Mycoplasma gallisepticum MGA_0676 is a membrane-associated cytotoxic nuclease with a staphylococcal nuclease region essential for nuclear translocation and apoptosis induction in chicken cells

Neutrophil extracellular traps in bacterial infections and evasion strategies

Neutrophils are innate immune cells that have a vital role in host defense systems. Neutrophil extracellular traps (NETs) are one of neutrophils' defense mechanisms against pathogens. NETs comprise an ejected lattice of chromatin associated with histones, granular proteins, and cytosolic proteins. They are thought to be an efficient strategy to capture and/or kill bacteria and received intensive research interest in the recent years. However, soon after NETs were identified, it was observed that certain bacteria were able to evade NET entrapment through many different mechanisms. Here, we outline the recent progress of NETs in bacterial infections and the strategies employed by bacteria to evade or withstand NETs. Identifying the molecules and mechanisms that modulate NET release will improve our understanding of the functions of NETs in infections and provide new avenues for the prevention and treatment of bacterial diseases.

Combining Fusion of Cells with CRISPR-Cas9 Editing for the Cloning of Large DNA Fragments or Complete Bacterial Genomes in Yeast

The genetic engineering of genome fragments larger than 100 kbp is challenging and requires both specific methods and cloning hosts. The yeast Saccharomyces cerevisiae is considered as a host of choice for cloning and engineering whole or partial genomes from viruses, bacteria, and algae. Several methods are now available to perform these manipulations, each with its own limitations. In order to extend the range of yeast cloning strategies, a new approach combining two already described methods, Fusion cloning and CReasPy-Cloning, was developed. The CReasPy-Fusion method allows the simultaneous cloning and engineering of megabase-sized genomes in yeast by the fusion of bacterial cells with yeast spheroplasts carrying the CRISPR-Cas9 system. With this new approach, we demonstrate the feasibility of cloning and editing whole genomes from several Mycoplasma species belonging to different phylogenetic groups. We also show that CReasPy-Fusion allows the capture of large genome fragments with high efficacy, resulting in the successful cloning of selected loci in yeast. We finally identify bacterial nuclease encoding genes as barriers for CReasPy-Fusion by showing that their removal from the donor genome improves the cloning efficacy.

Mycoplasma synoviae lipid-associated membrane proteins identification and expression changes when exposed to chicken cells

Mycoplasma synoviae is a significant cause of respiratory disease and synovitis among chickens, and has an adverse economic impact on broiler breeding efforts. The present study was designed to develop a systematic understanding of the role that M. synoviae lipid-associated membrane proteins (LAMPs) may play in the virulence of this pathogen. Bioinformatics tools were used to identify 146 predicted membrane proteins and lipoproteins in the M. synoviae proteome. Then, Triton X-114 was used to extract LAMPs that were subsequently identified via LC-MS/MS. This approach enabled the detection of potential LAMPs, and the top 200 most abundant proteins detected using this strategy were subject to further analysis. M. synoviae cells (100 MOI) were exposed to chicken fibroblasts (DF-1) and macrophages (HD-11) in a 1:1 mixed culture. Analysis of LAMP transcripts identified 72 up-regulated LAMP genes which were analyzed in depth by bioinformatics. GO analysis revealed these genes to be enriched in the nucleotide binding, sulfur amino acid transmembrane transporter activity, tRNA binding, rRNA modification, and transition metal ion transport pathways. Moreover, KEGG enrichment analysis suggested that these genes were enriched in the biosynthesis of secondary metabolites, carbon metabolism, glycolysis/gluconeogenesis, and nitrogen metabolism pathways.

