A complex role for the gamma-butyrolactone SCB1 in regulating antibiotic production in Streptomyces coelicolor A3(2).

Many streptomycetes produce extracellular gamma-butyrolactones. In several cases, these have been shown to act as signals for the onset of antibiotic production. Synthesis of these molecules appears to require a member of the AfsA family of proteins (AfsA is required for A-factor synthesis of the gamma-butyrolactone A-factor and consequently for streptomycin production in Streptomyces griseus). An afsA homologue, scbA, was identified in Streptomyces coelicolor A3(2) and was found to lie adjacent to a divergently transcribed gene, scbR, which encodes a gamma-butyrolactone binding protein. Gel retardation assays and DNase I footprinting studies revealed DNA binding sites for ScbR at - 4 to - 33 nt with respect to the scbA transcriptional start site, and at - 42 to - 68 nt with respect to the scbR transcriptional start site. Addition of the gamma-butyrolactone SCB1 of S. coelicolor resulted in loss of the DNA-binding ability of ScbR. A scbA mutant produced no gamma-butyrolactones, yet overproduced two antibiotics, actinorhodin (Act) and undecylprodigiosin (Red), whereas a deletion mutant of scbR also failed to make gamma-butyrolactones and showed delayed Red production. These phenotypes differ markedly from those expected by analogy with the S. griseus A-factor system. Furthermore, transcription of scbR increased, and that of scbA was abolished, in an scbR mutant, indicating that ScbR represses its own expression while activating that of scbA. In the scbA mutant, expression of both genes was greatly reduced. Addition of SCB1 to the scbA mutant induced transcription of scbR, but did not restore scbA expression, indicating that the deficiency in scbA transcription in the scbA mutant is not solely due to the inability to produce SCB1, and that ScbA is a positive autoregulator in addition to being required for gamma-butyrolactone production. Overall, these results indicate a complex mechanism for gamma-butyrolactone-mediated regulation of antibiotic biosynthesis in S. coelicolor.