General scheme of production, degradation, mechanism of action, and physiological target processes of the second messenger c-di-GMP. The figure introduces current concepts of c-di-GMP signaling and serves as a guideline, rather than reflecting all the complexities of the many systems currently under study. For instance, while many DGCs and PDEs are membrane attached via their N-terminal sensory domains, others are soluble in the cytoplasm. For further details, see the text.