Purification, characterization, gene cloning, sequencing, and overexpression of aminopeptidase N from Streptococcus thermophilus A

Parasite Metalo-aminopeptidases as Targets in Human Infectious Diseases

BACKGROUND

Parasitic human infectious diseases are a worldwide health problem due to the increased resistance to conventional drugs. For this reason, the identification of novel molecular targets and the discovery of new chemotherapeutic agents are urgently required. Metalo- aminopeptidases are promising targets in parasitic infections. They participate in crucial processes for parasite growth and pathogenesis.

OBJECTIVE

In this review, we describe the structural, functional and kinetic properties, and inhibitors, of several parasite metalo-aminopeptidases, for their use as targets in parasitic diseases.

CONCLUSION

Plasmodium falciparum M1 and M17 aminopeptidases are essential enzymes for parasite development, and M18 aminopeptidase could be involved in hemoglobin digestion and erythrocyte invasion and egression. Trypanosoma cruzi, T. brucei and Leishmania major acidic M17 aminopeptidases can play a nutritional role. T. brucei basic M17 aminopeptidase down-regulation delays the cytokinesis. The inhibition of Leishmania basic M17 aminopeptidase could affect parasite viability. L. donovani methionyl aminopeptidase inhibition prevents apoptosis but not the parasite death. Decrease in Acanthamoeba castellanii M17 aminopeptidase activity produces cell wall structural modifications and encystation inhibition. Inhibition of Babesia bovis growth is probably related to the inhibition of the parasite M17 aminopeptidase, probably involved in host hemoglobin degradation. Schistosoma mansoni M17 aminopeptidases inhibition may affect parasite development, since they could participate in hemoglobin degradation, surface membrane remodeling and eggs hatching. Toxoplasma gondii M17 aminopeptidase inhibition could attenuate parasite virulence, since it is apparently involved in the hydrolysis of cathepsin Cs- or proteasome-produced dipeptides and/or cell attachment/invasion processes. These data are relevant to validate these enzymes as targets.

Streptococcus pneumoniae aminopeptidase N contributes to bacterial virulence and elicits a strong innate immune response through MAPK and PI3K/AKT signaling

Streptococcus pneumoniae is a Gram-positive pathogen with high morbidity and mortality globally but some of its pathogenesis remains unknown. Previous research has provided evidence that aminopeptidase N (PepN) is most likely a virulence factor of S. pneumoniae. However, its role in S. pneumoniae virulence and its interaction with the host remains to be confirmed. We generated a pepN gene deficient mutant strain and found that its virulence for mice was significantly attenuated as were in vitro adhesion and invasion of host cells. The PepN protein could induce a strong innate immune response in vivo and in vitro and induced secretion of IL-6 and TNF-α by primary peritoneal macrophages via the rapid phosphorylation of MAPK and PI3K/AKT signaling pathways and this was confirmed using specific pathway inhibitors. In conclusion, PepN is a novel virulence factor that is essential for the virulence of S. pneumoniae and induces host innate immunity via MAPK and PI3K/AKT signaling.

PepN is a non-essential, cell wall-localized protein that contributes to neutrophil elastase-mediated killing of Streptococcus pneumoniae

Streptococcus pneumoniae (Spn) is an asymptomatic colonizer of the human nasopharynx but can also cause disease in the inner ear, meninges, lung and blood. Although various mechanisms contribute to the effective clearance of Spn, opsonophagocytosis by neutrophils is perhaps most critical. Upon phagocytosis, Spn is exposed to various degradative molecules, including a family of neutrophil serine proteases (NSPs) that are stored within intracellular granules. Despite the critical importance of NSPs in killing Spn, the bacterial proteins that are degraded by NSPs leading to Spn death are still unknown. In this report, we identify a 90kDa protein in a purified cell wall (CW) preparation, aminopeptidase N (PepN) that is degraded by the NSP neutrophil elastase (NE). Since PepN lacked a canonical signal sequence or LPxTG motif, we created a mutant expressing a FLAG tagged version of the protein and confirmed its localization to the CW compartment. We determined that not only is PepN a CW-localized protein, but also is a substrate of NE in the context of intact Spn cells. Furthermore, in comparison to wild-type TIGR4 Spn, a mutant strain lacking PepN demonstrated a significant hyper-resistance phenotype in vitro in the presence of purified NE as well as in opsonophagocytic assays with purified human neutrophils ex vivo. Taken together, this is the first study to demonstrate that PepN is a CW-localized protein and a substrate of NE that contributes to the effective killing of Spn by NSPs and human neutrophils.

