Redox control of antibiotic biosynthesis in Streptomyces formicae

Streptomyces bacteria make diverse specialised metabolites that form the basis of ~55% of clinically used antibiotics. Despite this, only 3% of their encoded specialised metabolites have been matched to molecules and understanding how their biosynthesis is controlled is essential to fully exploit their potential. Here we use Streptomyces formicae and the formicamycin biosynthetic pathway as a model to understand the complex regulation of specialised metabolism. We analysed all three pathway-specific regulators and found that biosynthesis is subject to negative feedback and redox control via two MarR-family proteins while activation of the pathway is dependent on a cytoplasmic two-component system. Like many Streptomyces antibiotics, formicamycins are only produced in solid culture and biosynthesis is switched off in aerated liquid cultures. Here, we demonstrate that a redox-sensitive repressor named ForJ senses oxygen via a single cysteine residue that is required to repress formicamycin biosynthesis in liquid cultures.