RNA cyclases are a family of RNA-modifying enzymes that are conserved in eukaryotes, bacteria and archaea. RNA 3'-terminal phosphate cyclase [9184239, 2199762] 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
These enzymes might be responsible for production of the cyclic phosphate RNA ends that are known to be required by many RNA ligases in both prokaryotes and eukaryotes.
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 insert domain disrupts the large domain, and uses a similar secondary structure element with different topology, observed in many other proteins such as thioredoxin . 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 .
This entry represents the small insert domain that interrupts the large repetitive domain.