Genetic separation of serum opacity factor from M protein of group A streptococci.

Mutagenized cultures of three strains of group A streptococci which were M positive and produced the extracellular serum opacity factor (OF) were screened for mutants which failed to exhibit OF activity on serum agar plates. All cell fractions from three mutants studied in detail, including culture supernatants, protoplast membranes, and cytoplasmic fractions, were completely devoid of activity. Two OF- isolates also lacked OF antigen, whereas the third continued to produce cross-reacting antigen. All three mutants were resistant to phagocytosis and yielded acid-extractable M antigen, indicating that the M protein of each strain was unaltered by the mutations. The separation of the OF+ and M+ phenotypes by mutation establishes that genes which code for the M protein and OF are distinct; therefore, the activities and antigenic determinants of each must be associated with separate, distinct protein molecules.