Novel mycoplasma nucleomodulin MbovP475 decreased cell viability by regulating expression of CRYAB and MCF2L2

Nucleomodulins are secreted bacterial proteins whose molecular targets are located in host cell nuclei. They are gaining attention as critical virulence factors that either modify the epigenetics of host cells or directly regulate host gene expression. Mycoplasma bovis is a major veterinary pathogen that secretes several potential virulence factors. The aim of this study was to determine whether any of their secreted proteins might function as nucleomodulins. After an initial in silico screening, the nuclear localization of the secreted putative lipoprotein MbovP475 of M. bovis was demonstrated in bovine macrophage cell line (BoMac) experimentally infected with M. bovis. Through combined application of ChIP-seq, Electrophoretic mobility shift assay (EMSA) and surface plasmon resonance (SPR) analysis, MbovP475 was determined to bind the promoter regions of the cell cycle central regulatory genes CRYAB and MCF2L2. MbovP475 has similar secondary structures with the transcription activator-like effectors (TALEs). Screening of various mutants affecting the potential DNA binding sites indicated that the residues ²⁴²NI²⁴³ within MbovP475 loop region of the helix-loop-helix domain were essential to its DNA binding activity, thereby contributing to decrease in BoMac cell viability. In conclusion, this is the first report to confirm M. bovis secretes a conserved TALE-like nucleomodulin that binds the promoters of CRYAB and MCF2L2 genes, and subsequently down-regulates their expression and decreases BoMac cell viability. Therefore, this study offers a new understanding of mycoplasma pathogenesis.

Eating the Enemy: Mycoplasma Strategies to Evade Neutrophil Extracellular Traps (NETs) Promoting Bacterial Nucleotides Uptake and Inflammatory Damage

Neutrophils are effector cells involved in the innate immune response against infection; they kill infectious agents in the intracellular compartment (phagocytosis) or in the extracellular milieu (degranulation). Moreover, neutrophils release neutrophil extracellular traps (NETs), complex structures composed of a scaffold of decondensed DNA associated with histones and antimicrobial compounds; NETs entrap infectious agents, preventing their spread and promoting their clearance. NET formation is triggered by microbial compounds, but many microorganisms have evolved several strategies for NET evasion. In addition, the dysregulated production of NETs is associated with chronic inflammatory diseases. Mycoplasmas are reduced genome bacteria, able to induce chronic infections with recurrent inflammatory symptoms. Mycoplasmas' parasitic lifestyle relies on metabolite uptake from the host. Mycoplasmas induce NET release, but their surface or secreted nucleases digest the NETs' DNA scaffold, allowing them to escape from entrapment and providing essential nucleotide precursors, thus promoting the infection. The presence of Mycoplasma species has been associated with chronic inflammatory disorders, such as systemic lupus erythematosus, rheumatoid arthritis, inflammatory bowel disease, Crohn's disease, and cancer. The persistence of mycoplasma infection and prolonged NET release may contribute to the onset of chronic inflammatory diseases and needs further investigation and insights.

Traditional Protocols and Optimization Methods Lead to Absent Expression in a Mycoplasma Cell-Free Gene Expression Platform

Cell-free expression (CFE) systems are one of the main platforms for building synthetic cells. A major drawback is the orthogonality of cell-free systems across species. To generate a CFE system compatible with recently established minimal cell constructs, we attempted to optimize a Mycoplasma bacterium-based CFE system using lysates of the genome-minimized cell JCVI-syn3A (Syn3A) and its close phylogenetic relative Mycoplasma capricolum (Mcap). To produce mycoplasma-derived crude lysates, we systematically tested methods commonly used for bacteria, based on the S30 protocol of Escherichia coli. Unexpectedly, after numerous attempts to optimize lysate production methods or composition of feeding buffer, none of the Mcap or Syn3A lysates supported cell-free gene expression. Only modest levels of in vitro transcription of RNA aptamers were observed. While our experimental systems were intended to perform transcription and translation, our assays focused on RNA. Further investigations identified persistently high ribonuclease (RNase) activity in all lysates, despite removal of recognizable nucleases from the respective genomes and attempts to inhibit nuclease activities in assorted CFE preparations. An alternative method using digitonin to permeabilize the mycoplasma cell membrane produced a lysate with diminished RNase activity yet still was unable to support cell-free gene expression. We found that intact mycoplasma cells poisoned E. coli cell-free extracts by degrading ribosomal RNAs, indicating that the mycoplasma cells, even the minimal cell, have a surface-associated RNase activity. However, it is not clear which gene encodes the RNase. This work summarizes attempts to produce mycoplasma-based CFE and serves as a cautionary tale for researchers entering this field.

