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Journal of Bacteriology, February 2003, p. 1208-1217, Vol. 185, No. 4
0021-9193/03/$08.00+0     DOI: 10.1128/JB.185.4.1208-1217.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Molecular Genetic Analysis of a Group A Streptococcus Operon Encoding Serum Opacity Factor and a Novel Fibronectin-Binding Protein, SfbX

Arthur Jeng,1 Varja Sakota,2 Zhongya Li,2 Vivekananda Datta,1 Bernard Beall,2* and Victor Nizet1*

Department of Pediatrics, Division of Pediatric Infectious Diseases, University of California, San Diego, School of Medicine, La Jolla, California,1 Respiratory Diseases Branch, Division of Bacterial and Mycotic Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia2

Received 17 July 2002/ Accepted 22 November 2002

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 ~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.

* Corresponding author. Mailing address for Bernard Beall: Respiratory Diseases Branch, Division of Bacterial and Mycotic Diseases, Centers for Disease Control and Prevention, 1600 Clifton Rd., N.E., MS-C02, Atlanta, GA 30333. Phone: (404) 639-1237. Fax: (404) 639-3123. E-mail: bbeall{at}cdc.gov.

* Corresponding author. Mailing address for Victor Nizet: Division of Infectious Diseases, 0672, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093. Phone: (858) 534-7408. Fax: (858) 534-7411. E-mail: vnizet{at}ucsd.edu.

Journal of Bacteriology, February 2003, p. 1208-1217, Vol. 185, No. 4
0021-9193/03/$08.00+0     DOI: 10.1128/JB.185.4.1208-1217.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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