Nucleic acid molecule having an RNA substrate cleaving enzymatic activity which cleaves a separate RNA substrate at a cleavage site. The nucleic acid molecule includes an RNA substrate binding portion which base pairs with the RNA substrate only 3′ of the cleavage site, and an enzymatic portion (which may include a part or all of the RNA substrate binding portion) having the enzymatic activity. The nucleic acid molecule is able to base pair with the RNA substrate only 3′ of the cleavage site, and cause cleavage of the RNA substrate at that cleavage site. The nucleic acid molecule can be either linear or circular. A general method for forming circular RNA in vivo and in vitro is provided.