Molecular advances in microbial aminopeptidases

Aminopeptidases are exopeptidases that catalyze the hydrolysis of amino acid residues from the N terminus of peptides and proteins. They are widely and diversely used for protein hydrolysis in industrial and research applications. They form a large enzyme family in microorganisms and most of the sequenced microbial genomes contain several aminopeptidase coding genes. Various approaches are being used to enhance the yield and desired properties of these enzymes to make it more suited for industrial applications. Novel aminopeptidases are being developed by site directed mutagenesis and recombinant DNA technology with improved substrate specificity and stability. This review focuses on its classification and recent advancements in the molecular studies pertaining to this enzyme.

A Novel Function for the Streptococcus pneumoniae Aminopeptidase N: Inhibition of T Cell Effector Function through Regulation of TCR Signaling

Streptococcus pneumoniae (Spn) causes a variety of disease states including fatal bacterial pneumonia. Our previous finding that introduction of Spn into an animal with ongoing influenza virus infection resulted in a CD8⁺ T cell population with reduced effector function gave rise to the possibility of direct regulation by pneumococcal components. Here, we show that treatment of effector T cells with lysate derived from Spn resulted in inhibition of IFNγ and tumor necrosis factor α production as well as of cytolytic granule release. Spn aminopeptidase N (PepN) was identified as the inhibitory bacterial component and surprisingly, this property was independent of the peptidase activity found in this family of proteins. Inhibitory activity was associated with reduced activation of ZAP-70, ERK1/2, c-Jun N-terminal kinase, and p38, demonstrating the ability of PepN to negatively regulate TCR signaling at multiple points in the cascade. These results reveal a novel immune regulatory function for a bacterial aminopeptidase.

Genes involved in protein metabolism of the probiotic lactic acid bacterium Lactobacillus delbrueckii UFV H2b20

A basic requirement for the prediction of the potential use of lactic acid bacteria (LAB) in the dairy industry is the identification of specific genes involved in flavour-forming pathways. The probiotic Lactobacillus delbrueckii UFV H2b20 was submitted to a genetic characterisation and phylogenetic analysis of genes involved in protein catabolism. Eight genes belonging to this system were identified, which possess a closely phylogenetic relationship to NCFM strains representative, as it was demonstrated for oppC and oppBII, encoding oligopeptide transport system components. PepC, PepN, and PepX might be essential for growth of LAB, probiotic or not, since the correspondent genes are always present, including in L. delbrueckii UFV H2b20 genome. For pepX gene, a probable link between carbohydrate catabolism and PepX expression may exists, where it is regulated by PepR1/CcpA-like, a common feature between Lactobacillus strains and also in L. delbrueckii UFV H2b20. The well conserved evolutionary history of the ilvE gene is evidence that the pathways leading to branched-chain amino acid degradation, such as isoleucine and valine, are similar among L. delbrueckii subsp. bulgaricus strains and L. delbrueckii UFV H2b20. Thus, the involvement of succinate in flavour formation can be attributed to IlvE activity. The presence of aminopeptidase G in L. delbrueckii UFV H2b20 genome, which is absent in several strains, might improve the proteolytic activity and effectiveness. The nucleotide sequence encoding PepG revealed that it is a cysteine endopeptidase, belonging to Peptidase C1 superfamily; sequence analysis showed 99% identity with L. delbrueckii subsp. bulgaricus ATCC 11842 pepG, whereas protein sequence analysis revealed 100% similarity with PepG from the same organism. The present study proposes a schematic model to explain how the proteolytic system of the probiotic L. delbrueckii UFV H2b20 works, based on the components identified so far.

Plasmodium falciparum: new molecular targets with potential for antimalarial drug development

Malaria remains one of the world's most devastating infectious diseases. Drug resistance to all classes of antimalarial agents has now been observed, highlighting the need for new agents that act against novel parasite targets. The complete sequencing of the Plasmodium falciparum genome has allowed the identification of new molecular targets within the parasite that may be amenable to chemotherapeutic intervention. In this review, we investigate four possible targets for the future development of new classes of antimalarial agents. These targets include histone deacetylase, the aspartic proteases or plasmepsins, aminopeptidases and the purine salvage enzyme hypoxanthine-xanthine-guanine phosphoribosyltransferase.

