afsB stimulates transcription of the actinorhodin biosynthetic pathway in Streptomyces coelicolor A3(2) and Streptomyces lividans.

The pleiotropic regulatory gene, afsB, from Streptomyces coelicolor A3(2), possibly encoding a DNA-binding protein, is required for actinorhodin production in this organism. Northern blot hybridization using a DNA fragment covering part of the set of cloned actinorhodin biosynthetic gene cluster (act) as the probe showed lack of the act transcripts in an afsB-negative mutant of S. coelicolor A3(2); the transcripts were restored on introduction of a cloned afsB gene. Introduction of the cloned afsB gene into Streptomyces lividans stimulated transcription of the act genes under conditions in which they are normally silent in this strain, leading to production of actinorhodin in large quantity. These data show that afsB exerts its positive regulatory effect by means of transcriptional stimulation of its target genes.