glucose, fructose, etc., in a catabolite repressor (CcpA)-independent mechanism. However, the catabolite co-repressor P-Ser-HPr was found to inhibit the activity of PrfA. In an hprKV267F mutant, in which most of the HPr is transformed into P-Ser-HPr, PrfA was barely active. The ptsH1 mutation (Ser-46 of HPr replaced with an alanine) prevented the inhibitory effect of the hprKV267F mutation. Interestingly, disruption of ccpA also inhibited PrfA activity. This effect is probably also mediated via P-Ser-HPr, since ccpA disruption leads to elevated amounts of P-Ser-HPr. Indeed, a ccpA ptsH1 double mutant exhibited normal PrfA activity. In S. pyogenes, the expression of several virulence genes depends on the transcription activator Mga. Interestingly, the mga promoter is preceded by an operator site, which serves as target for the CcpA/P-Ser-HPr complex. Numerous Gram-negative pathogens also contain hprK, which is often organised in an operon with transcription regulators necessary for the development of virulence, indicating that in these organisms P-Ser-HPr also plays a role in pathogenesis. Indeed, inactivation of Neisseria meningitidis hprK strongly diminished cell adhesion of this pathogen.

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.

Engineering Lactococcus lactis for production of mannitol: high yields from food-grade strains deficient in lactate dehydrogenase and the mannitol transport system

Mannitol is a sugar polyol claimed to have health-promoting properties. A mannitol-producing strain of Lactococcus lactis was obtained by disruption of two genes of the phosphoenolpyruvate (PEP)-mannitol phosphotransferase system (PTS(Mtl)). Genes mtlA and mtlF were independently deleted by double-crossover recombination in strain L. lactis FI9630 (a food-grade lactate dehydrogenase-deficient strain derived from MG1363), yielding two mutant (Delta ldh Delta mtlA and Delta ldh Delta mtlF) strains. The new strains, FI10091 and FI10089, respectively, do not possess any selection marker and are suitable for use in the food industry. The metabolism of glucose in nongrowing cell suspensions of the mutant strains was characterized by in vivo (13)C-nuclear magnetic resonance. The intermediate metabolite, mannitol-1-phosphate, accumulated intracellularly to high levels (up to 76 mM). Mannitol was a major end product, one-third of glucose being converted to this hexitol. The double mutants, in contrast to the parent strain, were unable to utilize mannitol even after glucose depletion, showing that mannitol was taken up exclusively by PEP-PTS(Mtl). Disruption of this system completely blocked mannitol transport in L. lactis, as intended. In addition to mannitol, approximately equimolar amounts of ethanol, 2,3-butanediol, and lactate were produced. A mixed-acid fermentation (formate, ethanol, and acetate) was also observed during growth under controlled conditions of pH and temperature, but mannitol production was low. The reasons for the alteration in the pattern of end products under nongrowing and growing conditions are discussed, and strategies to improve mannitol production during growth are proposed.

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.

Proteome analyses of heme-dependent respiration in Lactococcus lactis: involvement of the proteolytic system

Sugar fermentation was long considered the sole means of energy metabolism available to lactic acid bacteria. We recently showed that metabolism of Lactococcus lactis shifts progressively from fermentation to respiration during growth when oxygen and heme are available. To provide insights into this phenomenon, we compared the proteomic profiles of L. lactis under fermentative and respiratory growth conditions in rich medium. We identified 21 proteins whose levels differed significantly between these conditions. Two major groups of proteins were distinguished, one involved in carbon metabolism and the second in nitrogen metabolism. Unexpectedly, enzymes of the proteolytic system (PepO1 and PepC) which are repressed in rich medium in fermentation growth were induced under respiratory conditions despite the availability of free amino acids. A triple mutant (dtpT dtpP oppA) deficient in oligopeptide transport displayed normal respiration, showing that increased proteolytic activity is not an absolute requirement for respiratory metabolism. Transcriptional analysis confirmed that pepO1 is induced under respiration-permissive conditions. This induction was independent of CodY, the major regulator of proteolytic functions in L. lactis. We also observed that pepO1 induction is redox sensitive. In a codY mutant, pepO1 expression was increased twofold in aeration and eightfold in respiration-permissive conditions compared to static conditions. These observations suggest that new regulators activate proteolysis in L. lactis, which help to maintain the energetic needs of L. lactis during respiration.

CcpA regulation of aerobic and respiration growth in Lactococcus lactis

The catabolic control protein CcpA is the highly conserved regulator of carbon metabolism in Gram-positive bacteria. We recently showed that Lactococcus lactis, a fermenting bacterium in the family of Streptococcaceae, is capable of respiration late in growth when haem is added to aerated cultures. As the start of respiration coincides with glucose depletion from the medium, we hypothesized that CcpA is involved in this metabolic switch and investigated its role in lactococcal growth under aeration and respiration conditions. Compared with modest changes observed in fermentation growth, inactivation of ccpA shifts metabolism to mixed acid fermentation under aeration conditions. This shift is due to a modification of the redox balance via derepression of NADH oxidase, which eliminates oxygen and decreases the NADH pool. CcpA also plays a decisive role in respiration metabolism. Haem addition to lag phase ccpA cells results in growth arrest and cell mortality. Toxicity is due to oxidative stress provoked by precocious haem uptake. We identify the repressor of the haem transport system and show that it is a target of CcpA activation. We propose that CcpA-mediated repression of haem uptake is a means of preventing oxidative damage at the start of exponential growth. CcpA thus appears to govern a regulatory network that coordinates oxygen, iron and carbon metabolism.

