Updated project metadata. DarB is a cyclic di-AMP binding protein consisting of two CBS (Cystathionine-beta synthase) domains. To reveal the global role of the second-messenger, understanding the function of the receptor proteins is fundamental. In order to do so, we aimed to identify potential binding partners of DarB in vitro with an unbiased protein pulldown experiment. In this study, we identified the (p)ppGpp synthetase/ hydrolase RelA as the binding partner of DarB and could confirm these findings with an in vivo interaction experiment.