Specificity of peptide transport systems in Lactococcus lactis: evidence for a third system which transports hydrophobic di- and tripeptides

The growth-promoting effects of protein hydrolysates and their derived peptides on probiotics: structure-activity relationships, mechanisms and future perspectives

Lactic acid bacteria (LAB) are the main probiotics currently available in the markets and are essential for maintaining gut health. To guarantee probiotic function, it is imperative to boost the culture yield of probiotic organisms, ensure the sufficient viable cells in commercial products, or develop effective prebiotics. Recent studies have shown that protein hydrolysates and their derived peptides promote the proliferation of probiotic in vitro and the abundance of gut flora. This article comprehensively reviews different sources of protein hydrolysates and their derived peptides as growth-promoting factors for probiotics including Lactobacillus, Bifidobacterium, and Saccharomyces. We also provide a preliminary analysis of the characteristics of LAB proteolytic systems focusing on the correlation between their elements and growth-promoting activities. The structure-activity relationship and underlying mechanisms of growth-promoting peptides and their research perspectives are thoroughly discussed. Overall, this review provides valuable insights into growth-promoting protein hydrolysates and their derived peptides for proliferating probiotics in vivo or in vitro, which may inspire researchers to explore new options for industrial probiotics proliferation, dairy products fermentation, and novel prebiotics development in the future.

Characteristics of the Proteolytic Enzymes Produced by Lactic Acid Bacteria

Over the past several decades, we have observed a very rapid development in the biotechnological use of lactic acid bacteria (LAB) in various branches of the food industry. All such areas of activity of these bacteria are very important and promise enormous economic and industrial successes. LAB are a numerous group of microorganisms that have the ability to ferment sugars into lactic acid and to produce proteolytic enzymes. LAB proteolytic enzymes play an important role in supplying cells with the nitrogen compounds necessary for their growth. Their nutritional requirements in this regard are very high. Lactic acid bacteria require many free amino acids to grow. The available amount of such compounds in the natural environment is usually small, hence the main function of these enzymes is the hydrolysis of proteins to components absorbed by bacterial cells. Enzymes are synthesized inside bacterial cells and are mostly secreted outside the cell. This type of proteinase remains linked to the cell wall structure by covalent bonds. Thanks to advances in enzymology, it is possible to obtain and design new enzymes and their preparations that can be widely used in various biotechnological processes. This article characterizes the proteolytic activity, describes LAB nitrogen metabolism and details the characteristics of the peptide transport system. Potential applications of proteolytic enzymes in many industries are also presented, including the food industry.

Insight into the bovine milk peptide LPcin-YK3 selection in the proteolytic system of Lactobacillus species

Antimicrobial peptides are class of small, positively charged peptides known for their broad-spectrum antimicrobial activity. Antimicrobial activities for most antimicrobial peptides have largely remained elusive, particularly in the lactic acid bacteria. However, recently our investigation using LPcin-YK3, an antimicrobial peptide from bovine milk, suggests that in vitro antimicrobial activity was reduced over 100-fold compared with pathogenic bacteria. Additionally, for the structural study of how antimicrobial peptide undergoes its reaction at the proteolytic pathway of lactic acid bacteria based on degradation assay and propidium iodide staining, we performed molecular docking for interaction between oligopeptide-binding protein A and LPcin-YK3 peptide. Given that degradation related to the LPcin-YK3 peptide in lactic acid bacteria proteolytic system, the inhibitory inactivity of LPcin-YK3 against beneficial lactic acid bacteria strains may be one of the primary pharmacological properties of recombinant peptide discovered in bovine milk. These results provide structural and functional insights into the proteolytic mechanism and possibility as a putative substrate of oligopeptide-binding protein A in respect of LPcin-YK3 peptide.

The potential of proteins, hydrolysates and peptides as growth factors forLactobacillusandBifidobacterium: current research and future perspectives

Probiotics are live microorganisms that provide health benefits to the host when consumed in adequate concentrations.

Bioprospecting for Bioactive Peptide Production by Lactic Acid Bacteria Isolated from Fermented Dairy Food

With rapidly ageing populations, the world is experiencing unsustainable healthcare from chronic diseases such as metabolic, cardiovascular, neurodegenerative, and cancer disorders. Healthy diet and lifestyle might contribute to prevent these diseases and potentially enhance health outcomes in patients during and after therapy. Fermented dairy foods (FDFs) found their origin concurrently with human civilization for increasing milk shelf-life and enhancing sensorial attributes. Although the probiotic concept has been developed more recently, FDFs, such as milks and yoghurt, have been unconsciously associated with health-promoting effects since ancient times. These health benefits rely not only on the occurrence of fermentation-associated live microbes (mainly lactic acid bacteria; LAB), but also on the pro-health molecules (PHMs) mostly derived from microbial conversion of food compounds. Therefore, there is a renaissance of interest toward traditional fermented food as a reservoir of novel microbes producing PHMs, and “hyperfoods” can be tailored to deliver these healthy molecules to humans. In FDFs, the main PHMs are bioactive peptides (BPs) released from milk proteins by microbial proteolysis. BPs display a pattern of biofunctions such as anti-hypertensive, antioxidant, immuno-modulatory, and anti-microbial activities. Here, we summarized the BPs most frequently encountered in dairy food and their biological activities; we reviewed the main studies exploring the potential of dairy microbiota to release BPs; and delineated the main effectors of the proteolytic LAB systems responsible for BPs release.

pH- and Temperature-Dependent Peptide Binding to the Lactococcus lactis Oligopeptide-Binding Protein A Measured with a Fluorescence Anisotropy Assay

Bacterial ATP-binding cassette transporters are a superfamily of transport systems involved in the import of various molecules including amino acids, ions, sugars, and peptides. In the lactic acid bacteria Lactococcus lactis, the oligopeptide-binding protein A (OppA) binds peptides for import to support nitrogen metabolism and cell growth. The OppA protein is of great interest because it can bind peptides over a broad variety of lengths and sequences; however, current methods to study peptide binding have employed low throughput, endpoint, or low dynamic range techniques. Therefore, in this study, we developed a fluorescence anisotropy-based peptide-binding assay that can be readily employed to quantify OppA function. To test the utility of our assay, we characterized the pH dependence of oligopeptide binding because L. lactis is commonly used in fermentation and often must survive in low pH environments caused by lactic acid export. We determined that OppA affinity increases as pH or temperature decreases, and circular dichroism spectroscopy further indicated that acidic conditions increase the thermal stability of the protein, increasing the unfolding transition temperature by 10 °C from pH 8 to pH 6. Thus, our fluorescence anisotropy assay provides an easy technique to measure peptide binding, and it can be used to understand molecular aspects of OppA function under stress conditions experienced during fermentation and other biotechnology applications.

