Mutational and Comparative Analysis of Streptolysin O, an Oxygen-labile Streptococcal Hemolysin

Abstract

The structural gene of streptolysin O was cloned from Streptococcus pyogenes strain Sa and S. equisimilis H46A, and the nucleotide sequences were compared with those of strain Richards. To obtain the minimal active fragment of the toxin and to elucidate structure-function relationships in hemolytic function, streptolysin O mutants deleted in N- and C-terminal regions were constructed. Internal amino acid residues were also replaced by introduction of point mutations. Analyses of these mutants showed that considerable activity was retained even after deletion of the N-terminal 107 residues, but genetic removal of the ultimate C-terminal residue resulted in a marked decrease in hemolytic function. By removal in succession, hemolytic activity declined exponentially, and only 0.002% of the activity remained after deletion of the C-terminal four residues. Nucleotide replacement experiments indicated pivotal roles of I₂₀₂, V₂₁₇, D₃₂₄-L₃₂₅, V₃₃₉, and H₄₆₉ residues in hemolysis.