The many roles of cyclic di-AMP to control the physiology of .

The dinucleotide cyclic di-AMP (c-di-AMP) is synthesized as a second messenger in the Gram-positive model bacterium as well as in many bacteria and archaea. possesses three diadenylate cyclases and two phosphodiesterases that synthesize and degrade the molecule, respectively. Among the second messengers, c-di-AMP is unique since it is essential for on the one hand but toxic upon accumulation on the other. This role as an "essential poison" is related to the function of c-di-AMP in the control of potassium homeostasis. C-di-AMP inhibits the expression and activity of potassium uptake systems by binding to riboswitches and transporters and activates the activity of potassium exporters. In this way, c-di-AMP allows the adjustment of uptake and export systems to achieve a balanced intracellular potassium concentration. C-di-AMP also binds to two dedicated signal transduction proteins, DarA and DarB. Both proteins seem to interact with other proteins in their apo state, i.e. in the absence of c-di-AMP. For DarB, the (p)ppGpp synthetase/hydrolase Rel and the pyruvate carboxylase PycA have been identified as targets. The interactions trigger the synthesis of the alarmone (p)ppGpp and of the acceptor molecule for the citric acid cycle, oxaloacetate, respectively. In the absence of c-di-AMP, many amino acids inhibit the growth of . This feature can be used to identify novel players in amino acid homeostasis. In this review, we discuss the different functions of c-di-AMP and their physiological relevance.