RNA cyclase is a protein of from 36 to 42 kDa. The best conserved region is a glycine-rich stretch of residues located in the central part of the sequence and which is reminiscent of various ATP, GTP or AMP glycine-rich loops.

The crystal structure of RNA 3'-terminal phosphate cyclase shows that each molecule consists of two domains. The larger domain contains three repeats of a folding unit comprising two parallel alpha helices and a four-stranded beta sheet; this fold was previously identified in translation initiation factor 3 (IF3). The large domain is similar to one of the two domains of 5-enolpyruvylshikimate-3-phosphate synthase and UDP-N-acetylglucosamine enolpyruvyl transferase. The smaller domain uses a similar secondary structure element with different topology, observed in many other proteins such as thioredoxin [PMID:10673421]. Although the active site of this enzyme could not be unambiguously assigned, it can be mapped to a region surrounding His309, an adenylate acceptor, in which a number of amino acids are highly conserved in the enzyme from different sources [PMID:10673421].