Utilization of (15)N-labelled yeast hydrolysate in Lactococcus lactis IL1403 culture indicates co-consumption of peptide-bound and free amino acids with simultaneous efflux of free amino acids

Lactococcus lactis subsp. lactis IL1403 was grown in medium containing unlabelled free amino acids and (15)N-labelled yeast hydrolysate to gain insight into the role of peptides as a source of amino acids under conditions where free amino acids are abundant. A mathematical model was composed to estimate the fluxes of free and peptide-derived amino acids into and out of the intracellular amino acid pool. We observed co-consumption of peptides and free amino acids and a considerable efflux of most free amino acids during growth. We did not observe significant differences between the peptide consumption patterns of essential and non-essential amino acids, which suggests that the incorporation of a particular amino acid is more dependent on its availability in a readily assimilated form than the organism's auxotrophy for it. For most amino acids the contribution of peptide-bound forms to the formation of biomass was initially between 30 and 60 % with the remainder originating from free amino acids. During the later stages of fermentation we observed a decrease in the utilization of peptide-bound amino acids, thus indicating that the more readily assimilated peptides are gradually exhausted from the medium during growth.

The Hyb hydrogenase permits hydrogen-dependent respiratory growth of Salmonella enterica serovar Typhimurium

Salmonella enterica serovar Typhimurium contains three distinct respiratory hydrogenases, all of which contribute to virulence. Addition of H(2) significantly enhanced the growth rate and yield of S. Typhimurium in an amino acid-containing medium; this occurred with three different terminal respiratory electron acceptors. Based on studies with site-specific double-hydrogenase mutant strains, most of this H(2)-dependent growth increase was attributed to the Hyb hydrogenase, rather than to the Hya or Hyd respiratory H(2)-oxidizing enzymes. The wild type strain with H(2) had 4.0-fold greater uptake of (14)C-labeled amino acids over a period of minutes than did cells incubated without H(2). The double-uptake hydrogenase mutant containing only the Hyb hydrogenase transported amino acids H(2) dependently like the wild type. The Hyb-only-containing strain produced a membrane potential comparable to that of the wild type. The H(2)-stimulated amino acid uptake of the wild type and the Hyb-only strain was inhibited by the protonophore carbonyl cyanide m-chlorophenylhydrazone but was less affected by the ATP synthase inhibitor sodium orthovanadate. In the wild type, proteins TonB and ExbD, which are known to couple proton motive force (PMF) to transport processes, were induced by H(2) exposure, as were the genes corresponding to these periplasmic PMF-coupling factors. However, studies on tonB and exbD single mutant strains could not confirm a major role for these proteins in amino acid transport. The results link H(2) oxidation via the Hyb enzyme to growth, amino acid transport, and expression of periplasmic proteins that facilitate PMF-mediated transport across the outer membrane.

Genetic characterization of oligopeptide uptake systems in Sinorhizobium meliloti

The genetic characterization of three ABC transport systems involved in oligopeptide uptake by Sinorhizobium meliloti is reported. Oligopeptide permease (Opp) encoded by the pSymB oppABCD operon, is required for uptake of tetrapeptides and certain tripeptides like 3Ala and bialaphos. The chromosomally encoded dipeptide permease (Dpp1), also able to import the toxic tripeptide bialaphos, is required for utilization of dipeptides and tripeptides like 3Gly and GlyGlyAla, with minor importance for utilization of 3Ala and tetrapeptides. The ttp (tri and tetrapeptide uptake) operon, encodes a third ABC system (Ttp) unable of transporting bialaphos and with minor role in the utilization of tetrapeptides and tripeptides like 3-Ala. Despite the overlapping substrate specificities of these ABC transporters, the corresponding gene operons displayed distinct expression profiles: dpp1 showed high constitutive expression levels under all conditions tested, in contrast to the low expression levels of ttp, whereas opp was maximally expressed upon entry into stationary phase. Nevertheless, complex interactions among the three systems at the transcriptional level were observed: opp was negatively autoregulated via OppA and positively regulated via DppA1, whereas dpp1 seems negatively autoregulated via DppA1. The expression of both opp and dppl was reduced in Ttp mutants. The ABC transport systems characterized in this work are not essential for the establishment of nitrogen-fixing symbiosis with alfalfa.

Carbohydrate starvation causes a metabolically active but nonculturable state in Lactococcus lactis

This study characterized the ability of lactococci to become nonculturable under carbohydrate starvation while maintaining metabolic activity. We determined the changes in physiological parameters and extracellular substrate levels of multiple lactococcal strains under a number of environmental conditions along with whole-genome expression profiles. Three distinct phases were observed, logarithmic growth, sugar exhaustion, and nonculturability. Shortly after carbohydrate starvation, each lactococcal strain lost the ability to form colonies on solid media but maintained an intact cell membrane and metabolic activity for over 3.5 years. ML3, a strain that metabolized lactose rapidly, reached nonculturability within 1 week. Strains that metabolized lactose slowly (SK11) or not at all (IL1403) required 1 to 3 months to become nonculturable. In all cases, the cells contained at least 100 pM of intracellular ATP after 6 months of starvation and remained at that level for the remainder of the study. Aminopeptidase and lipase/esterase activities decreased below detection limits during the nonculturable phase. During sugar exhaustion and entry into nonculturability, serine and methionine were produced, while glutamine and arginine were depleted from the medium. The cells retained the ability to transport amino acids via proton motive force and peptides via ATP-driven translocation. The addition of branched-chain amino acids to the culture medium resulted in increased intracellular ATP levels and new metabolic products, indicating that branched-chain amino acid catabolism resulted in energy and metabolic products to support survival during starvation. Gene expression analysis showed that the genes responsible for sugar metabolism were repressed as the cells entered nonculturability. The genes responsible for cell division were repressed, while autolysis and cell wall metabolism genes were induced neither at starvation nor during nonculturability. Taken together, these observations verify that carbohydrate-starved lactococci attain a nonculturable state wherein sugar metabolism, cell division, and autolysis are repressed, allowing the cells to maintain transcription, metabolic activity, and energy production during a state that produces new metabolites not associated with logarithmic growth.

