Reversal of T4 RNA ligase

Abstract
Unexpected products detected in oligoribonucleotide synthesis reactions catalyzed by T4 RNA ligase are a result of a partial reversal of the enzyme reaction. A transfer assay for the reversal of the 3rd step in the RNA ligase reaction mechanism and an exchange assay for the reversal of both the 2nd and 3rd steps are described. Reversal is confirmed by the formation of the expected covalent intermediates, adenylylated donor and adenylylligase, from a reaction containing 5''-AMP, unadenylylated ligase and the tetranucleotide (Ap)3Cp. In the reverse reaction, RNA ligase shows a strong preference for hydrolysis of the 3''-terminal phosphodiester bonds of oligoribonucleotides which terminate in a 3''-phosphate. Several strategies are discussed to minimize the effects of reversal in the enzymatic synthesis of oligoribonucleotides.