Plasmodium falciparum neutral aminopeptidases: new targets for anti-malarials

The neutral aminopeptidases M1 alanyl aminopeptidase (PfM1AAP) and M17 leucine aminopeptidase (PfM17LAP) of the human malaria parasite Plasmodium falciparum are targets for the development of novel anti-malarial drugs. Although the functions of these enzymes remain unknown, they are believed to act in the terminal stages of haemoglobin degradation, generating amino acids essential for parasite growth and development. Inhibitors of both enzymes are lethal to P. falciparum in culture and kill the murine malaria P. chabaudi in vivo. Recent biochemical, structural and functional studies provide the substrate specificity and mechanistic binding data needed to guide the development of more potent anti-malarial drugs. Together with biological studies, these data form the rationale for choosing PfM1AAP and PfM17LAP as targets for anti-malarial development.

Possible promoter regions within the proteolytic system in Streptococcus thermophilus and their interaction with the CodY homolog

Possible promoter regions preceding 14 genes belonging to the proteolytic system of Streptococcus thermophilus KLDS 3.0503 were predicted by a promoter analysis software nnpp. The 14 genes included an extracellular protease gene prtS, an oligopeptide ABC transport system gene amiA1, and 12 genes, respectively, encoding peptidases pepA, pepS, pepN, pepC, pepB, pepQ, pepV, pepT, pepM, pepXP, pepP, and pepO. These predicted promoter sequences were cloned and inserted into the upstream of a promoterless Escherichia coli gusA (beta-glucuronidase) gene in a promoter probe vector pNZ273. The resulting vectors were, respectively, introduced into S. thermophilus KLDS 3.0503 and all 14 predicted promoter sequences were able to drive gusA expression, which indicated that these sequences were functional promoters. These promoters were able to interact with the S. thermophilus CodY homolog in an in vitro DNA binding assay but they did not contain a conserved CodY-box sequence identified in Lactococcus lactis. These results were useful for further studies on the regulation of protein metabolism in S. thermophilus.

Structural basis for the unusual specificity of Escherichia coli aminopeptidase N

Aminopeptidase N from Escherichia coli is a M1 class aminopeptidase with the active-site region related to that of thermolysin. The enzyme has unusual specificity, cleaving adjacent to the large, nonpolar amino acids Phe and Tyr but also cleaving next to the polar residues Lys and Arg. To try to understand the structural basis for this pattern of hydrolysis, the structure of the enzyme was determined in complex with the amino acids L-arginine, L-lysine, L-phenylalanine, L-tryptophan, and L-tyrosine. These amino acids all bind with their backbone atoms close to the active-site zinc ion and their side chain occupying the S1 subsite. This subsite is in the form of a cylinder, about 10 A in cross-section and 12 A in length. The bottom of the cylinder includes the zinc ion and a number of polar side chains that make multiple hydrogen-bonding and other interactions with the alpha-amino group and the alpha-carboxylate of the bound amino acid. The walls of the S1 cylinder are hydrophobic and accommodate the nonpolar or largely nonpolar side chains of Phe and Tyr. The top of the cylinder is polar in character and includes bound water molecules. The epsilon-amino group of the bound lysine side chain and the guanidinium group of arginine both make multiple hydrogen bonds to this part of the S1 site. At the same time, the hydrocarbon part of the lysine and arginine side chains is accommodated within the nonpolar walls of the S1 cylinder. This combination of hydrophobic and hydrophilic binding surfaces explains the ability of ePepN to cleave Lys, Arg, Phe, and Tyr. Another favored substrate has Ala at the P1 position. The short, nonpolar side chain of this residue can clearly be bound within the hydrophobic part of the S1 cylinder, but the reason for its facile hydrolysis remains uncertain.

The intracellular proteolytic system of Yarrowia lipolytica and characterization of an aminopeptidase