Proteolytic systems of lactic acid bacteria

Lactic acid bacteria (LAB) have a very long history of use in the manufacturing processes of fermented foods and a great deal of effort was made to investigate and manipulate the role of LAB in these processes. Today, the diverse group of LAB includes species that are among the best-studied microorganisms and proteolysis is one of the particular physiological traits of LAB of which detailed knowledge was obtained. The proteolytic system involved in casein utilization provides cells with essential amino acids during growth in milk and is also of industrial importance due to its contribution to the development of the organoleptic properties of fermented milk products. For the most extensively studied LAB, Lactococcus lactis, a model for casein proteolysis, transport, peptidolysis, and regulation thereof is now established. In addition to nutrient processing, cellular proteolysis plays a critical role in polypeptide quality control and in many regulatory circuits by keeping basal levels of regulatory proteins low and removing them when they are no longer needed. As part of the industrial processes, LAB are challenged by various stress conditions that are likely to affect metabolic activities, including proteolysis. While environmental stress responses of LAB have received increasing interest in recent years, our current knowledge on stress-related proteolysis in LAB is almost exclusively based on studies on L. lactis. This review provides the current status in the research of proteolytic systems of LAB with industrial relevance.

Identification and functional characterization of the Lactococcus lactis CodY-regulated branched-chain amino acid permease BcaP (CtrA)

Transcriptome analyses have previously revealed that a gene encoding the putative amino acid transporter CtrA (YhdG) is one of the major targets of the pleiotropic regulator CodY in Lactococcus lactis and Bacillus subtilis. The role of ctrA in L. lactis was further investigated with respect to both transport activity as well as CodY-mediated regulation. CtrA is required for optimal growth in media containing free amino acids as the only amino acid source. Amino acid transport studies showed that ctrA encodes a secondary amino acid transport system that is specific for branched-chain amino acids (BCAAs) (isoleucine, leucine, and valine) and methionine, which is in disagreement with its previously proposed function (a cationic amino acid transporter), which was assigned based on homology. We propose to rename CtrA BcaP, for branched-chain amino acid permease. BcaP is a member of a group of conserved transport systems, as homologs are widely distributed among gram-positive bacteria. Deletion of bcaP resulted in the loss of most of the BCAA uptake activity of L. lactis, indicating that BcaP is the major BCAA carrier of this organism. Deletion of bcaP together with a second (putative) BCAA permease, encoded by brnQ, further reduced the viability of the strain. DNA microarray analysis showed that deletion of bcaP predominantly affects genes belonging to the regulons of the transcriptional regulator CodY, which is involved in global nitrogen metabolism and needs BCAAs for its activation, and of CmbR, which is involved in sulfur amino acid metabolism.

The Lactococcus lactis CodY regulon: identification of a conserved cis-regulatory element

CodY of Lactococcus lactis MG1363 is a transcriptional regulator that represses the expression of several genes encoding proteins of the proteolytic system. DNA microarray analysis, comparing the expression profiles of L. lactis MG1363 and an isogenic strain in which codY was mutated, was used to determine the CodY regulon. In peptide-rich medium and exponentially growing cells, where CodY exerts strong repressing activity, the expression of over 30 genes was significantly increased upon removal of codY. The differentially expressed genes included those predominantly involved in amino acid transport and metabolism. In addition, several genes belonging to other functional categories were derepressed, stressing the pleiotropic role of CodY. Scrutinizing the transcriptome data with bioinformatics tools revealed the presence of a novel over-represented motif in the upstream regions of several of the genes derepressed in L. lactis MG1363DeltacodY. Evidence is presented that this 15-bp cis-sequence, AATTTTCWGAAAATT, serves as a high affinity binding site for CodY, as shown by electrophoretic mobility shift assays and DNase I footprinting analyses. The presence of this CodY-box is sufficient to evoke CodY-mediated regulation in vivo. A copy of this motif is also present in the upstream region of codY itself. It is shown that CodY regulates its own synthesis and requires the CodY-box and branched-chain amino acids to interact with its promoter.

Specificity and selectivity determinants of peptide transport in Lactococcus lactis and other microorganisms

Peptide transport in microorganisms is important for nutrition of the cell and various signalling processes including regulation of gene expression, sporulation, chemotaxis, competence and virulence development. Peptide transport is mediated via different combinations of ion-linked and ATP-binding cassette (ABC) transporters, the latter utilizing single or multiple peptide-binding proteins with overlapping specificities. The paradigm for research on peptide transport is Lactococcus lactis, in which the uptake of peptides containing essential amino acids is vital for growth on milk proteins. Differential expression and characteristics of peptide-binding proteins in several Lactococcus lactis strains resulted in apparent conflicts with older literature. Recent developments and new data now make the pieces of the puzzle fall back into place again and confirm the view that the oligopeptide-binding proteins determine the uptake selectivity of their cognate ABC transporters. Besides reviewing the current data on binding specificity and transport selectivity of peptide transporters in L. lactis, the possible implications for peptide utilization by other bacterial species are discussed.

Specificity of the second binding protein of the peptide ABC-transporter (Dpp) ofLactococcus lactisIL1403

The genome sequence of Lactococcus lactis IL1403 revealed the presence of a putative peptide-binding protein-dependent ABC-transporter (Dpp). The genes for two peptide-binding proteins (dppA and dppP) precede the membrane components, which include two transmembrane protein genes (dppB and dppC) and two ATP-binding protein genes (dppD and dppF). In this work, the gene specifying the second peptide-binding protein (DppP) was cloned under the control of the nisin promoter. The protein fused to a carboxyl-terminal histidine tag (DppP-His(6)) was purified and its binding properties were determined by monitoring the intrinsic fluorescence changes observed upon ligand binding. The major features of peptide binding to DppP-His(6) include: (i) a requirement for a free N-terminal alpha-amino group in the ligand; (ii) a high affinity for di-, tri- and tetra-peptides; (iii) affinity constants for peptide binding independent of pH; and (iv) a high affinity for D-isomer-containing peptides. Remarkably, the features (ii), (iii) and (iv) differ from those previously reported for DppA-His(6), suggesting that DppP-His(6) is a more versatile peptide-binding protein that could have additional functions.

