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A clonal variant of serotype M1 group A streptococcus, strain 90-131, disseminated to several continents, where it was associated with severe systemic infections and toxic shock. Although this strain harbours the speA gene and is efficiently internalized by human epithelial cells, clinical isolates often fail to express the erythrogenic toxin under laboratory growth conditions. Cultures of strain 90-131 were observed to phase vary between small, dry, compact and larger, more mucoid colonies. The former were shown to be poorly internalized by epithelial cells. Analysis of RNA by Northern hybridization demonstrated that the emm1, hasA and speA genes were weakly transcribed in cultures derived from the small colonies and highly transcribed in those derived from the large colonies. An insertion mutation in mga (the multigene activator) downregulated the invasion of epithelial cells and the transcription of emm1 and hasA, but had little impact on the transcription of speA. These are the first data to suggest the existence of a common regulatory circuit linking intracellular invasion, M protein, hyaluronic acid capsule and erythrogenic toxin expression by group A streptococcus. Moreover, the genetic instability of toxin expression exhibited by this serotype may impact on laboratory studies that attempt to associate toxin production with toxic shock.