c-di-GMP Arms an Anti-σ to Control Progression of Multicellular Differentiation in Streptomyces

Streptomyces are our primary source of antibiotics, produced concomitantly with the transition from vegetative growth to sporulation in a complex developmental life cycle. We previously showed that the signaling molecule c-di-GMP binds BldD, a master repressor, to control initiation of development. Here we demonstrate that c-di-GMP also intervenes later in development to control differentiation of the reproductive hyphae into spores by arming a novel anti-σ (RsiG) to bind and sequester a sporulation-specific σ factor (σ(WhiG)). We present the structure of the RsiG-(c-di-GMP)(2)-σ(WhiG) complex, revealing an unusual, partially intercalated c-di-GMP dimer bound at the RsiG-σ(WhiG) interface. RsiG binds c-di-GMP in the absence of σ(WhiG), employing a novel E(X)(3)S(X)(2)R(X)(3)Q(X)(3)D motif repeated on each helix of a coiled coil. Further studies demonstrate that c-di-GMP is essential for RsiG to inhibit σ(WhiG). These findings reveal a newly described control mechanism for σ-anti-σ complex formation and establish c-di-GMP as the central integrator of Streptomyces development.