The trigger enzyme system of Escherichia coli. The global c-di-GMP pool is partially controlled by the DGC YegE and the PDE YhjH. When the c-di-GMP pool is low, the trigger enzyme and PDE YciR bind the DGC YdaM and its target, the transcription factor MlrA (bold arrows), preventing YdaM activation of MlrA. When the c-di-GMP pool is high, YciR degrades c-di-GMP and releases YdaM and MlrA in the process, leading to YdaM activation of MlrA and further contribution to the global c-di-GMP pool (blue arrow). Adapted from Lindenberg et al. (38).