RNA cyclases are a family of RNA-modifying enzymes that are conserved in all cellular organisms. They catalyse the ATP-dependent conversion of the 3'-phosphate to the 2',3'-cyclic phosphodiester at the end of RNA, in a reaction involving formation of the covalent AMP-cyclase intermediate [1]. The structure of RTC demonstrates that RTCs are comprised two domain. The larger domain contains an insert domain of approximately 100 amino acids [1].

1: Crystal structure of RNA 3'-terminal phosphate cyclase, a ubiquitous enzyme with unusual topology. Palm GJ, Billy E, Filipowicz W, Wlodawer A; Structure Fold Des 2000;8:13-23. PMID:10673421