-!- The enzyme converts the 3'-terminal phosphate of various RNA substrates into the 2',3'-cyclic phosphodiester in an ATP-dependent reaction. -!- Catalysis occurs by a three-step mechanism, starting with the activation of the enzyme by ATP, forming a phosphoramide bond between adenylate and a histidine residue. -!- The adenylate group is then transferred to the 3'-phosphate terminus of the substrate, forming the capped structure (RNA)- 3'-(5'-diphosphoadenosine). -!- Finally, the enzyme catalyzes an attack of the vicinal O-2' on the 3'-phosphorus, which results in formation of cyclic phosphate and release of the adenylate. -!- The enzyme also has a polynucleotide 5' adenylation activity. -!- Cf. EC 6.5.1.5.