Figure 2.

The cleavage and ligation mechanism of small nucleolytic ribozymes. The 2′ OH attacks nucleophilically the neighbouring 3′,5′-phosphodiester bond. Upon passing through a trigonal bipyramidal transition state, the cleavage reaction yields a 5′ product with a 2′,3′-cyclic phosphate and a 3′ product with a 5′ OH. In the reverse ligation reaction, this 5′ hydroxyl group attacks the 2′,3′-cyclic phosphate, and, passing through the same transition state, the two RNAs are joined by a conventional 3′, 5′ phosphodiester.