Charged casein-derived oligopeptides competitively inhibit the transport of a reporter oligopeptide by Lactococcus lactis

AIM

To study the effect of casein-derived peptides, accumulated during growth of Lactococcus lactis in milk, on its oligopeptide transport (Opp) function.

METHODS AND RESULTS

This effect was estimated by analysing the ability of casein-derived peptides to compete for the transport of a reporter peptide by whole L. lactis cells. The transport of the reported peptide was monitored by determining the intracellular concentrations of the corresponding amino acids by means of reverse-phase high-performance liquid chromatography (HPLC). Uptake of the reporter peptide was competitively inhibited by casein-derived peptides. The competition was only because of charged casein-derived peptides, including anionic peptides. The design of specific pure peptides made it possible to evidence for a positive (or negative) influence exerted by the positively (or negatively) charged side chain of the N-terminal amino acid on the competition.

CONCLUSIONS

Charged casein-derived peptides impaired the oligopeptide transport function of L. lactis.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results demonstrate an inhibition of Opp when too many peptides are produced by the proteinase. Peptide transport by Opp therefore represents a bottleneck for increasing the growth rate of L. lactis in milk.

Identification and characterization of an oligopeptide transport system in Leuconostoc mesenteroides subsp. mesenteroides CNRZ 1463

AIMS

To identify and characterize an oligopeptide transport system in Leuconostoc mesenteroides CNRZ 1473.

METHODS AND RESULTS

The uptake of a model substrate was monitored by determining intracellular concentrations of the corresponding amino acids by means of reversed-phase HPLC analysis. The oligopeptide transport system is specific for peptides containing at least four amino acid residues and operative under physiological conditions of growth. It is expressed maximally in the presence of oligopeptides, enhanced in the presence of Mg2+ or Ca2+ ions, and driven by ATP or a related energy-rich phosphorylated intermediate.

CONCLUSIONS

The study showed evidence for and characterized the oligopeptide transport system of Leuc. mesenteroides for the first time.

SIGNIFICANCE AND IMPACT OF THE STUDY

The potential of the findings is discussed with reference to the growth of Leuc. mesenteroides in mixed-strain cultures for the dairy industry.

Three oligopeptide-binding proteins are involved in the oligopeptide transport of Streptococcus thermophilus

The functions necessary for bacterial growth strongly depend on the features of the bacteria and the components of the growth media. Our objective was to identify the functions essential to the optimum growth of Streptococcus thermophilus in milk. Using random insertional mutagenesis on a S. thermophilus strain chosen for its ability to grow rapidly in milk, we obtained several mutants incapable of rapid growth in milk. We isolated and characterized one of these mutants in which an amiA1 gene encoding an oligopeptide-binding protein (OBP) was interrupted. This gene was a part of an operon containing all the components of an ATP binding cassette transporter. Three highly homologous amiA genes encoding OBPs work with the same components of the ATP transport system. Their simultaneous inactivation led to a drastic diminution in the growth rate in milk and the absence of growth in chemically defined medium containing peptides as the nitrogen source. We constructed single and multiple negative mutants for AmiAs and cell wall proteinase (PrtS), the only proteinase capable of hydrolyzing casein oligopeptides outside the cell. Growth experiments in chemically defined medium containing peptides indicated that AmiA1, AmiA2, and AmiA3 exhibited overlapping substrate specificities, and that the whole system allows the transport of peptides containing from 3 to 23 residues.

Transcriptional pattern of genes coding for the proteolytic system of Lactococcus lactis and evidence for coordinated regulation of key enzymes by peptide supply

The transcription of 16 genes encoding 12 peptidases (pepC, pepN, pepX, pepP, pepA, pepF2, pepDA1, pepDA2, pepQ, pepT, pepM, and pepO1), P(I) and P(III) proteinases (prtP1 and prtP3), and three transport systems (dtpT, dtpP, and opp-pepO1) of Lactococcus lactis MG1363 was analyzed in response to different environmental factors. Promoter fusions with luciferase reporter genes and/or mRNA analysis were used to study the effects of sugar sources, growth at 37 degrees C, and peptide supply on the transcription of these genes. Only transcription of the pepP gene is modulated by the source of sugar. The presence of potential catabolite-responsive element (CRE) boxes in its promoter region suggests that expression of this gene is directly controlled by catabolic repression. Elevated temperature had no significant effect on the level of transcription of these genes. prtP1, prtP3, pepC, pepN, pepX, and the opp-pepO1 operon are the most highly expressed genes in chemically defined medium, and their expression is repressed 5- to 150-fold by addition of peptide sources such as Casitone in the medium. Moreover, the transcription of prtP1, prtP3, pepC, pepN, and the opp-pepO1 operon is repressed two- to eight-fold by the dipeptides leucylproline and prolylleucine. The transcription of pepDA2 might also be repressed by the peptide sources, but this effect is not observed on the regulation of dtpT, pepP, pepA, pepF2, pepDA1, pepQ, pepT, pepM, and the dtpP operon. The significance of these results with respect to the functions of different components of the proteolytic system in L. lactis are discussed.

