Sequence logos of the catalytic region from each of the nucleotide cyclase classes. The sequences in each class were aligned using PROBE with a purge value of 500 to obtain the class models. The average information at conserved positions for each class model (all motifs) is class 1 (A): 1.234 bits, class 2 (B): 1.366 bits, class 3 (C): 1.458 bits, class 4 (D): 1.264 bits, and class 5 (E): 1.096 bits. The class sequences were scanned with the class models at a purge value of 1000 to make the class logos. The class models are aligned vertically with respect to the catalytic asparagine and arginine. A residue that contributes to substrate specificity is indicated with +, and the catalytic asparagine and arginine are indicated with * (see text). The significance of several highly conserved residues amino-terminal to the catalytic asparagine and arginine in A, B, and C is unknown and is likely because these classes consist mainly of closely related sequences.