-!- A widely distributed family of related enzymes found in protozoans, plants, bacteria, animals and viruses that cleave ssRNA 3'-phosphate group with little base specificity. -!- The enzyme catalyzes a two-stage endonucleolytic cleavage. -!- The first reaction produces 5'-hydroxy-phosphooligonucletides and 3'-phosphooligonucleotides ending with a 2',3'-cyclic phosphodiester, which are released from the enzyme. -!- The enzyme then hydrolyzes the cyclic products in a second reaction that takes place only when all the susceptible 3',5'-phosphodiester bonds have been cyclised. -!- The second reaction is a reversal of the first reaction using the hydroxyl group of water instead of the 5'-hydroxyl group of ribose. -!- The overall process is that of a phosphorus-oxygen lyase followed by hydrolysis to form the 3'-nucleotides. -!- Formerly EC 2.7.7.17, EC 3.1.4.23, and EC 3.1.27.1.