Differential contributions of two SARP family regulatory genes to indigoidine biosynthesis in Streptomyces lavendulae FRI-5

The Streptomyces antibiotic regulatory protein (SARP) family regulators have been shown to control the production of secondary metabolites in many Streptomyces species as the most downstream regulators in the regulatory cascade. Streptomyces lavendulae FRI-5 produces a blue pigment (indigoidine) together with two types of antibiotics: D-cycloserine and the nucleoside antibiotics. The production of these secondary metabolites is governed by a signaling system consisting of a gamma-butyrolactone, IM-2 [(2R,3R,1'R)-2-1'-hydroxybutyl-3-hydroxymethyl-gamma-butanolide], and its cognate receptor, FarA. Here, we characterized two regulatory genes of the SARP family, farR3 and farR4, which are tandemly located in the proximal region of farA. farR3 is transcribed both as a monocistronic RNA and as a bicistronic farR4-farR3 mRNA, and the expression profile is tightly controlled by the IM-2/FarA system. Loss of farR3 delayed and decreased the production of indigoidine without any changes in the transcriptional profile of other far regulatory genes, indicating that FarR3 positively controls the biosynthesis of indigoidine and is positioned in the downstream region of the IM-2/FarA signaling system. Meanwhile, loss of farR4 induced the early production of IM-2 by increasing transcription of an IM-2 biosynthetic gene, farX, indicating that FarR4 negatively controls the biosynthesis of IM-2. Thus, our results suggested differential contributions of the SARP family regulators to the regulation of secondary metabolism in S. lavendulae FRI-5. This is the first report to show that an SARP family regulator is involved in the biosynthesis of a signaling molecule functioning at the most upstream region of the regulatory cascade for Streptomyces secondary metabolism.