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Streptococcal pyrogenic exotoxin B (SpeB) is an extracellular cysteine protease that is a critical virulence factor made by the major human pathogen group A Streptococcus (GAS). speB expression is dependent on the regulator of proteinase B (RopB) and is upregulated with increasing cell density and during infection. Because computer modelling suggested significant structural similarity between RopB and peptide‐sensing regulatory proteins made by other Gram‐positive bacteria, we hypothesized that speB expression is influenced by RopB–peptide interactions. Inactivation of the gene (vfr) encoding the virulence factor related (Vfr) protein resulted in increased speB transcript level during the exponential growth phase, whereas provision of only the amino‐terminal region of Vfr comprising the secretion signal sequence in trans restored a wild‐type speB expression profile. Addition of the culture supernatant from …