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Streptococcal toxic shock syndrome (STSS) is primarily caused by Streptococcus pyogenes, but it may also be caused by Streptococcus dysgalactiae subsp. equisimilis (SDSE). The analyses of S. pyogenes have revealed the important roles of NAD+‐glycohydrolase (Nga) and CovR/CovS, a two‐component regulatory system. We examined these factors in SDSE by analyzing mainly two isogenic SDSE strains (12‐10‐1 and 12‐10‐3) from the blood of a patient with STSS. The Nga activities were measured and the nucleotide sequences of covR and covS genes were determined. We detected one nucleotide difference between the covR gene of 12‐10‐1 and that of 12‐10‐3, and the Nga activity of 12‐10‐1 was approximately 6.8‐fold more than that of 12‐10‐3. The introduction of covR of 12‐10‐3 into 12‐10‐1 significantly reduced the Nga activity, but the introduction of 12‐10‐1 covR into itself had only a little effect. In addition, the knockout of covR or covS of 12‐10‐3 remarkably increased the Nga activity. We are the first to report that strains with wild‐type and mutated covR were isolated simultaneously from an SDSE STSS patient and that the CovR/CovS two‐component regulatory system is involved in the Nga activity in SDSE as well as in S. pyogenes. [ABSTRACT FROM AUTHOR]