Kinetic framework for ligation by an efficient RNA ligase ribozyme

The class I RNA ligase ribozyme, isolated previously from random sequences, performs an efficient RNA ligation reaction. It ligates two substrate RNAs, promoting the attack of the 3'-hydroxyl of one substrate upon the 5'-triphosphate of the other substrate with release of pyrophosphate. This ligation reaction has similarities to the reaction catalyzed by RNA polymerases. Using data from steady-state kinetic measurements and pulse-chase/pH-jump experiments, we have constructed minimal kinetic frameworks for two versions of the class I ligase, named 207t and 210t. For both ligases, as well as for the self-ligating parent ribozyme, the rate constant for the chemical step (k(c)) is log-linear with pH in the range 5.7-8.0. At physiological pH, the k(c) is 100 min(-1), a value similar to these reported for the fastest naturally occurring ribozymes. At higher pH, product release is limiting for both 207t and 210t. The 210t ribozyme, with its faster product release, attains multiple-turnover rates (k(cat) = 360 min(-1), pH 9.0) exceeding those of 207t and other reported ribozyme reactions. The kinetic framework for the 210t ribozyme describes the limits of this catalysis and suggests how key steps can be targeted for improvement using design or combinatorial approaches.