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Characterization and functional analysis of atl, a novel gene encoding autolysin in Streptococcus suis.

ABSTRACT: Streptococcus suis serotype 2 (S. suis 2) is an important swine and human pathogen responsible for septicemia and meningitis. A novel gene, designated atl and encoding a major autolysin of S. suis 2 virulent strain HA9801, was identified and characterized in this study. The Atl protein contains 1,025 amino acids with a predicted molecular mass of 113 kDa and has a conserved N-acetylmuramoyl-l-alanine amidase domain. Recombinant Atl was expressed in Escherichia coli, and its bacteriolytic and fibronectin-binding activities were confirmed by zymography and Western affinity blotting. Two bacteriolytic bands were shown in the sodium dodecyl sulfate extracts of HA9801, while both were absent from the atl inactivated mutant. Cell chains of the mutant strain became longer than that of the parental strain. In the autolysis assay, HA9801 decreased to 20% of the initial optical density (OD) value, while the mutant strain had almost no autolytic activity. The biofilm capacity of the atl mutant was reduced ?30% compared to the parental strain. In the zebrafish infection model, the 50% lethal dose of the mutant strain was increased up to 5-fold. Furthermore, the adherence to HEp-2 cells of the atl mutant was 50% less than that of the parental strain. Based on the functional analysis of the recombinant Atl and observed effects of atl inactivation on HA9801, we conclude that Atl is a major autolysin of HA9801. It takes part in cell autolysis, separation of daughter cells, biofilm formation, fibronectin-binding activity, cell adhesion, and pathogenesis of HA9801.

SUBMITTER: Ju CX

PROVIDER: S-EPMC3294825 | biostudies-literature | 2012 Mar

REPOSITORIES: biostudies-literature

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Characterization and functional analysis of atl, a novel gene encoding autolysin in Streptococcus suis.

Ju Cun-Xiang CX Gu Hong-Wei HW Lu Cheng-Ping CP

Journal of bacteriology 20120106 6

Streptococcus suis serotype 2 (S. suis 2) is an important swine and human pathogen responsible for septicemia and meningitis. A novel gene, designated atl and encoding a major autolysin of S. suis 2 virulent strain HA9801, was identified and characterized in this study. The Atl protein contains 1,025 amino acids with a predicted molecular mass of 113 kDa and has a conserved N-acetylmuramoyl-l-alanine amidase domain. Recombinant Atl was expressed in Escherichia coli, and its bacteriolytic and fib ...[more]

PMID: 22228730

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Project description:Given the lack of effective vaccines to control Streptococcus suis infection and the lack of a rapid and reliable molecular diagnostic assay to detect its infection, a polyclonal antibody was raised against the whole-cell protein of S. suis type 2 and used to screen an S. suis gene library in an effort to identify protective antigen(s) and antigens of diagnostic importance. A clone that produced a 45-kDa S. suis-specific protein was identified by Western blotting. Restriction analysis showed that the gene encoding the 45-kDa protein was present on a 1.6-kb pair DraI region on the cloned chromosomal fragment. The nucleotide sequence contained an open reading frame that encoded a polypeptide of 448 amino acid residues with a calculated molecular mass of 48.8 kDa, in close agreement with the size observed on Western blots. A GenBank database search revealed that the derived amino acid sequence is homologous to the sequence of glutamate dehydrogenase (GDH) protein isolated from various sources, including conserved motifs and functional domains typical of the family 1-type hexameric GDH proteins, thus placing it in that family. Because of these similarities, the protein was designated the GDH of S. suis. Hybridization studies showed that the gene is conserved among the S. suis type 2 strains tested. Antiserum raised against the purified recombinant protein was reactive with a protein of the same molecular size as the recombinant protein in S. suis strains, suggesting expression of the gene in all of the isolates and antigenic conservation of the protein. The recombinant protein was reactive with serum from pigs experimentally infected with a virulent strain of S. suis type 2, suggesting that the protein might serve as an antigen of diagnostic importance to detect S. suis infection. Activity staining showed that the S. suis GDH activity is NAD(P)H dependent but, unlike the NAD(P)H-dependent GDH from various other sources, that of S. suis utilizes L-glutamate rather than alpha-ketoglutarate as the substrate. Highly virulent strains of S. suis type 2 could be distinguished from moderately virulent and avirulent strains on the basis of their GDH protein profile following activity staining on a nondenaturing gel. We examined the cellular location of the protein using a whole-cell enzyme-linked immunosorbent assay and an immunogold-labeling technique. Results showed that the S. suis GDH protein is exposed at the surface of intact cells.

