Production of the antibiotic FR-008/candicidin in Streptomyces sp. FR-008 is co-regulated by two regulators, FscRI and FscRIV, from different transcription factor families.

In Streptomyces sp. FR-008, the biosynthetic gene cluster of the polyene antibiotic FR-008, also known as candicidin, consists of 21 genes, including four regulatory genes, fscRI-fscRIV. Our bioinformatics analyses indicate that FscRI has an N-terminal PAS domain, whereas the other three regulators have N-terminal AAA domains and are members of the LAL (large ATP-binding regulators of the LuxR type) family. Deletion of fscRI abolished the production of FR-008, with production restored in the complemented strain, supporting a critical role for FscRI in FR-008 biosynthesis. Consistent with these findings, transcription of genes involved in the biosynthesis and efflux of FR-008 was greatly downregulated in a ΔfscRI mutant. Interestingly, the regulatory gene fscRIV was also downregulated in the ΔfscRI mutant. Production of FR-008 was reduced, but not abrogated, in an fscRIV deletion mutant, and although structural genes were downregulated in ΔfscRIV, the changes were much less dramatic than in ΔfscRI, suggesting a stronger regulatory role for FscRI. Remarkably, transcription of fscRI was also decreased in ΔfscRIV. Expression of fscRI restored antibiotic production in a ΔfscRIV mutant, but not vice versa. Putative binding sequences for FscRI were identified upstream of fscRIV and the three structural genes fscA, fscB and fscD, which encode large modular polyketide synthases. Our findings suggest that fscRI and fscRIV are interregulatory, whereas expression of fscRII and fscRIII appears to be independent of fscRI and fscRIV. This study demonstrates that the regulation of polyene antibiotic synthesis can involve mutually regulated transcriptional activators that belong to different families.