A GAF-domain-regulated adenylyl cyclase from Anabaena is a self-activating cAMP switch

The gene cyaB1 from the cyanobacterium Anabaena sp. PCC 7120 codes for a protein consisting of two N‐terminal GAF domains (GAF‐A and GAF‐B), a PAS domain and a class III adenylyl cyclase catalytic domain. The catalytic domain is active as a homodimer, as demonstrated by reconstitution from complementary inactive point mutants. The specific activity of the holoenyzme increased exponentially with time because the product cAMP activated dose dependently and nucleotide specifically (half‐maximally at 1 μM), identifying cAMP as a novel GAF domain ligand. Using point mutants of either the GAF‐A or GAF‐B domain revealed that cAMP activated via the GAF‐B domain. We replaced the cyanobacterial GAF domain ensemble in cyaB1 with the tandem GAF‐A/GAF‐B assemblage from the rat cGMP‐stimulated phosphodiesterase type 2, and converted cyaB1 to a cGMP‐stimulated adenylyl cyclase. This demonstrated the functional conservation of the GAF domain ensemble since the divergence of bacterial and eukaryotic lineages >2 billion years ago. In cyanobacteria, cyaB1 may act as a cAMP switch to stabilize committed developmental decisions.