Characterization of PepB, a group B streptococcal oligopeptidase

Competence shut-off by intracellular pheromone degradation in salivarius streptococci

Competence for DNA transformation is a major strategy for bacterial adaptation and survival. Yet, this successful tactic is energy-consuming, shifts dramatically the metabolism, and transitory impairs the regular cell-cycle. In streptococci, complex regulatory pathways control competence deactivation to narrow its development to a sharp window of time, a process known as competence shut-off. Although characterized in streptococci whose competence is activated by the ComCDE signaling pathway, it remains unclear for those controlled by the ComRS system. In this work, we investigate competence shut-off in the major human gut commensal Streptococcus salivarius. Using a deterministic mathematical model of the ComRS system, we predicted a negative player under the control of the central regulator ComX as involved in ComS/XIP pheromone degradation through a negative feedback loop. The individual inactivation of peptidase genes belonging to the ComX regulon allowed the identification of PepF as an essential oligoendopeptidase in S. salivarius. By combining conditional mutants, transcriptional analyses, and biochemical characterization of pheromone degradation, we validated the reciprocal role of PepF and XIP in ComRS shut-off. Notably, engineering cleavage site residues generated ultra-resistant peptides producing high and long-lasting competence activation. Altogether, this study reveals a proteolytic shut-off mechanism of competence in the salivarius group and suggests that this mechanism could be shared by other ComRS-containing streptococci.

The human oral cavity is one of the most challenging ecological niches for bacteria. In this ecosystem, hundreds of species compete for food and survival in a physicochemical fluctuating environment. To outcompete, Streptococcus salivarius has developed a particular physiological state called competence during which antibacterial compounds are produced together with the uptake of external DNA that can be integrated in its own genome. Although this strategy is of main importance for evolution and adaptation, its short-term cost in terms of energy and metabolism reprogramming are important. To restrain competence activation to a sharp window of time, bacteria use a process known as shut-off. Although described in some species, this process is still mostly unknown in streptococci. In this work, we used predictive mathematical simulations to infer the role of a pheromone-degradation machinery involved in the exit from competence. We confirmed experimentally this mechanism by identifying PepF as a competence-induced oligoendopeptidase with a specific activity towards the XIP pheromone. Importantly, we show that this peptidase is not only shutting down competence but also preventing its development under inappropriate conditions.

Heterologous Expression, Purification and Characterization of an Oligopeptidase A from the Pathogen Leptospira interrogans

Oligopeptidases are enzymes involved in the degradation of short peptides (generally less than 30 amino acids in size) which help pathogens evade the host defence mechanisms. Leptospira is a zoonotic pathogen and causes leptospirosis in mammals. Proteome analysis of Leptospira revealed the presence of oligopeptidase A (OpdA) among other membrane proteins. To study the role of oligopeptidase in leptospirosis, the OpdA of L. interrogans was cloned and expressed in Escherichia coli with a histidine tag (His-tag). The protein showed maximum expression at 37 °C with 0.5 mM of IPTG after 2 h of induction. Recombinant OpdA protein was purified to homogeneity using Ni-affinity chromatography. The purified OpdA showed more than 80% inhibition with a serine protease inhibitor but the activity was reduced to 30% with the cysteine protease inhibitor. The peptidase activity was increased significantly in the presence of Zn²⁺ at a neutral pH. Inhibitor assay indicate the presence of more than one active sites for peptidase activity as reported with the OpdA of E. coli and Salmonella. Over-expression of OpdA in E. coli BL21 (DE3) did not cause any negative effects on normal cell growth and viability. The role of OpdA as virulence factor in Leptospira and its potential as a therapeutic and diagnostic target in leptospirosis is yet to be identified.

Degradation of bradykinin by a metalloendopeptidase from Streptococcus pyogenes

OBJECTIVES

Streptococcus pyogenes secretes streptococcal pyrogenic exotoxin B (SpeB), which cleaves kininogen to liberate bradykinin. In addition, this bacterium also has cell-associated bradykinin-degrading activity. Here, we characterized the bradykinin-degrading enzyme produced by S. pyogenes.

METHODS

The effects of various peptidase inhibitors on bradykinin degradation by intact S. pyogenes and cell lysates were assessed. Cleavage of bradykinin and other peptides by a recombinant putative metalloendopeptidase (Sp-Pep) from S. pyogenes was analyzed by mass spectrometry. The enhancement of vascular permeability induced by bradykinin (before and after treatment with Sp-Pep) was evaluated in guinea pig skin.

RESULTS

Various S. pyogenes strains expressed Sp-Pep. Immunoadsorption of S. pyogenes with an anti-Sp-Pep antibody showed that 80% of the bradykinin-degrading activity in S. pyogenes was due to Sp-Pep. Recombinant Sp-Pep cleaved bradykinin, and cleavage caused a loss of its extravasation-inducing potential. Sp-Pep-mediated degradation of bradykinin was 40 times more efficient than degradation of substance P and angiotensin II. While S. pyogenes secreted mature SpeB in stationary phase, this bacterium produced Sp-Pep during all tested growth phases.