Project description:Many surface proteins which are covalently linked to the cell wall of gram-positive bacteria have a consensus C-terminal motif, Leu-Pro-X-Thr-Gly (LPXTG). This sequence is cleaved, and the processed protein is attached to an amino group of a cross-bridge in the peptidoglycan by a specific enzyme called sortase. Using the type strain of Streptococcus suis, NCTC 10234, we found five genes encoding proteins that were homologous to sortases of other bacteria and determined the nucleotide sequences of the genetic regions. One gene, designated srtA, was linked to gyrA, as were the sortase and sortase-like genes of other streptococci. Three genes, designated srtB, srtC, and srtD, were tandemly clustered in a different location, where there were three segments of directly repeated sequences of approximately 110 bp in close vicinity. The remaining gene, designated srtE, was located separately on the chromosome with a pseudogene which may encode a transposase. The deduced amino acid sequences of the five Srt proteins showed 18 to 31% identity with the sortases of Streptococcus gordonii and Staphylococcus aureus, except that SrtA of S. suis had 65% identity with that of S. gordonii. Isogenic mutants deficient for srtA, srtBCD, or srtE were generated by allelic exchanges. The protein fraction which was released from partially purified cell walls by digestion with N-acetylmuramidase was profiled by two-dimensional gel electrophoresis. More than 15 of the protein spots were missing in the profile of the srtA mutant compared with that of the parent strain, and this phenotype was completely complemented by srtA cloned from S. suis. Four genes encoding proteins corresponding to such spots were identified and sequenced. The deduced translational products of the four genes possessed the LPXTG motif in their C-terminal regions. On the other hand, the protein spots that were missing in the srtA mutant appeared in the profiles of the srtBCD and srtE mutants. These results provide evidence that the cell wall sorting system involving srtA is also present in S. suis.

Project description:In our continued effort to search for a Streptococcus suis protein(s) that can serve as a vaccine candidate or a diagnostic reagent, we constructed and screened a gene library with a polyclonal antibody raised against the whole-cell protein of S. suis type 2. A clone that reacted with the antibody was identified and characterized. Analysis revealed that the gene encoding the protein is localized within a 2.0-kbp EcoRI DNA fragment. The nucleotide sequence contained an open reading frame that encoded a polypeptide of 445 amino acid residues with a calculated molecular mass of 46.4 kDa. By in vitro protein synthesis and Western blot experiments, the protein exhibited an electrophoretic mobility of approximately 38 kDa. At the amino acid level the deduced primary sequence shared homology with sequences of unknown function from Streptococcus pneumoniae (89%), Streptococcus mutans (86%), Lactococcus lactis (80%), Listeria monocytogenes (74%), and Clostridium perfringens (64%). Except for strains of serotypes 20, 26, 32, and 33, Southern hybridization analysis revealed the presence of the gene in strains of other S. suis serotypes and demonstrated restriction fragment length differences caused by a point mutation in the EcoRI recognition sequence. We confirmed expression of the 38-kDa protein in the hybridization-positive isolates using specific antiserum against the purified protein. The recombinant protein was reactive with serum from pigs experimentally infected with virulent strains of S. suis type 2, suggesting that the protein is immunogenic and may serve as an antigen of diagnostic importance for the detection of most S. suis infections. Pigs immunized with the recombinant 38-kDa protein mounted antibody responses to the protein and were completely protected against challenge with a strain of a homologous serotype, the wild-type virulent strain of S. suis type 2, suggesting that it may be a good candidate for the development of a vaccine that can be used as protection against S. suis infection. Analysis of the cellular fractions of the bacterium by Western blotting revealed that the protein was present in the surface and cell wall extracts. The functional role of the protein with respect to pathogenesis and whether antibodies against the antigen confer protective immunity against diseases caused by strains of other pathogenic S. suis capsular types remains to be determined.

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Characterization and functional analysis of atl, a novel gene encoding autolysin in Streptococcus suis.

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Characterization and functional analysis of atl, a novel gene encoding autolysin in Streptococcus suis.

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