Class III nucleotidyl cyclases are the largest, most diverse group of nucleotidyl cyclases (NC's) containing prokaryotic and eukaryotic proteins. They can be divided into two major groups; the mononucleotidyl cyclases (MNC's) and the diguanylate cyclases (DGC's). The MNC's, which include the adenylate cyclases (AC's) and the guanylate cyclases (GC's), have a conserved cyclase homology domain (CHD), while the DGC's have a conserved GGDEF domain, named after a conserved motif within this subgroup. Their products, cyclic guanylyl and adenylyl nucleotides, are second messengers that play important roles in eukaryotic signal transduction and prokaryotic sensory pathways.
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