FIG 1.

Enzymes involved in the turnover of second messengers cyclic di-GMP (A) and cyclic di-AMP (B). GGDEF domain proteins are cyclic di-GMP synthases (2), but a few members can preferentially synthesize cyclic GMP-AMP (31). Cyclic di-GMP is degraded by EAL domain and HD-GYP domain phosphodiesterases into 5′-pGpG and GMP, respectively (66). 5′-pGpG is further hydrolyzed to GMP by the oligoribonuclease Orn. Cyclic di-AMP is synthesized by the DAC domain, and hydrolysis by HD and DHH/DHHA1 domain proteins has been demonstrated. Major phenotypes affected upon cyclic dinucleotide synthesis in many bacteria are indicated.