Tpt1 catalyzes the last step of tRNA splicing in yeast. It transfers the splice junction 2'-phosphate from ligated tRNA to NAD, to produce ADP-ribose 1"-2"-cyclic phosphate. This is presumed to be followed by a transesterification step to release the RNA. The first step of this reaction is similar to that catalyzed by some bacterial toxins. E. coli KptA and mouse Tpt1 are likely to use the same reaction mechanism.