Graphical Abstract

The application of aptamer Apt-236 targeting PvpA protein in the detection of antibodies against Mycoplasma gallisepticum

Mycoplasma gallisepticum (M. gallisepticum) is the primary agent of chronic respiratory disease causing important economic losses in the poultry industry. Compared to antibodies, aptamers used to diagnose M. gallisepticum have many advantages, such as being chemically, animal-free produced and easily modifiable without affecting their affinity. Herein, a single-stranded DNA (ssDNA) aptamer Apt-236 which can specifically bind to PvpA protein of M. gallisepticum with a Kd of 1.30 ± 0.18 nM was selected successfully. An indirect blocking ELAA (ib-ELAA) for M. gallisepticum antibodies detection was also developed using Apt-236, in which M. gallisepticum antibodies would block the binding-position of aptamers. Therefor positive sera would prevent color development whereas negative sera will allow a strong color reaction. The ib-ELAA was consistent with other three widely used assays in terms of the growth and decline of the antibody response to M. gallisepticum, and showed substantial agreement with the results obtained using a commercial ELISA kit in clinical chicken sera samples. Therefore, the ib-ELAA developed in this study was a new format for aptamer application and would be an alternative method for the surveillance of M. gallisepticum.

MBOVPG45_0375 Encodes an IgG-Binding Protein and MBOVPG45_0376 Encodes an IgG-Cleaving Protein in Mycoplasma bovis

Mycoplasma bovis is a significant bacterial pathogen which is able to persist in cattle and cause chronic diseases. This phenomenon may relate to M. bovis evading the immune system of the host. Immunoglobulin-binding proteins are widely distributed in a variety of pathogenic bacteria, including some Mycoplasma species. These proteins are considered to help the bacteria evade the immune response of the host. Here we found M. bovis strain PG45 can bind to IgG from several animals. MBOVPG45_0375 encodes a putative membrane protein, has strong amino acid sequence similarity with Immunoglobulin G-binding protein in Mycoplasma mycoides subsp. capri. Hence, we constructed recombinant MBOVPG45_0375 (r0375) in the Escherichia coli expression system and demonstrated that r0375 can bind to IgG non-immunologically rather than specific binding similar to interaction of antigen and antibody. Moreover, r0375 can bind to the Fab fragment of IgG. Also, the binding of r0375 and IgG inhibits the formation of antigen-antibody union. Furthermore, MBOVPG45_0376 encodes an IgG-cleaving protein of M. bovis strain PG45. Nevertheless, r0375 binding to IgG is required for the cleavage activity of recombinant 0376 (r0376). The activity of r0376 is also affected by incubation time and temperature. In addition, we found both MBOVPG45_0375 and MBOVPG45_0376 are membrane proteins of M. bovis strain PG45. These results about MBOVPG45_0375 as an IgG-binding protein and MBOVPG45_0376 as an IgG-cleaving protein offer a new insight into the interaction between M. bovis and its host.

Infection strategies of mycoplasmas: Unraveling the panoply of virulence factors

Mycoplasmas, the smallest bacteria lacking a cell wall, can cause various diseases in both humans and animals. Mycoplasmas harbor a variety of virulence factors that enable them to overcome numerous barriers of entry into the host; using accessory proteins, mycoplasma adhesins can bind to the receptors or extracellular matrix of the host cell. Although the host immune system can eradicate the invading mycoplasma in most cases, a few sagacious mycoplasmas employ a series of invasion and immune escape strategies to ensure their continued survival within their hosts. For instance, capsular polysaccharides are crucial for anti-phagocytosis and immunomodulation. Invasive enzymes degrade reactive oxygen species, neutrophil extracellular traps, and immunoglobulins. Biofilm formation is important for establishing a persistent infection. During proliferation, successfully surviving mycoplasmas generate numerous metabolites, including hydrogen peroxide, ammonia and hydrogen sulfide; or secrete various exotoxins, such as community-acquired respiratory distress syndrome toxin, and hemolysins; and express various pathogenic enzymes, all of which have potent toxic effects on host cells. Furthermore, some inherent components of mycoplasmas, such as lipids, membrane lipoproteins, and even mycoplasma-generated superantigens, can exert a significant pathogenic impact on the host cells or the immune system. In this review, we describe the proposed virulence factors in the toolkit of notorious mycoplasmas to better understand the pathogenic features of these bacteria, along with their pathogenic mechanisms.

