Structural basis for the c-di-AMP - dependent regulation of the stringent response by DarB

The bacterial second messenger c-di-AMP controls various cellular processes including potassium and osmolyte homeostasis. The c-di-AMP receptor protein DarB of Bacillus subtilis binds to the Rel protein and triggers the Rel-dependent stringent response. Here we report crystal structures of DarB in the ligand-free state and of DarB complexes with c-di-AMP, 3´3´-cGAMP and AMP. DarB consists of two CBS domains and forms a homo-dimer with a parallel, head-to-head assembly of the monomers. The DarB dimer binds two cyclic di-nucleotide molecules or two AMP molecules. Only one adenine of bound c-di-AMP is specifically recognized by DarB, while the second one protrudes out of the donut-shaped protein. This enables DarB to bind also 3´3´-cGAMP, as only the adenine fits to the active site, but not the guanine. In absence of c-di-AMP DarB binds to Rel and stimulates (p)ppGpp synthesis, whereas the presence of c-di-AMP abolishes the interaction. The DarB crystal structures reveal no conformational changes upon c-di-AMP binding, hence, the regulatory function of DarB on Rel must be controlled directly by the bound c-di-AMP. A structural model of the DarB-Rel complex was erived from in silico docking, validated with a mass spectrometric analysis of the chemically cross-linked DarB-Rel complex and mutagenesis studies. Based on the predicted complex structure a mechanism of stringent response regulation by c-di-AMP is suggested.