In enzymology, a RNA-3â²-phosphate cyclase () is an enzyme that catalyzes the chemical reaction
Thus, the two substrates of this enzyme are ATP and RNA 3'-terminal-phosphate, whereas its 3 products are AMP, diphosphate, and RNA terminal-2',3'-cyclic-phosphate.
This enzyme belongs to the family of ligases, specifically those forming phosphoric-ester bonds. The systematic name of this enzyme class is RNA-3'-phosphate:RNA ligase (cyclizing, AMP-forming). This enzyme is also called RNA cyclase.
As of 2010, three structures have been solved for this class of enzymes, with PDB accession codes and , (un-adenylated) and (adenylated).