Structure and function of diadenylate cyclase DacM from Mycoplasma ovipneumoniae

Cyclic diadenosine monophosphate (c-di-AMP) is a second-messenger nucleotide that is produced by many bacteria. C-di-AMP can not only regulate bacterial growth, cell-wall homeostasis, ion transport and gene transcription, but can also be recognized by multiple sensor / receptor proteins in infected host cells to trigger an innate immune response. Mycoplasma ovipneumoniae causes non-progressive pneumonia in both sheep and goats. Here, we analyzed c-di-AMP signaling in M. ovipneumoniae, which is a genome-reduced obligately pathogenic bacterium. Our results demonstrate that these bacteria can produce c-di-AMP, and we could identify the diadenylate cyclase, which was named DacM. The enzyme was found to utilize both ATP and ADP to synthesize c-di-AMP, resembling CdaM from a novel family of diadenylate cyclases first found in Mycoplasma pneumoniae. Furthermore, we present the crystal structures of DacM in the apo state and substrate-bound state at 3 [A] and 1.9 [A] resolution, respectively. Mutation of residues Asp112, Gly113, Tyr128, Phe129, and Arg143 surrounding the active sites to Ala were lethal to DacM enzymatic activity. These structures provide valuable insights into the biochemistry of c-di-AMP, and offer a basis for the structure-based design of new drugs for animal husbandry.