Purification and cloning of a streptokinase from Streptococcus uberis

A bovine plasminogen activator was purified from the culture supernatant of the bovine pathogen Streptococcus uberis NCTC 3858. After the final reverse-phase high-performance liquid chromatography step a single protein with a molecular mass of 32 kDa was detected in the active fraction. A partial peptide map was established, and degenerate primers were designed and used for amplification of fragments of the gene encoding the activator. Inverse PCR was subsequently used for obtaining the full-length gene. The S. uberis plasminogen activator gene (skc) encodes a protein consisting of 286 amino acids including a signal peptide of 25 amino acids. In an amino acid sequence comparison the cloned activator showed an identity of approximately 26% to the streptokinases isolated from Streptococcus equisimilis and Streptococcus pyogenes. Interestingly, the activator from S. uberis was found to lack the C-terminal domain possessed by the streptokinase from S. equisimilis. This is apparently a general feature of the streptokinases of this species; biochemical and genetic analysis of 10 additional strains of S. uberis revealed that 9 of these were highly similar to strain NCTC 3858. Sequencing of the skc gene from three of these strains indicated that the amino acid sequence of the protein is highly conserved within the species.