Pleiotropic transcriptional repressor CodY senses the intracellular pool of branched-chain amino acids in Lactococcus lactis

Proteolysis is essential for supplying Lactococcus lactis with amino acids during growth in milk. Expression of the major components of the L. lactis proteolytic system, including the cell wall proteinase (PrtP), the oligopeptide transport system (Opp) and at least four intracellular peptidases (PepO1, PepN, PepC, PepDA2), was shown previously to be controlled negatively by a rich nitrogen source. The transcription of prtP, opp-pepO1, pepN and pepC genes is regulated by dipeptides in the medium. Random insertion mutants derepressed for nitrogen control in the expression of the oligopeptide transport system were isolated using an opp-lacZ fusion. A third of the mutants were targeted in the same locus. The product of the inactivated gene shared 48% identity with CodY from Bacillus subtilis, a pleiotropic repressor of the dipeptide permease operon (dpp) and several genes including genes involved in amino acid degradation and competence induction. The signal controlling CodY-dependent repression was searched for by analysing the response of the opp-lux fusion to the addition of 67 dipeptides with different amino acid compositions. Full correlation was found between the dipeptide content in branched-chain amino acids (BCAA; isoleucine, leucine or valine) and their ability to mediate the repression of opp-pepO1 expression. The repressive effect resulting from specific regulatory dipeptides was abolished in L. lactis mutants affected in terms of their transport or degradation into amino acids, showing that the signal was dependent on the BCAA pool in the cell. Lastly, the repression of opp-pepO1 expression was stronger in a mutant unable to degrade BCAAs, underlining the central role of BCAAs as a signal for CodY activity. This pattern of regulation suggests that, in L. lactis and possibly other Gram-positive bacteria, CodY is a pleiotropic repressor sensing nutritional supply as a function of the BCAA pool in the cell.

Lactococcus lactis LM0230 contains a single aminotransferase involved in aspartate biosynthesis, which is essential for growth in milk

Amino acid aminotransferases (ATases), which catalyse the last biosynthetic step of many amino acids, may have important physiological functions in Lactococcus lactis during growth in milk. In this study, the aspartate ATase gene (aspC) from L. lactis LM0230 was cloned by complementation into Escherichia coli DL39. One chromosomal fragment putatively encoding aspC was partially sequenced. A 1179 bp ORF was identified which could encode for a 393 aa, 43.2 kDa protein. The deduced amino acid sequence had high identity to other AspC sequences in GenBank and is a member of the Igamma family of ATases. Substrate-specificity studies suggested that the lactococcal AspC has ATase activity only with aspartic acid (Asp). An internal deletion was introduced into the L. lactis chromosomal copy of aspC by homologous recombination. The wild-type and mutant strain grew similarly in defined media containing all 20 amino acids and did not grow in minimal media unless supplemented with asparagine (Asn). The mutant strain was also unable to grow in or significantly acidify milk unless supplemented with Asp or Asn. These results suggest that only one lactococcal ATase is involved in the conversion of oxaloacetate to Asp, and Asp biosynthesis is required for the growth of L. lactis LM0230 in milk.

Peptide utilization by Lactococcus lactis and Leuconostoc mesenteroides

To explain the competition for nitrogenous nutrients observed in mixed strain cultures of Lactococcus lactis and Leuconostoc mesenteroides, the utilization of peptides as a source of essential amino acids for growth in a chemically defined medium was compared in 12 strains of dairy origin. Both species were multiple amino acid auxotrophs and harboured a large set of intracellular peptidases. Lactococcus lactis can use a wide variety of peptides up to 13 amino acid residues whereas Leuc. mesenteroides assimilated only shorter peptides containing up to seven amino acids. Growth was limited by the transport of peptides and not by their hydrolysis. The nutritional value of peptides varied with the strains and the composition of the peptides, L. lactis being advantaged over Leuc. mesenteroides.

The autoproteolysis of Lactococcus lactis lactocepin III affects its specificity towards beta-casein

The effect of autoproteolysis of Lactococcus lactis lactocepin III on its specificity towards beta-casein was investigated. beta-Casein degradation was performed by using either an autolysin-defective derivative of L. lactis MG1363 carrying the proteinase genes of L. lactis SK11, which was unable to transport oligopeptides, or autoproteolyzed enzyme purified from L. lactis SK11. Comparison of the peptide pools by high-performance liquid chromatography analysis revealed significant differences. To analyze these differences in more detail, the peptides released by the cell-anchored proteinase were identified by on-line coupling of liquid chromatography to mass spectrometry. More than 100 oligopeptides were released from beta-casein by the cell-anchored proteinase. Analysis of the cleavage sites indicated that the specificity of peptide bond cleavage by the cell-anchored proteinase differed significantly from that of the autoproteolyzed enzyme.

Kinetics and substrate specificity of membrane-reconstituted peptide transporter DtpT of Lactococcus lactis

The peptide transport protein DtpT of Lactococcus lactis was purified and reconstituted into detergent-destabilized liposomes. The kinetics and substrate specificity of the transporter in the proteoliposomal system were determined, using Pro-[(14)C]Ala as a reporter peptide in the presence of various peptides or peptide mimetics. The DtpT protein appears to be specific for di- and tripeptides, with the highest affinities for peptides with at least one hydrophobic residue. The effect of the hydrophobicity, size, or charge of the amino acid was different for the amino- and carboxyl-terminal positions of dipeptides. Free amino acids, omega-amino fatty acid compounds, or peptides with more than three amino acid residues do not interact with DtpT. For high-affinity interaction with DtpT, the peptides need to have free amino and carboxyl termini, amino acids in the L configuration, and trans-peptide bonds. Comparison of the specificity of DtpT with that of the eukaryotic homologues PepT(1) and PepT(2) shows that the bacterial transporter is more restrictive in its substrate recognition.

Kinetics and structural requirements for the binding protein of the Di-tripeptide transport system of Lactococcus lactis

The gene (dppA) encoding the binding protein of the di-tripeptide ABC transporter of Lactococcus lactis (DppA) was cloned under the control of the nisin promoter. Amplified expression ( approximately 200-fold increase) of the protein fused to a carboxyl-terminal six-histidine tag allowed the purification of DppA-(His)(6) by nickel-chelate affinity and anion-exchange chromatography. Ligand binding to DppA-(His)(6) elicited an electrophoretic mobility shift, a decrease in the intrinsic fluorescence, and a blue shift of the emission maximum. Each of these parameters detected conformational changes in the protein that reflect ligand binding, and these were used to determine the structural requirements of DppA-(His)(6) for binding peptides. The major features of peptide binding include (i) high affinity for di- and tripeptides, (ii) requirement of a free N-terminal alpha-amino group and an alpha-peptide bound contiguous with the N-terminal amino group, (iii) stereospecificity for L-isomers, and (iv) preference for dipeptides containing methionine or arginine, followed by hydrophobic tripeptides consisting of leucine or valine residues. Maximal binding affinity was detected at pH 6.0, and the K(d) for binding increased 1 order of magnitude for every unit increase in pH. This suggests that the ionization of protein residues (pK > 6.0) in or in close proximity to the binding site is critical in the binding mechanism.