MHO_0730 as a Surface-Exposed Calcium-Dependent Nuclease of Mycoplasma hominis Promoting Neutrophil Extracellular Trap Formation and Escape

Mycoplasma lipoproteins play a relevant role in pathogenicity and directly interact with the host immune system. Among human mycoplasmas, Mycoplasma hominis is described as a commensal bacterium that can be associated with a number of genital and extragenital conditions. Mechanisms of M. hominis pathogenicity are still largely obscure, and only a limited number of proteins have been associated with virulence. The current study focused on investigating the role of MHO_0730 as a virulence factor and demonstrated that MHO_0730 is a surface lipoprotein, potentially expressed in vivo during natural infection, acting both as a nuclease with its amino acidic portion and as a potent inducer of Neutrophil extracellular trapsosis with its N-terminal lipid moiety. Evidence for M. hominis neutrophil extracellular trap escape is also presented. Results highlight the relevance of MHO_0730 in promoting infection and modulation and evasion of innate immunity and provide additional knowledge on M. hominis virulence and survival in the host.

Multiple differences in pathogen-host cell interactions following a bacterial host shift

Novel disease emergence is often associated with changes in pathogen traits that enable pathogen colonisation, persistence and transmission in the novel host environment. While understanding the mechanisms underlying disease emergence is likely to have critical implications for preventing infectious outbreaks, such knowledge is often based on studies of viral pathogens, despite the fact that bacterial pathogens may exhibit very different life histories. Here, we investigate the ability of epizootic outbreak strains of the bacterial pathogen, Mycoplasma gallisepticum, which jumped from poultry into North American house finches (Haemorhous mexicanus), to interact with model avian cells. We found that house finch epizootic outbreak strains of M. gallisepticum displayed a greater ability to adhere to, invade, persist within and exit from cultured chicken embryonic fibroblasts, than the reference virulent (R_low) and attenuated (R_high) poultry strains. Furthermore, unlike the poultry strains, the house finch epizootic outbreak strain HF_1994 displayed a striking lack of cytotoxicity, even exerting a cytoprotective effect on avian cells. Our results suggest that, at epizootic outbreak in house finches, M. gallisepticum was particularly adept at using the intra-cellular environment, which may have facilitated colonisation, dissemination and immune evasion within the novel finch host. Whether this high-invasion phenotype is similarly displayed in interactions with house finch cells, and whether it contributed to the success of the host shift, remains to be determined.

Extracellular DNA: A Nutritional Trigger of Mycoplasma bovis Cytotoxicity

Microbial access to host nutrients is a key factor of the host-pathogen interplay. With their nearly minimal genome, wall-less bacteria of the class Mollicutes have limited metabolic capacities and largely depend on host nutrients for their survival. Despite these limitations, host-restricted mycoplasmas are widely distributed in nature and many species are pathogenic for humans and animals. Yet, only partial information is available regarding the mechanisms evolved by these minimal pathogens to meet their nutrients and the contribution of these mechanisms to virulence. By using the ruminant pathogen Mycoplasma bovis as a model system, extracellular DNA (eDNA) was identified as a limiting nutrient for mycoplasma proliferation under cell culture conditions. Remarkably, the growth-promoting effect induced by supplementation with eDNA was associated with important cytotoxicity for actively dividing host cells, but not confluent monolayers. To identify biological functions mediating M. bovis cytotoxicity, we produced a library of transposon knockout mutants and identified three critical genomic regions whose disruption was associated with a non-cytopathic phenotype. The coding sequences (CDS) disrupted in these regions pointed towards pyruvate metabolism as contributing to M. bovis cytotoxicity. Hydrogen peroxide was found responsible for eDNA-mediated M. bovis cytotoxicity, and non-cytopathic mutants were unable to produce this toxic metabolic compound. In our experimental conditions, no contact between M. bovis and host cells was required for cytotoxicity. Further analyses revealed important intra-species differences in eDNA-mediated cytotoxicity and H₂O₂ production, with some strains displaying a cytopathic phenotype despite no H₂O₂ production. Interestingly, the genome of strains PG45 and HB0801 were characterized by the occurrence of insertion sequences (IS) at close proximity to several CDSs found disrupted in non-cytopathic mutants. Since PG45 and HB0801 produced no or limited amount of H₂O₂, IS-elements might influence H₂O₂ production in M. bovis. These results confirm the multifaceted role of eDNA in microbial communities and further identify this ubiquitous material as a nutritional trigger of M. bovis cytotoxicity. M. bovis may thus take advantage of the multiple sources of eDNA in vivo to modulate its interaction with host cells, a way for this minimal pathogen to overcome its limited coding capacity.

