InterPro: IPR000228 RNA 3'-terminal phosphate cyclase-like

RNA cyclases are a family of RNA-modifying enzymes that are conserved in eukaryotes, bacteria and archaea. RNA 3'-terminal phosphate cyclase (EC:6.5.1.4) [1, 2] catalyses the conversion of 3'-phosphate to a 2',3'-cyclic phosphodiester at the end of RNA.

ATP + RNA 3'-terminal-phosphate = AMP + diphosphate + RNA terminal-2',3'-cyclic-phosphate

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 [3]. 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 [3].