Enzymatic mechanism of an RNA ligase from wheat germ

We have characterized the mechanism of action of a wheat germ RNA ligase which has been partially purified on the basis of its ability to participate in in vitro splicing of yeast tRNA precursors (Gegenheimer, P., Gabius, H-J., Peebles, C.L., and Abelson, J. (1983) J. Biol. Chem. 258, 8365-8373). The preparation catalyzes the ligation of oligoribonucleotide substrates forming a 2'-phosphomonoester, 3',5'-phosphodiester linkage. The 5' terminus of an RNA substrate can have either a 5'-hydroxyl or a 5'-phosphate. The 5'-phosphate, which for a 5'-hydroxyl substrate can be introduced by a polynucleotide kinase activity in the preparation, is incorporated into the ligated junction. The 3' terminus can have either a 2',3'-cyclic phosphate or a 2'-phosphate. 2',3'-Cyclic phosphates can be converted into 2'-phosphates by a 2',3'-cyclic phosphate, 3'-phosphodiesterase activity in the preparation. The 2'-phosphate of the ligated product is derived from the phosphate at the 3' terminus of the substrate. Ligation proceeds with the adenylylation of the 5'-phosphorylated terminus to form an intermediate with a 5',5'-phosphoanhydride bond.