Nucleases of bacterial pathogens as virulence factors, therapeutic targets and diagnostic markers

New frontiers of therapy are being explored against the upcoming bacterial diseases rendered untreatable due to multiple, extreme and pan- antibiotic resistance. Nucleases are ubiquitous in bacterial pathogens performing various functions like acquiring nucleotide nutrients, allowing or preventing uptake of foreign DNA, controlling biofilm formation/dispersal/architecture, invading host by tissue damage, evading immune defence by degrading DNA matrix of neutrophil extracellular traps (NETs) and immunomodulating the host immune response. Secretory nucleases also provide means of survival to other bacteria like iron-reducing Shewanella and such functions help them adapt and survive proficiently. Other than their pro-pathogen roles in survival, nucleases can be used directly as therapeutics. One of the powerful armours of pathogens is the formation of biofilms, thus helping them resist and persist in the harshest of environments. As eDNA forms the structural and binding component of biofilm, nucleases can be used against the adhering component, thus increasing the permeability of antimicrobial agents. Nucleases have recently become a model system of intense study for their biological functions and medical applications in diagnosis, immunoprophylaxis and therapy. Rational implications of these enzymes can impact human medicine positively in future by opening new ways for therapeutics which have otherwise reached saturation due to multi drug resistance.

Mycoplasma hyopneumoniae Mhp597 is a cytotoxicity, inflammation and immunosuppression associated nuclease

Nucleases are ubiquitously recognized as essential proteins in mycoplasmas because these organisms lack most capacities for de novo synthesis of nucleotides. Some of these proteins were proved to be important pathogenic factors during the infection of mycoplasms. In this study, the protein Mhp597 from M. hyopneumoniae was expressed and purified in Escherichia coli. Analysis of nuclease activity showed that recombinant Mhp597 (rMhp597) was a Ca₂₊ or Mg₂₊ dependent thermostable nuclease with very high activity and neutrophil extracellular traps (NETs) induced by M. hyopneumoniae were completely degraded by this nuclease. In addition, when PK15 cells were incubated with different concentrations of rMhp597, their viability was reduced and cell apoptosis was observed in a dose-dependent manner. To further investigate the host immune system response, we report that rMhp597 up-regulated the exression of inflammatory genes showing that TLR4/MyD88/NF-κB signal pathway was involved. On the other hand, rMhp597 down-regulated the expression of type I IFN (IFN-α/β) and promoted the multiplication of porcine reproductive and respiratory syndrome virus (PRRSV). Recombinant rMhp597δ^315-377 lacking C-terminal 63 amino acids exhibited all biological functions mentioned above except for nuclease activity. In summary, Mhp597 is a dynamic secreted nuclease involved in cytotoxicity, inflammation and immunosuppression.