CONCLUSIONS

S. pyogenes produces a cell-associated metalloendopeptidase that degrades bradykinin.

Characterization of a novel PepF-like oligopeptidase secreted by Bacillus amyloliquefaciens 23-7A

An oligopeptidase from Bacillus amyloliquefaciens 23-7A was characterized along with its biochemical activities and structural gene. The protein's amino acid sequence and enzymatic activities were similar to those of other bacterial PepFs, which belong to metallopeptidase family M3. While most bacterial PepFs are cytoplasmic endopeptidases, the identified PepFBa oligopeptidase is a secreted protein and may facilitate the process of sporulation.

Stress-responsive proteins are upregulated in Streptococcus mutans during acid tolerance

Streptococcus mutansis an important pathogen in the initiation of dental caries as the bacterium remains metabolically active when the environment becomes acidic. The mechanisms underlying this ability to survive and proliferate at low pH remain an area of intense investigation. Differential two-dimensional electrophoretic proteome analysis ofS. mutansgrown at steady state in continuous culture at pH 7·0 or pH 5·0 enabled the resolution of 199 cellular and extracellular protein spots with altered levels of expression. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry identified 167 of these protein spots. Sixty-one were associated with stress-responsive pathways involved in DNA replication, transcription, translation, protein folding and proteolysis. The 61 protein spots represented isoforms or cleavage products of 30 different proteins, of which 25 were either upregulated or uniquely expressed during acid-tolerant growth at pH 5·0. Among the unique and upregulated proteins were five that have not been previously identified as being associated with acid tolerance inS. mutansand/or which have not been studied in any detail in oral streptococci. These were the single-stranded DNA-binding protein, Ssb, the transcription elongation factor, GreA, the RNA exonuclease, polyribonucleotide nucleotidyltransferase (PnpA), and two proteinases, the ATP-binding subunit, ClpL, of the Clp family of proteinases and a proteinase encoded by thepepgene family with properties similar to the dipeptidase, PepD, ofLactobacillus helveticus. The identification of these and other differentially expressed proteins associated with an acid-tolerant-growth phenotype provides new information on targets for mutagenic studies that will allow the future assessment of their physiological significance in the survival and proliferation ofS. mutansin low pH environments.

Genome sequence of Streptococcus agalactiae, a pathogen causing invasive neonatal disease

Streptococcus agalactiae is a commensal bacterium colonizing the intestinal tract of a significant proportion of the human population. However, it is also a pathogen which is the leading cause of invasive infections in neonates and causes septicaemia, meningitis and pneumonia. We sequenced the genome of the serogroup III strain NEM316, responsible for a fatal case of septicaemia. The genome is 2 211 485 base pairs long and contains 2118 protein coding genes. Fifty-five per cent of the predicted genes have an ortholog in the Streptococcus pyogenes genome, representing a conserved backbone between these two streptococci. Among the genes in S. agalactiae that lack an ortholog in S. pyogenes, 50% are clustered within 14 islands. These islands contain known and putative virulence genes, mostly encoding surface proteins as well as a number of genes related to mobile elements. Some of these islands could therefore be considered as pathogenicity islands. Compared with other pathogenic streptococci, S. agalactiae shows the unique feature that pathogenicity islands may have an important role in virulence acquisition and in genetic diversity.

Life in protein-rich environments: the relA-independent response of Streptococcus pyogenes to amino acid starvation

Considering that group A streptococci are multiple auxotrophs that may encounter shortage of amino acids during specific stages of the infectious process, we studied their adaptive response to amino acid deprivation. We found that, in addition to the (p)ppGpp-mediated stringent response characterized previously, Streptococcus pyogenes exhibits a relA-independent response comprising transcriptional modulation of a specific subset of genes involved in pathogenesis. Genes/operons transcriptionally upregulated during starvation of both wild type and relA mutants included the two-component signal transduction system covRS, the positive regulator (ropB) of the pyrogenic exotoxin B gene, speB, the oligopeptide (opp) and dipeptide (dpp) permease systems and the pepB gene putatively involved in the intracellular processing of oligopeptides. Upregulation of covRS was accompanied by downregulation of ska, one of the target genes of the negative CovR regulator, and the net effect of amino acid starvation also favoured repression of speB. A significant feature of upregulated opp expression was stimulated readthrough transcription of the operon-internal oppA terminator, leading to increased mRNA levels for synthesis of the translocator complex relative to the substrate-binding protein. Based on these and previous results, a stimulus-response network is proposed that counteracts the stringent response and may enable the pathogen to mount a dynamic response to the protein-rich environment provided by its human host.