Intracellular proteases of Yarrowia lipolytica have been scarcely studied. These enzymes may play an important role in nitrogen metabolism, posttranslational processing, nutritional stress, dimorphism, etc.; biochemical and genetic control of these enzymes can help in obtaining high-level expression of recombinant proteins in heterologous systems. In this study, we report the presence of three proteases: aminopeptidase yylAPE, carboxypeptidase yylCP and dipeptidyl aminopeptidase yylDAP, measured under several nutritional conditions. Yarrowia lipolytica produced the highest level of intracellular proteolytic enzymes, i.e. yylAPE, yylCP and yylDAP, in media with peptone during stationary growth phase. When soluble extracts were subjected to PAGE, and the three activities were revealed in gels with the corresponding substrates, only one band of activity was detected for each one. The three enzymes were affected by serine protease inhibitors. Chelating agents affected mainly APE activity. The aminopeptidase was purified by selective fractionation with ammonium sulfate and three chromatographic steps (anion exchange, hydrophobic interaction and gel filtration chromatography). The enzyme had a molecular mass of 97 kDa; optimal pH and temperature were 7.0 and 37 degrees C, respectively. The aminopeptidase showed a preference for lysine in the N-position. The K(m) value was 0.86 microM and V(max) value was 990.8 micromoL min(-1) mg(-1) for Lys-pNA.

Biotechnological methods to accelerate cheddar cheese ripening

Cheese is one of the dairy products that can result from the enzymatic coagulation of milk. The basic steps of the transformation of milk into cheese are coagulation, draining, and ripening. Ripening is the complex process required for the development of a cheese's flavor, texture and aroma. Proteolysis, lipolysis and glycolysis are the three main biochemical reactions that are responsible for the basic changes during the maturation period. As ripening is a relatively expensive process for the cheese industry, reducing maturation time without destroying the quality of the ripened cheese has economic and technological benefits. Elevated ripening temperatures, addition of enzymes, addition of cheese slurry, attenuated starters, adjunct cultures, genetically engineered starters and recombinant enzymes and microencapsulation of ripening enzymes are traditional and modern methods used to accelerate cheese ripening. In this context, an up to date review of Cheddar cheese ripening is presented.

Isolation of aminopeptidase N genes of food associated propionibacteria and observation of their transcription in skim milk and acid whey

In this study consensus oligonucleotides PN5/PN3 were designed by aligning the aminopeptidase N genes (pepN) of various actinobacteria and applied to the isolation of the pepN genes of dairy propionibacteria (PAB) and closely related species associated with food. This allowed sequencing of a pepN gene region from Propionibacterium jensenii LMG 16541. The sequence of this gene was completed by inverse PCR. Consensus primer pairs NU1/D1 and NU2/D1 were derived from the alignment of the new sequence with its homologues in Propionibacterium acnes and other actinobacteria; these were used to start sequencing of the pepN genes of Propionibacterium freudenreichii, Propionibacterium thoenii, Propionibacterium microaerophilum, Propionibacterium acidipropionici, Propioni bacterium cyclohexanicum and Propionibacterium microaerophilum. Reverse transcription coupled with PN5/PN3 and NU1/D1 PCR tests indicated that the pepN genes of P. jensenii and P. freudenreichii are expressed during growth in skim milk and acid whey.

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.

Over-expression, purification, and characterization of aminopeptidase N from Escherichia coli

The gene from Escherichia coli encoding aminopeptidase N (PepN) was subcloned into pET-26b, and PepN was over-expressed in BL21(DE3) E. coli and purified using Q-Sepharose chromatography. This protocol yielded over 17 mg of purified, recombinant PepN per liter of growth culture under optimum conditions. Gel filtration chromatography revealed that recombinant PepN exists as a monomer. MALDI-TOF mass spectra showed that the enzyme has a molecular mass of 98,750 Da, and steady-state kinetic studies revealed that as-isolated, recombinant PepN exhibits a k(cat) of 354 +/- 11s(-1) and a K(m) of 376 +/- 39 microM when using L-alanine-p-nitroanilide as the substrate. Metal analyses demonstrated that as-isolated, recombinant PepN binds 0.5 and <0.1 equivalents of iron and zinc, respectively. The addition of Zn(II) to recombinant PepN inhibits catalytic activity, while the addition of iron causes a slight decrease or no change in activity. Further metal binding studies revealed that recombinant PepN tightly binds 5 equivalents of iron and <0.1 equivalents of Zn(II). By using this over-expression and purification system, E. coli PepN can now be obtained in quantities necessary for structural characterization and possibly inhibitor design efforts.