Mechanism of Apoptosis Induction by Mycoplasmal Nuclease MGA_0676 in Chicken Embryo Fibroblasts

MGA_0676 has been characterized as a Mycoplasma gallisepticum nuclease that can induce apoptosis of chicken cells. However, the mechanism by which MGA_0676 induces apoptosis has remained unclear. In this study, we evaluated MGA_0676-induced apoptosis and internalization in immortalized chicken embryo fibroblasts (DF-1) and cancer cell lines. The internalization of MGA_0676 was proven through caveolin-mediated endocytosis by blocking the endocytosis with specific inhibitors or with siRNA. We identified the Thif domain of NEDD8-activating enzyme E1 regulatory subunit (NAE) in DF-1 as the target region interacting with the SNC domain of MGA_0676. The interaction between the Thif and SNC domains was observed co-located in the perinuclear and nuclear of DF-1. We found that the interaction between NAE and MGA_0676 increased the ability of apoptosis and accelerated the process of cullin neddylation in DF-1 cells, in turn activating NF-κB. This resulted in the observed aggregation of NF-κB in the nuclei of DF-1 cells. Moreover, the apoptosis induced by MGA_0676 decreased significantly when NF-κB was inhibited by siRNA or BAY 11-7082 or when NAE was silenced by siRNA. Overall, our results demonstrate that MGA_0676 is internalized through caveolin-mediated endocytosis, interacts with SNC-dependent Thif to accelerate the process of cullin neddylation and activates NF-κB in DF-1 cells, ultimately playing a key role in apoptosis in chicken cells. Our results indicate MGA_0676 constitutes a critical etiological virulence factor of the respiratory disease caused by M. gallisepticum. This study also opens a venue to investigate MGA_0676 as a potential candidate as pro-apoptotic drug in cancer studies.

Validation of a novel Mho microarray for a comprehensive characterisation of the Mycoplasma hominis action in HeLa cell infection

Mycoplasma hominis is the second smallest facultative pathogen of the human urogenital tract. With less than 600 protein-encoding genes, it represents an ideal model organism for the study of host-pathogen interactions. For a comprehensive characterisation of the M. hominis action in infection a customized Mho microarray, which was based on two genome sequences (PG21 and LBD-4), was designed to analyze the dynamics of the mycoplasma transcriptome during infection and validated for M. hominis strain FBG. RNA preparation was evaluated and adapted to ensure the highest recovery of mycoplasmal mRNAs from in vitro HeLa cell infection assays. Following cRNA hybridization, the read-out strategy of the hybridization results was optimized and confirmed by RT-PCR. A statistically robust infection assay with M. hominis strain FBG enabled the identification of differentially regulated key effector molecules such as critical cytoadhesins (4 h post infection (pI)), invasins (48 h pI) and proteins associated with establishing chronic infection of the host (336 h pI). Of the 294 differentially regulated genes (>2-fold) 128 (43.5%) encoded hypothetical proteins, including lipoproteins that seem to play a central role as virulence factors at each stage of infection: P75 as a novel cytoadhesin candidate, which is also differentially upregulated in chronic infection; the MHO_2100 protein, a postulated invasin and the MHO_730-protein, a novel ecto-nuclease and domain of an ABC transporter, the function of which in chronic infection has still to be elucidated. Implementation of the M. hominis microarray strategy led to a comprehensive identification of to date unknown candidates for virulence factors at relevant stages of host cell infection.

Ureaplasma diversum Genome Provides New Insights about the Interaction of the Surface Molecules of This Bacterium with the Host

