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DNA sequence of the serum opacity factor of group A streptococci: identification of a fibronectin-binding repeat domain.

ABSTRACT: The serum opacity factor (SOF) is a group A streptococcal protein that induces opacity of mammalian serum. The serum opacity factor 22 gene (sof22) from an M type 22 strain was cloned from an EMBL4 library by screening for plaques exhibiting serum opacity activity. DNA sequencing yielded an open reading frame of 3,075 bp. Its deduced amino acid sequence predicts a protein of 1,025 residues with a molecular weight of 112,735, a size that approximates that of the SOF22 protein isolated from both the original streptococcal strain and Escherichia coli harboring the cloned sof22 gene. The molecule is composed of three domains: an N-terminal domain responsible for the opacity reaction (opacity domain), a repeat domain with fibronectin-binding (Fn-binding) activity, and a C-terminal cell attachment domain. The C-terminal end of SOF22 is characterized by a hexameric LPXTGX motif, an adjacent hydrophobic region, and a charged C terminus, which are the hallmarks of cell-bound surface proteins found on nearly all gram-positive bacteria. Immediately upstream of this cell anchor region, SOF22 contains four tandem repeat sequence blocks, flanked by prolinerich segments. The repeats share up to 50% identity with a repeated motif found in other group A streptococcal Fn-binding proteins and exhibit Fn-binding activity, as shown by subcloning experiments. According to deletion analysis, the opacity domain is confined to the region N terminal to the repeat segment. Thus, SOF22 is unique among the known Fn-binding proteins from gram-positive bacteria in containing an independent module with a defined function in its N-terminal portion. Southern blot analysis with a probe from this N-terminal region indicates that the opacity domain of SOF varies extensively among different SOF-producing M types.

SUBMITTER: Rakonjac JV

PROVIDER: S-EPMC173041 | biostudies-other | 1995 Feb

REPOSITORIES: biostudies-other

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DNA sequence of the serum opacity factor of group A streptococci: identification of a fibronectin-binding repeat domain.

Rakonjac J V JV Robbins J C JC Fischetti V A VA

Infection and immunity 19950201 2

The serum opacity factor (SOF) is a group A streptococcal protein that induces opacity of mammalian serum. The serum opacity factor 22 gene (sof22) from an M type 22 strain was cloned from an EMBL4 library by screening for plaques exhibiting serum opacity activity. DNA sequencing yielded an open reading frame of 3,075 bp. Its deduced amino acid sequence predicts a protein of 1,025 residues with a molecular weight of 112,735, a size that approximates that of the SOF22 protein isolated from both t ...[more]

PMID: 7822031

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Project description:The group A Streptococcus (GAS) sof gene encodes the serum opacity factor protein, which is capable of opacifying mammalian sera and binding at least two host proteins, fibronectin and fibrinogen. The sof gene exists in approximately 50% of clinical isolates, and there is a classical association of so-called nephritogenic strains with the opacity factor-positive phenotype. In both a type emm49 strain and a type emm12 strain, the sequences upstream of the 5' end of sof and downstream of the putative terminator were determined to be nearly identical to a region in the M type 1 genome approximately 10 kb upstream of the emm1 gene. This close genetic linkage is likely reflected in the strict correlation of opacity factor phenotype with specific emm genotypes. A new fibronectin-binding protein gene, sfbX, was discovered immediately downstream of sof in emm12 and emm49 strains and in several other sof-positive strains. The sof and sfbX genes were found to be expressed on the same transcription unit, which was correlated with the putative promoter and rho-independant terminator sequences that flank these two genes. The sfbX genes from different emm types are predicted to encode approximately 650-residue surface-bound proteins sharing 89 to 92% sequence identity. SfbX residues approximately 1 to 480 are not highly similar to those of other known proteins, with the closest match being the Staphylococcus aureus coagulase protein. The remaining portions of these proteins (residues 481 to 650) contain four putative fibronectin-binding repeats highly similar to those of other streptococcal fibronectin-binding proteins and a potential LP(X)SG cell wall anchor motif. Targeted in-frame allelic-exchange mutagenesis, complementation, and heterologous-expression studies found that serum opacification is encoded by sof alone and that sfbX encodes a fibronectin-binding function. A recombinant SfbX protein was found to bind immobilized fibronectin and to partially inhibit GAS adherence to fibronectin. The sfbX gene was found to be present only in sof-positive strains, and together these genes could influence the spectrum of tissues colonized by sof-positive GAS.

Project description:Viridans Group Streptococci (VGS) species-level identification is fundamental for patients management. Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) has been used for VGS identification but discrimination within the Mitis group resulted difficult. In this study, VGS identifications with two MALDI-TOF instruments, the Biotyper (Bruker) and the VITEK MS (bioMérieux) have been compared to those derived from tuf, soda and rpoB genes sequencing. VGS isolates were clustered and a dendrogram constructed using the Biotyper 3.0 software (Bruker). RpoB gene sequencing resulted the most sensitive and specific molecular method for S. pneumonia identification and was used as reference method. The sensitivity and the specificity of the VITEK MS in S. pneumonia identification were 100%, while the Biotyper resulted less specific (92.4%). In non pneumococcal VGS strains, the group-level correlation between rpoB and the Biotyper was 100%, while the species-level correlation was 61% after database upgrading (than 37% before upgrading). The group-level correlation between rpoB and the VITEK MS was 100%, while the species-level correlation was 36% and increases at 69% if isolates identified as S. mitis/S. oralis are included. The less accurate performance of the VITEK MS in VGS identification within the Mitis group was due to the inability to discriminate between S. mitis and S. oralis. Conversely, the Biotyper, after the release of the upgraded database, was able to discriminate between the two species. In the dendrogram, VGS strains from the same group were grouped into the same cluster and had a good correspondence with the gene-based clustering reported by other authors, thus confirming the validity of the upgraded version of the database. Data from this study demonstrated that MALDI-TOF technique can represent a rapid and cost saving method for VGS identification even within the Mitis group but improvements of spectra database are still recommended.

Dataset Information

DNA sequence of the serum opacity factor of group A streptococci: identification of a fibronectin-binding repeat domain.

Publications

DNA sequence of the serum opacity factor of group A streptococci: identification of a fibronectin-binding repeat domain.

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