A fibronectin-binding protein from Streptococcus equisimilis: characterization of the gene and identification of the binding domain.

We have cloned and characterized a gene, fnb, from S. equisimilis, encoding a fibronectin-binding protein, FnB. A genomic library containing chromosomal DNA from S. equisimilis strain Se165 in the pUC18 vector in E. coli TG1 was screened using a DNA fragment of the gene fnbB from S. dysgalactiae strain S2 as probe. The complete gene was sequenced. The molecular mass of the protein, calculated from the deduced amino acid sequence is 120 kDa, which coincides with that determined by gel electrophoresis and Western blotting of the gene product. The fibronectin-binding activity was localized to a region of three repeated units, each 36 amino acids long. The COOH-terminal part of FnB from S. equisimilis, including the repeated Fn-binding domain, is very similar to the corresponding part of FnBB from S. dysgalactiae, indicating a common origin of the regions encoding Fn-binding activity of the respective genes.