Whole genome sequencing and analyses of Ureaplasma diversum ATCC 49782 was undertaken as a step towards understanding U. diversum biology and pathogenicity. The complete genome showed 973,501 bp in a single circular chromosome, with 28.2% of G+C content. A total of 782 coding DNA sequences (CDSs), and 6 rRNA and 32 tRNA genes were predicted and annotated. The metabolic pathways are identical to other human ureaplasmas, including the production of ATP via hydrolysis of the urea. Genes related to pathogenicity, such as urease, phospholipase, hemolysin, and a Mycoplasma Ig binding protein (MIB)-Mycoplasma Ig protease (MIP) system were identified. More interestingly, a large number of genes (n = 40) encoding surface molecules were annotated in the genome (lipoproteins, multiple-banded antigen like protein, membrane nuclease lipoprotein and variable surface antigens lipoprotein). In addition, a gene encoding glycosyltransferase was also found. This enzyme has been associated with the production of capsule in mycoplasmas and ureaplasma. We then sought to detect the presence of a capsule in this organism. A polysaccharide capsule from 11 to 17 nm of U. diversum was observed trough electron microscopy and using specific dyes. This structure contained arabinose, xylose, mannose, galactose and glucose. In order to understand the inflammatory response against these surface molecules, we evaluated the response of murine macrophages J774 against viable and non-viable U. diversum. As with viable bacteria, non-viable bacteria were capable of promoting a significant inflammatory response by activation of Toll like receptor 2 (TLR2), indicating that surface molecules are important for the activation of inflammatory response. Furthermore, a cascade of genes related to the inflammasome pathway of macrophages was also up-regulated during infection with viable organisms when compared to non-infected cells. In conclusion, U. diversum has a typical ureaplasma genome and metabolism, and its surface molecules, including the identified capsular material, represent major components of the organism immunopathogenesis.

Mycoplasma bovis MBOV_RS02825 Encodes a Secretory Nuclease Associated with Cytotoxicity

This study aimed to determine the activity of one Mycoplasma bovis nuclease encoded by MBOV_RS02825 and its association with cytotoxicity. The bioinformatics analysis predicted that it encodes a Ca²⁺-dependent nuclease based on existence of enzymatic sites in a TNASE_3 domain derived from a Staphylococcus aureus thermonuclease (SNc). We cloned and purified the recombinant MbovNase (rMbovNase), and demonstrated its nuclease activity by digesting bovine macrophage linear DNA and RNA, and closed circular plasmid DNA in the presence of 10 mM Ca²⁺ at 22–65 °C. In addition, this MbovNase was localized in membrane and rMbovNase able to degrade DNA matrix of neutrophil extracellular traps (NETs). When incubated with macrophages, rMbovNase bound to and invaded the cells localizing to both the cytoplasm and nuclei. These cells experienced apoptosis and the viability was significantly reduced. The apoptosis was confirmed by activated expression of phosphorylated NF-κB p65 and Bax, and inhibition of Iκβα and Bcl-2. In contrast, rMbovNase^Δ181–342 without TNASE_3 domain exhibited deficiency in all the biological functions. Furthermore, rMbovNase was also demonstrated to be secreted. In conclusion, it is a first report that MbovNase is an active nuclease, both secretory and membrane protein with ability to degrade NETs and induce apoptosis.

Mm19, a Mycoplasma meleagridis Major Surface Nuclease that Is Related to the RE_AlwI Superfamily of Endonucleases

Mycoplasma meleagridis infection is widespread in turkeys, causing poor growth and feathering, airsacculitis, osteodystrophy, and reduction in hatchability. Like most mycoplasma species, M. meleagridis is characterized by its inability to synthesize purine and pyrimidine nucleotides de novo. Consistent with this intrinsic deficiency, we here report the cloning, expression, and characterization of a M. meleagridis gene sequence encoding a major surface nuclease, referred to as Mm19. Mm19 consists of a 1941-bp ORF encoding a 646-amino-acid polypeptide with a predicted molecular mass of 74,825 kDa. BLASTP analysis revealed a significant match with the catalytic/dimerization domain of type II restriction enzymes of the RE_AlwI superfamily. This finding is consistent with the genomic location of Mm19 sequence, which dispalys characteristics of a typical type II restriction-modification locus. Like intact M. meleagridis cells, the E. coli-expressed Mm19 fusion product was found to exhibit a nuclease activity against plasmid DNA, double-stranded DNA, single-stranded DNA, and RNA. The Mm19-associated nuclease activity was consistently enhanced with Mg2+ divalent cations, a hallmark of type II restriction enzymes. A rabbit hyperimmune antiserum raised against the bacterially expressed Mm19 strongly reacted with M. meleagridis intact cells and fully neutralized the surface-bound nuclease activity. Collectively, the results show that M. meleagridis expresses a strong surface-bound nuclease activity, which is the product of a single gene sequence that is related to the RE_AlwI superfamily of endonucleases.