Very little knowledge is available about the occurrence and function of cyclic nucleotides as second messengers in Archaea. We have demonstrated that the extremely halophilic archaeon Haloferax volcanii produces c-di-AMP and now want to understand its function in this organism. First experiments hint at a function in osmoregulation as an H. volcanii strain producing less c-di-AMP swells up under hyposalt conditions (Braun et al., 2019). Moreover, the euryarchaeal diadenylate cyclase exhibits an additional N-terminal domain otherwise found in pyruvate kinases. We want to study the influence of this domain on the function of the diadenylate cyclase.