Activation of the SoxR Regulon in Streptomyces coelicolor by the Extracellular Form of the Pigmented Antibiotic Actinorhodin

The redox-sensitive transcription factor SoxR in enteric bacteria senses and regulates the cellular response to superoxide and nitric oxide. In other bacterial groups, however, it may respond to redox-active small molecules, as demonstrated for pyocyanin sensing in pseudomonads. The antibiotic-producing soil bacterium Streptomyces coelicolor contains a gene for an SoxR homologue (SCO1697) whose DNA recognition helix is identical to that of Escherichia coli SoxR. Using the E. coli SoxR binding sequence, we predicted five candidate genes of the SoxR regulon and demonstrated that SoxR binds to their promoter regions and activates their expression concurrently with the production of the blue antibiotic actinorhodin (a benzoisochromanequinone). These genes encode a probable NADPH-dependent flavin reductase (SCO2478), an NADPH-dependent quinone reductase (SCO4266), an ABC transporter (SCO7008), a monooxygenase (SCO1909), and a hypothetical protein (SCO1178). Addition of actinorhodin to exponentially growing cells activated the expression of SoxR target genes in an SoxR-dependent manner. The secreted gamma-actinorhodin was over 10-fold more effective in activation than the intracellular form of actinorhodin, suggesting that SoxR is specified to respond more to exogenous signals than to intracellular metabolites. The Delta soxR mutant was not compromised in resistance against oxidants but was slow in forming aerial mycelium on R2YE medium with reduced sporulation, and its production of actinorhodin and undecylprodigiosin was lowered by about 50% and 30%, respectively, compared to that of the wild type. These results support the proposal that SoxR senses redox-active molecules, such as actinorhodin in S. coelicolor, and induces a protective function against them. It also functions to ensure that cells undergo optimal differentiation and secondary metabolite production.