The Legionella pneumophila iraAB locus is required for iron assimilation, intracellular infection, and virulence

Legionella pneumophila, a facultative intracellular parasite of human alveolar macrophages and protozoa, causes Legionnaires' disease. Using mini-Tn10 mutagenesis, we previously isolated a L. pneumophila mutant that was hypersensitive to iron chelators. This mutant, NU216, and its allelic equivalent, NU216R, were also defective for intracellular infection, particularly in iron-deficient host cells. To determine whether NU216R was attenuated for virulence, we assessed its ability to cause disease in guinea pigs following intratracheal inoculation. NU216R-infected animals yielded 1,000-fold fewer bacteria from their lungs and spleen compared to wild-type-130b-infected animals that had received a 50-fold-lower dose. Moreover, NU216R-infected animals subsequently cleared the bacteria from these sites. While infection with 130b resulted in high fever, weight loss, and ruffled fur, inoculation with NU216R did not elicit any signs of disease. DNA sequence analysis revealed that the transposon insertion in NU216R lies in the first open reading frame of a two-gene operon. This open reading frame (iraA) encodes a 272-amino-acid protein that shows sequence similarity to methyltransferases. The second open reading frame (iraB) encodes a 501-amino-acid protein that is highly similar to di- and tripeptide transporters from both prokaryotes and eukaryotes. Southern hybridization analyses determined that the iraAB locus was largely limited to strains of L. pneumophila, the most pathogenic of the Legionella species. A newly derived mutant containing a targeted disruption of iraB showed reduced ability to grow under iron-depleted extracellular conditions, but it did not have an infectivity defect in the macrophage-like U937 cells. These data suggest that iraA is critical for virulence of L. pneumophila while iraB is involved in a novel method of iron acquisition which may utilize iron-loaded peptides.

Kinetics and specificity of peptide uptake by the oligopeptide transport system of Lactococcus lactis

To obtain amino acids for growth, Lactococcus lactis uses a proteolytic system to degrade exogenous proteins such as caseins. The extracellular cell wall-attached proteinase PrtP and the oligopeptide transport system Opp mediate the first two steps in the utilization of caseins. beta-Casein is degraded by PrtP to fragments of 5-30 amino acid residues, and only a limited number of peptides are selected from this pool for uptake via Opp. To study the specificity of Opp and the kinetics of peptide uptake in L. lactis in detail, we used the following strategy: (i) the Opp system was overexpressed; (ii) a 4-fold peptidase mutant was used that is unable to degrade KYGK; (iii) iodinated KYGK was used as the reporter peptide; (iv) libraries of peptides, in which one amino acid position is systematically varied, were used as competitive peptides; and (v) peptides were synthesized on the basis of the beta-casein degradation products, their inhibition of KYGK uptake was determined, and the uptake of these peptides was followed by high-performance liquid chromatography (HPLC). These studies indicate that (i) the Opp system can transport a broad range of peptides from 4 up to at least 18 residues with very little preference for particular side chains and (ii) the kinetics of peptide uptake differ for different substrates tested. Whereas class I peptides such as KYGK exhibit normal Michaelis-Menten kinetics, the level of uptake of the majority of peptides (class II) increases sigmoidally with concentration. Different models for explaining the apparent cooperative effects that are observed for peptide uptake are discussed.

The group A streptococcal dipeptide permease (Dpp) is involved in the uptake of essential amino acids and affects the expression of cysteine protease

The majority of characterized bacterial dipeptide permeases (Dpp) are membrane-associated complexes of five proteins belonging to the ABC-transporter family. They have been found to be involved in the uptake of essential amino acids, haem production, chemotaxis and sporulation. A 5.8 kb genomic DNA fragment of the serotype M49 group A streptococcal (GAS) strain CS101 was sequenced and found to contain five putative GAS Dpp genes (dppA to dppE). Deduced amino acid sequences exhibited 17-54% similarity to corresponding ABC-transporter sequences. The operon organization of the five genes was confirmed by transcriptional analysis, and a shorter, more abundant, dppA-only transcript was detected similar to that found in the GAS oligopeptide permease (Opp) system. Insertional inactivation was used to create serotype M2 and M49 strains that did not express the dppD and dppEATPase genes or nearly the entire operon. In feeding experiments with di- to hexapeptides, the wild-type strain grew with each peptide tested. The dpp mutants were unable to grow on dipeptides, whereas hexapeptides did not sustain the growth of opp mutants. Expression of the dpp operon was induced approximately fourfold in late exponential growth phase. In addition, a striking increase in the dppA to dppA-E ratio from 5:1 to more than 20:1 occurred during late exponential growth phase in complex medium. Growth in chemically defined medium (CDM) supplemented with various dipeptides specifically induced the expression of dpp and reduced both the dppA to dppA-E and oppA to oppA-F mRNA ratios. Expression of the virulence factor SpeB (major cysteine protease) was reduced eightfold in dpp mutants, whereas dpp expression was decreased about fourfold in a Mga virulence regulator mutant. Taken together, these data indicate a correlation between levels of intracellular essential amino acids and the regulation of virulence factor expression.

Specificity of milk peptide utilization by Lactococcus lactis

To study the substrate specificity of the oligopeptide transport system of Lactococcus lactis for its natural substrates, the growth of L. lactis MG1363 was studied in a chemically defined medium containing milk peptides or a tryptic digest of alpha s2-casein as the source of amino acids. Peptides were separated into acidic, neutral, and basic pools by solid-phase extraction or by cation-exchange liquid chromatogrpaphy. Their ability to sustain growth and the time course of their utilization demonstrated the preferential use of hydrophobic basic peptides with molecular masses ranging between 600 and 1,100 Da by L. lactis MG1363 and the inability to use large, acidic peptides. These peptide utilization preferences reflect the substrate specificity of the oligopeptide transport system of the strain, since no significant cell lysis was inferred. Considering the free amino acid content of milk and these findings on peptide utilization, it was demonstrated that the cessation of growth of L. lactis MG1363 in milk was due to deprivation of leucine and methionine.

