Dual-function enzyme acts as a global c-di-GMP sink and local anti sigma factor antagonist to drive cellular differentiation

Abstract

Soil bacteria of the genus Streptomyces are natural producers of over two-thirds of clinically used antibiotics. Their ability to generate these valuable metabolites is tightly linked to a developmental program involving the transition from vegetative hyphae to spores. The second messenger cyclic di-GMP (c-di-GMP) stabilizes effector complexes that block sporulation, including the RsiG-σWhiG complex leading to sequestration of the developmental sigma factor by its anti sigma factor. How signal termination and disruption of effector complexes is achieved to allow sporulation, remains poorly understood. Here, we identify the phosphodiesterase RmdB as a dual-function regulator that terminates c-di-GMP signaling both globally and locally. We show that deletion of the rmdB gene leads to increase of the global c-di-GMP pool and delayed development. Using genetic complementation, we demonstrate that both the EAL motif and the GGDEF domain are essential for the physiological function of RmdB. Our co-immunoprecipitation and co-elution assays revealed that RmdB interacts directly with the sigma factor σWhiG via its GGDEF domain, thus preventing binding of the anti sigma factor RsiG to σWhiG and promoting sporulation. Our bacterial two-hybrid analyses identify RmdB as an interaction hub connecting to multiple diguanylate cyclases (DGCs), including CdgE, which also interacts with σWhiG. These findings establish a novel principle of bacterial signaling in which a phosphodiesterase serves as an antagonist of an anti sigma factor, integrating global second messenger degradation with local effector complex formation to control cell fate decisions.