PepN is the major aminopeptidase in Escherichia coli: insights on substrate specificity and role during sodium-salicylate-induced stress

PepN and its homologues are involved in the ATP-independent steps (downstream processing) during cytosolic protein degradation. To obtain insights into the contribution of PepN to the peptidase activity in Escherichia coli, the hydrolysis of a selection of endopeptidase and exopeptidase substrates was studied in extracts of wild-type strains and two pepN mutants, 9218 and DH5alphaDeltapepN. Hydrolysis of three of the seven endopeptidase substrates tested was reduced in both pepN mutants. Similar studies revealed that hydrolysis of 10 of 14 exopeptidase substrates studied was greatly reduced in both pepN mutants. This decreased ability to cleave these substrates is pepN-specific as there is no reduction in the ability to hydrolyse exopeptidase substrates in E. coli mutants lacking other peptidases, pepA, pepB or pepE. PepN overexpression complemented the hydrolysis of the affected exopeptidase substrates. These results suggest that PepN is responsible for the majority of aminopeptidase activity in E. coli. Further in vitro studies with purified PepN revealed a preference to cleave basic and small amino acids as aminopeptidase substrates. Kinetic characterization revealed the aminopeptidase cleavage preference of E. coli PepN to be Arg>Ala>Lys>Gly. Finally, it was shown that PepN is a negative regulator of the sodium-salicylate-induced stress in E. coli, demonstrating a physiological role for this aminoendopeptidase under some stress conditions.

PepN, the major Suc-LLVY-AMC-hydrolyzing enzyme in Escherichia coli, displays functional similarity with downstream processing enzymes in Archaea and eukarya. Implications in cytosolic protein degradation

Succinyl-Leu-Leu-Val-Tyr-7-amido-4-methylcoumarin (Suc-LLVY-AMC), a fluorogenic endopeptidase substrate, is used to detect 20 S proteasomal activity from Archaea to mammals. An o-phenanthroline-sensitive Suc-LLVY-AMC hydrolyzing activity was detected in Escherichia coli although it lacks 20 S proteasomes. We identified PepN, previously characterized as the sole alanine aminopeptidase in E. coli, to be responsible for the hydrolysis of Suc-LLVY-AMC. PepN is an aminoendopeptidase. First, extracts from an ethyl methanesulfonate-derived PepN mutant, 9218, did not cleave Suc-LLVY-AMC and L-Ala-para-nitroanilide (pNA). Second, biochemically purified PepN cleaves a wide variety of both aminopeptidase and endopeptidase substrates, and L-Ala-pNA is cleaved more efficiently than other substrates. Studies with bestatin, an aminopeptidase-specific inhibitor, suggest differences in the mechanisms of cleavage of aminopeptidase and endopeptidase substrates. Third, PepN hydrolyzes whole proteins, casein and albumin. Finally, an E. coli strain with a targeted deletion in PepN also lacks the ability to cleave Suc-LLVY-AMC and L-Ala-pNA, and expression of wild type PepN in this mutant rescues both activities. In addition, we identified a low molecular weight Suc-LLVY-AMC-cleaving peptidase in Mycobacterium smegmatis, a eubacteria harboring 20 S proteasomes, to be an aminopeptidase homologous to E. coli PepN, by mass spectrometry analysis. "Sequence-based homologues" of PepN include well characterized aminopeptidases, e.g. Tricorn interacting factors F2 and F3 in Archaea and puromycin-sensitive aminopeptidase in mammals. However, our results suggest that eubacterial PepN and its homologues displaying aminoendopeptidase activities may be "functionally similar" to enzymes important in downstream processing of proteins in the cytosol: Tricorn-F1-F2-F3 complex in Archaea and TPPII/Multicorn in eukaryotes.

Production, purification and characterization of intracellular alanylaminopeptidase of Pseudomonas sp

The soil bacterium Pseudomonas sp. was found to synthesize an aminopeptidase that prefers Ala-beta-naphtylamide as substrate. The enzyme was purified 660-fold by ammonium sulfate fractionation, preparative electrophoresis, ion exchange chromatography on Protein-Pak Q 8 HR and molecular sieving chromatography on Zorbax SE-250. When purified to homogeneity, the enzyme was shown to be a monomeric protein with a molar mass of 65 kDa; it showed a maximum activity at pH 7.5 and 45 degrees C.

Cloning and sequencing of the gene encoding X-prolyl-dipeptidyl aminopeptidase (PepX) from Streptococcus thermophilus strain ACA-DC 4

AIMS

To clone and sequence the pepX gene from Streptococcus thermophilus.

METHODS AND RESULTS

Three pairs of primers were used in polymerase chain reactions using as template the total DNA from Strep. thermophilus ACA-DC 4 in order to amplify, clone and sequence the pepX gene. Sequence analysis revealed an open reading frame of 2268 nucleotides encoding a protein of 755 amino acids. The calculated molecular mass of 85 632 Da agreed well with the apparent molecular mass of 80 000 Da previously determined by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and gel filtration for the monomeric form of the purified enzyme.