Import and metabolism of glutathione by Streptococcus mutans

Glutathione (gamma-GluCysGly, GSH) is not found in most gram-positive bacteria, but some appear to synthesize it and others, including Streptococcus mutans ATCC 33402, import it from their growth medium. Import of oxidized glutathione (GSSG) by S. mutans 33402 in 7H9 medium was shown to require glucose and to occur with an apparent Km of 18+/-5 microM. GSSG, GSH, S-methylglutathione, and homocysteine-glutathione mixed disulfide (hCySSG) were imported at comparable rates (measured by depletion of substrate in the medium), as was the disulfide of gamma-GluCys. In contrast, the disulfide of CysGly was not taken up at a measurable rate, indicating that the gamma-Glu residue is important for efficient transport. During incubation with GSSG, little GSSG was detected in cells but GSH and gamma-GluCys accumulated during the first 30 min and then declined. No significant intracellular accumulation of Cys or sulfide was found. Transient intracellular accumulation of D/L-homocysteine, as well as GSH and gamma-GluCys, was observed during import of hCySSG. Although substantial levels of GSH were found in cells when S. mutans was grown on media containing glutathione, such GSH accumulation had no effect on the growth rate. However, the presence of cellular GSH did protect against growth inhibition by the thiol-oxidizing agent diamide. Import of glutathione by S. mutans ATCC 25175, which like strain 33402 does not synthesize glutathione, occurred at a rate comparable to that of strain 33402, but three species which appear to synthesize glutathione (S. agalactiae ATCC 12927, S. pyogenes ATCC 8668, and Enterococcus faecalis ATCC 29212) imported glutathione at negligible or markedly lower rates.

Utilization of oligopeptides by Listeria monocytogenes Scott A

For effective utilization of peptides, Listeria monocytogenes possesses two different peptide transport systems. The first one is the previously described proton motive force (PMF)-driven di- and tripeptide transport system (A. Verheul, A. Hagting, M.-R. Amezaga, I. R. Booth, F. M. Rombouts, and T. Abee, Appl. Environ. Microbiol, 61:226-233, 1995). The present results reveal that L. monocytogenes possesses an oligopeptide transport system, presumably requiring ATP rather than the PMF as the driving force for translocation. Experiments to determine growth in a defined medium containing peptides of various lengths suggested that the oligopeptide permease transports peptides of up to 8 amino acid residues. Peptidase activities towards several oligopeptides were demonstrated in cell extract from L. monocytogenes, which indicates that upon internalization, the oligopeptides are hydrolyzed to serve as sources of amino acids for growth. The peptide transporters of the nonproteolytic L. monocytogenes might play an important role in foods that harbor indigenous proteinases and/or proteolytic microorganisms, since Pseudomonas fragi as well as Bacillus cereus was found to enhance the growth of L. monocytogenes to a large extent in a medium in which the milk protein casein was the sole source of nitrogen. In addition, growth stimulation was elicited in this medium when casein was hydrolyzed by using purified protease from Bacillus licheniformis. The possible contribution of the oligopeptide transport system in the establishment of high numbers of L. monocytogenes cells in fermented milk products is discussed.

Pathway for uptake and degradation of X-prolyl tripeptides in Streptococcus mutans VA-29R and Streptococcus sanguis ATCC 10556

The growth of Streptococcus mutans and Streptococcus sanguis in the oral environment requires that these micro-organisms be able to degrade salivary proteins and to assimilate the resulting peptides as an amino nitrogen source. Our research is aimed at the definition of the proteolytic enzyme systems in these oral streptococci which allow them to utilize such substrates. In the present work, the nature of the hydrolytic activity expressed by S. mutans VA-29R and S. sanguis ATCC 10556 against X-Pro4-nitroanilide and X-Pro-Y tripeptide substrates was investigated. This activity was predominantly associated with a cytoplasmic dipeptidyl peptidase which preferentially catalyzes the release of an N-terminal dipeptide from substrates in which proline is the penultimate residue. These streptococci also possess a second cytoplasmic peptidase, pepD, which catalyzes the hydrolysis of X-Pro dipeptides. We found that Gly-Pro-Ala or Ala-Pro-Gly were transported into the bacterial cells only when an energy source such as glucose was present. Peptide uptake was time-dependent, and selective exodus of peptide-derived amino acids from the bacterial cells occurred during peptide uptake. Results from these studies provide evidence that S. mutans VA-29R and S. sanguis ATCC 10556 possess a pathway for the complete degradation of X-Pro tripeptides. Transport of the peptides into cells prior to hydrolysis provides an efficient way by which all amino acids of a peptide may be obtained at an energy expense equivalent to that associated with the transport of just one amino acid. In light of the abundance of proline in salivary polypeptides, this degradative pathway could be an important component in the proteolytic pathway for salivary polypeptide utilization in these oral streptococci.

Amplified expression, purification and functional reconstitution of the dipeptide and tripeptide transport protein of Lactococcus lactis

Transport of hydrophilic dipeptides and tripeptides into Lactococcus lactis is mediated by a proton-motive-force-driven peptide-transport protein (DtpT) that shares similarity to eukaryotic peptide transporters, e.g. from yeasts, plants, and the kidney and small intestine of rabbit, man and rat. The expression level of DtpT protein in L. lactis was increased (20-40-fold) to approximately 10% of total integral membrane protein by means of a low-copy-number vector and selecting the appropriate growth conditions. Membrane vesicles bearing the DtpT-His6 protein (containing a C-terminal factor-Xa cleavage site and a six-histidine-tag) showed a Pro-Ala uptake activity that was half that of membranes containing the wild-type protein. The activity in the DtpT-His6 membrane vesicles increased at least 50% upon removal of the His6 tag from the protein. More than 95% DtpT was solubilized from L. lactis membranes in the presence of 1% (mass/vol.) n-dodecyl-beta-D-maltoside, and approximately 2 mg DtpT-His6 was purified by Ni2+-chelate affinity chromatography from 100 mg membrane protein. Purified DtpT-His6 was reconstituted unidirectionally into detergent-saturated formed liposomes, which were prepared from Escherichia coli phospholipid and egg phosphatidylcholine; the detergent was removed by adsorption to polystyrene beads. The highest uptake activities were obtained when DtpT was incorporated into liposomes that were treated with a low amount of n-dodecyl-beta-D-maltoside (onset of liposome solubilization). The uptake activity could be improved by addition of NaCl (200 mM) and lipids (2 mg/ml) during the solubilization, purification and reconstitution steps.

