Abstract: A prototype RNA cyclase ribozyme that allows efficient production of circular RNA. Methods for modifying the prototype to produce a wide variety of custom circular RNA are detailed. The method utilizes a new plasmid which enables production of a wide variety of imaginable RNA sequences in a covalent, circular form free from intron sequences in vitro. At a particular site in the plasmid, a sequence coding for the desired circular RNA is inserted to create a new RNA cyclase ribozyme gene. RNA transcribed from RNA cyclase ribozyme genes autocatalytically converts the desired RNA sequence it contains into circular form. RNA cyclase genes may be placed into appropriate expression vectors for synthesis of circular RNA in vivo as part of ribozyme or antisense gene regulation approaches to genetic engineering.