A‐factor as a microbial hormone that controls cellular differentiation and secondary metabolism in Streptomyces griseus

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Abstract
A‐factor, containing a γ‐butyrolactone in its structure, is an autoregulatory factor or a‘microbiai hormone’controlling secondary metabolism and cellular differentiation in Streptomyces griseus. A‐factor exerts its regulatory role by binding to a specific receptor protein which, in the absence of A‐factor, acts as a repressor‐type regulator for morphological and physiological differentiation, in the signal relay leading to streptomycin production in S. griseus, the A‐factor signal is transferred from the A‐factor receptor to the upstream activation sequence of a regulatory gene, strR, in the streptomycin biosynthetic gene cluster via an A‐factor‐dependent protein that serves as a transcription factor for strR. The StrR protein thus Induced appears to activate the transcription of other streptomycin‐production genes. The presence of A‐factor homologues in a wide variety of Streptomyces species and distantly related bacteria implies the generality of γ‐butyrolactones as chemical cellular signalling molecules in microorganisms.