The complete nucleotide sequence of the cya gene from E. coli was determined. The gene encodes a polypeptide consisting of 848 amino acid residues with a calculated molecular weight of 97,542. The deduced protein structure reveals that cyclase is comprised of two domains, an amino-terminal region exhibiting catalytic activity and a carboxy-terminal region possibly carrying regulatory function. The frequent appearance of rare codons in the beginning of the gene as well as the sequence duplication in the promoter-initiator region suggest possible regulation(s) at the translational level. An unknown gene (cyaX) which seems to code for a very hydrophobic protein was found following the cya gene. Sequence analysis suggests that the cyax is a part of the cya operon.