Catabolism of mannitol in Lactococcus lactis MG1363 and a mutant defective in lactate dehydrogenase

Mannitol metabolism in Lactococcus lactis MG1363 and in a derivative strain deficient in lactate dehydrogenase (LDH(d)) was characterized. Both strains had the ability to grow on mannitol as an energy source, although this polyol was a poorer substrate for growth than glucose. When compared to glucose, the metabolism of mannitol caused an NADH burden due to formation of an additional NADH molecule at the reaction catalysed by mannitol-1-phosphate dehydrogenase (Mtl1PDH). This resulted in a prominent accumulation of mannitol 1-phosphate (Mtl1P) both in growing and resting cells, suggesting the existence of a severe bottleneck at Mtl1PDH. Growth on mannitol induced the activity of Mtl1PDH in both the LDH(d) and MG1363 strains. The lower accumulation of Mtl1P in mannitol-grown cells when compared to glucose-grown LDH(d) cells, as monitored by in vivo (13)C-NMR, reflects this induction. A clear shift towards the production of ethanol was observed on mannitol, indicating pressure to regenerate NAD(+) when this substrate was used. A strategy to obtain a mannitol-overproducing strain is proposed.

Pyrophosphate-producing protein dephosphorylation by HPr kinase/phosphorylase: a relic of early life?

In most Gram-positive bacteria, serine-46-phosphorylated HPr (P-Ser-HPr) controls the expression of numerous catabolic genes ( approximately 10% of their genome) by acting as catabolite corepressor. HPr kinase/phosphorylase (HprK/P), the bifunctional sensor enzyme for catabolite repression, phosphorylates HPr, a phosphocarrier protein of the sugar-transporting phosphoenolpyruvate/glycose phosphotransferase system, in the presence of ATP and fructose-1,6-bisphosphate but dephosphorylates P-Ser-HPr when phosphate prevails over ATP and fructose-1,6-bisphosphate. We demonstrate here that P-Ser-HPr dephosphorylation leads to the formation of HPr and pyrophosphate. HprK/P, which binds phosphate at the same site as the beta phosphate of ATP, probably uses the inorganic phosphate to carry out a nucleophilic attack on the phosphoryl bond in P-Ser-HPr. HprK/P is the first enzyme known to catalyze P-protein dephosphorylation via this phospho-phosphorolysis mechanism. This reaction is reversible, and at elevated pyrophosphate concentrations, HprK/P can use pyrophosphate to phosphorylate HPr. Growth of Bacillus subtilis on glucose increased intracellular pyrophosphate to concentrations ( approximately 6 mM), which in in vitro tests allowed efficient pyrophosphate-dependent HPr phosphorylation. To effectively dephosphorylate P-Ser-HPr when glucose is exhausted, the pyrophosphate concentration in the cells is lowered to 1 mM. In B. subtilis, this might be achieved by YvoE. This protein exhibits pyrophosphatase activity, and its gene is organized in an operon with hprK.

Carbon catabolite repression in bacteria: choice of the carbon source and autoregulatory limitation of sugar utilization

Carbon catabolite repression (CCR) in bacteria is generally regarded as a regulatory mechanism to ensure sequential utilization of carbohydrates. Selection of the carbon sources is mainly made at the level of carbohydrate-specific induction. Since virtually all carbohydrate catabolic genes or operons are regulated by specific control proteins and require inducers for high level expression, direct control of the activity of regulators or control of inducer formation is an efficient measure to keep them silent. By these mechanisms, bacteria are able to establish a hierarchy of sugar utilization. In addition to the control of induction processes by CCR, bacteria have developed global transcriptional regulation circuits, in which pleiotropic regulators are activated. These global control proteins, the catabolite gene activator protein (CAP), also known as cAMP receptor protein, in Escherichia coli or the catabolite control protein (CcpA) in Gram-positive bacteria with low GC content, act upon a large number of catabolic genes/operons. Since practically any carbon source is able to trigger global transcriptional control, expression of sugar utilization genes is restricted even in the sole presence of their cognate substrates. Consequently, CAP- or CcpA-dependent catabolite repression serves as an autoregulatory device to keep sugar utilization at a certain level rather than to establish preferential utilization of certain carbon sources. Together with other autoregulatory mechanisms that are not acting at the gene expression level, CCR helps bacteria to adjust sugar utilization to their metabolic capacities. Therefore, catabolic/metabolic balance would perhaps better describe the physiological role of this regulatory network than the term catabolite repression.