CONCLUSIONS

The pepX gene from Strep. thermophilus ACA-DC 4 was cloned and sequenced. The PepX protein showed significant sequence similarity with PepX enzymes from other lactic acid bacteria and contained a motif which was almost identical with the active site motif of the serine-dependent PepX family.

SIGNIFICANCE AND IMPACT OF THE STUDY

There are economic and technological incentives for accelerating and controlling the process of cheese ripening. To achieve this, starters may be modified by introducing appropriate genes from other food-grade bacteria. New or additional peptidase activities may alter or improve the proteolytic properties of lactic acid bacteria.

Purification, characterization, and genetic analysis of a leucine aminopeptidase from Aspergillus sojae

Extracellular leucine aminopeptidase (LAP) from Aspergillus sojae was purified to protein homogeneity by sequential fast protein liquid chromatography steps. LAP had an apparent molecular mass of 37 kDa, of which approximately 3% was contributed by N-glycosylated carbohydrate. The purified enzyme was most active at pH 9 and 70 degrees C for 30 min. The enzyme preferentially hydrolyzed leucine p-nitroanilide followed by Phe, Lys, and Arg derivatives. The LAP activity was strongly inhibited by metal-chelating agents, and was largely restored by divalent cations like Zn(2+) and Co(2+). The lap gene and its corresponding cDNA fragment of the A. sojae were cloned using degenerated primers derived from internal amino acid sequences of the purified enzyme. lap is interrupted by three introns and is transcribed in a 1.3-kb mRNA that encodes a 377-amino-acid protein with a calculated molecular mass of 41.061 kDa. The mature LAP is preceded by a leader peptide of 77 amino acids, predicted to include an 18-amino-acid signal peptide and an extra sequence of 59 amino acids. Two putative N-glycosylation sites are identified in Asn-87 and Asn-288. Southern blot analysis suggested that lap is a single-copy gene in the A. sojae genome. The deduced amino acid sequence of A. sojae LAP shares only 11-33.1% identity with those of LAPs from 18 organisms.

Regulation of the activity of intracellular alanylaminopeptidase synthesized by Pseudomonas sp

Activity of purified alanylaminopeptidase of Pseudomonas sp. measured in the presence of the alanine derivative of 2-naphthoic acid (NA-Ala) is inhibited by 1,10-phenanthroline, EDTA, bestatin and amastatin; this finding supports the conclusion that this enzyme is a metallo-aminopeptidase. A decrease of its activity in the presence of iodoacetamide and its activation by thiols points to the significant role of -SH groups in the regulation of its activity. Co2+, Ca2+ and Mg2+ ions increased the enzyme activity while Zn2+, Cd2+ and Pb2+ markedly inhibited the enzyme even at low concentrations. A high thermal stability of alanylaminopeptidase depended on the presence of 1 mmol/L Co2+ and of 1 mmol/L L-cysteine in the incubation mixture.

Advances in the genetics of thermophilic lactic acid bacteria

Molecular genetics of thermophilic lactic acid bacteria has advanced in several directions: exploitation of the milk proteins and sugars; primary and secondary metabolism; stress response; and molecular ecology of bacteria and their phages. These have singularly contributed to open new avenues of scientific interest in the field: comparative phage genomics; horizontal gene transfer events in bacterial or phage populations; and genetics of external polysaccharide production.

Purification, characterisation, cloning and sequencing of the gene encoding oligopeptidase PepO from Streptococcus thermophilus A

The oligopeptidase PepO from Streptococcus thermophilus A was purified to protein homogeneity by a five-step chromatography procedure. It was estimated to be a serine metallopeptidase of 70 kDa, with maximal activity at pH 6.5 and 41 degrees C. PepO has endopeptidase activity on oligopeptides composed of between five and 30 amino acids. PepO was demonstrated to be active and stable at the pH, temperature and salt concentrations found in Swiss-type cheese during ripening. Using a battery of PCR techniques, the pepO gene was amplified, subcloned and sequenced, revealing an open reading frame of 1893 nucleotides. The amino acid sequence analysis of the pepO gene-translation product shows homology with PepO enzymes from other lactic acid bacteria and contains the signature sequence of the metallopeptidase family.