Membrane topology of the di- and tripeptide transport protein of Lactococcus lactis

Transport of hydrophilic di- and tripeptides into Lactococcus lactis is mediated by a proton motive force-driven peptide transport protein (DtpT) that shares similarity with eukaryotic peptide transporters, e.g., from kidney and small intestine of rabbit, man, and rat. Hydropathy profiling in combination with the "positive inside rule" predicts for most of the homologous proteins an alpha-helical bundle of 12 transmembrane segments, but the positions of these transmembrane segments and the location of the amino and carboxyl termini are by no means conclusive. The secondary structure of DtpT was investigated by analyzing 42 DtpT-alkaline phosphatase fusion proteins, generated by random or directed fusions of the corresponding genes. These studies confirm the presence of 12 transmembrane segments but refute several other predictions made of the secondary structure. Data obtained from the fusion proteins were substantiated by studying the accessibility of single cysteine mutants in putative cytoplasmic or extracellular loops by membrane (im)permeant sulfhydryl reagents. The deduced topology model of DtpT consists of a bundle of 12 alpha-helixes with a short amino and a large carboxyl terminus, both located at the cytoplasmic site of the membrane. On the basis of sequence comparisons with DtpT, it seems likely that the structure model of the amino-terminal half of DtpT also holds for the eukaryotic peptide transporters, whereas the carboxyl-terminal half is largely different.

Interaction between proteolytic strains of Lactococcus lactis influenced by different types of proteinase during growth in milk

The influence of the type of cell envelope-located proteinase (PI versus PIII) on the associative growth of Lactococcus lactis in milk was studied. Two genetically engineered strains, differing only by the type of proteinase, were first used as a model study. An interaction occurred during the second exponential growth phase of the mixed culture and resulted in a decrease in growth rate of the PI-type proteinase strain, whereas that of the PIII-type proteinase strain remained unaffected. The reduction in proteolytic activity of the PI-type proteinase strain (presumably resulting from an inhibition of the synthesis of the enzyme) due to the peptides released by the PIII-type proteinase was found to be partly responsible for this interaction. Extension of the study to wild-type proteinase-positive L. lactis strains showed a systematic imbalance of the mixture of the two strains in favor of the PIII-type proteinase strain.

Cloning and functional expression in Escherichia coli of the gene encoding the di- and tripeptide transport protein of Lactobacillus helveticus

The gene encoding the di- and tripeptide transport protein (DtpT) of Lactobacillus helveticus (DtpTLH) was cloned with the aid of the inverse PCR technique and used to complement the dipeptide transport-deficient and proline-auxotrophic Escherichia coli E1772. Functional expression of the peptide transporter was shown by the uptake of prolyl-[14C] alanine in whole cells and membrane vesicles. Peptide transport via DtpT in membrane vesicles is driven by the proton motive force. The system has specificity for di- and tripeptides but not for amino acids or tetrapeptides. The dtpTLH gene consists of 1,491 bp, which translates into a 497-amino-acid polypeptide. DtpTLH shows 34% identity to the di- and tripeptide transport protein of Lactococcus lactis and is also homologous to various peptide transporters of eukaryotic origin, but the similarity between these proteins is confined mainly to the N-terminal halves.

Bacterial metabolism in dental biofilms

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

The proteolytic systems of lactic acid bacteria

Proteolysis in dairy lactic acid bacteria has been studied in great detail by genetic, biochemical and ultrastructural methods. From these studies the picture emerges that the proteolytic systems of lactococci and lactobacilli are remarkably similar in their components and mode of action. The proteolytic system consists of an extracellularly located serine-proteinase, transport systems specific for di-tripeptides and oligopeptides (> 3 residues), and a multitude of intracellular peptidases. This review describes the properties and regulation of individual components as well as studies that have led to identification of their cellular localization. Targeted mutational techniques developed in recent years have made it possible to investigate the role of individual and combinations of enzymes in vivo. Based on these results as well as in vitro studies of the enzymes and transporters, a model for the proteolytic pathway is proposed. The main features are: (i) proteinases have a broad specificity and are capable of releasing a large number of different oligopeptides, of which a large fraction falls in the range of 4 to 8 amino acid residues; (ii) oligopeptide transport is the main route for nitrogen entry into the cell; (iii) all peptidases are located intracellularly and concerted action of peptidases is required for complete degradation of accumulated peptides.

Multiple-peptidase mutants of Lactococcus lactis are severely impaired in their ability to grow in milk

To examine the contribution of peptidases to the growth of lactococcus lactis in milk, 16 single- and multiple-deletion mutants were constructed. In successive rounds of chromosomal gene replacement mutagenesis, up to all five of the following peptidase genes were inactivated (fivefold mutant): pepX, pepO, pepT, pepC, and pepN. Multiple mutations led to slower growth rates in milk, the general trend being that growth rates decreased when more peptidases were inactivated. The fivefold mutant grew more than 10 times more slowly in milk than the wild-type strain. In one of the fourfold mutants and in the fivefold mutant, the intracellular pools of amino acids were lower than those of the wild type, whereas peptides had accumulated inside the cell. No significant differences in the activities of the cell envelope-associated proteinase and of the oligopeptide transport system were observed. Also, the expression of the peptidases still present in the various mutants was not detectably affected. Thus, the lower growth rates can directly be attributed to the inability of the mutants to degrade casein-derived peptides. These results supply the first direct evidence for the functioning of lactococcal peptidases in the degradation of milk proteins. Furthermore, the study provides critical information about the relative importance of the peptidases for growth in milk, the order of events in the proteolytic pathway, and the regulation of its individual components.