NAD+-dependent synthesis of a 5'-phospho-ADP-ribosylated RNA/DNA cap by RNA 2'-phosphotransferase Tpt1.

RNA 2'-phosphotransferase Tpt1 converts an internal RNA 2'-monophosphate to a 2'-OH via a two-step NAD+-dependent mechanism in which: (i) the 2'-phosphate attacks the C1″ of NAD+ to expel nicotinamide and form a 2'-phospho-ADP-ribosylated RNA intermediate; and (ii) the ADP-ribose O2″ attacks the phosphate of the RNA 2'-phospho-ADPR intermediate to expel the RNA 2'-OH and generate ADP-ribose 1″-2″ cyclic phosphate. Tpt1 is an essential component of the fungal tRNA splicing pathway that generates a unique 2'-PO4, 3'-5' phosphodiester splice junction during tRNA ligation. The wide distribution of Tpt1 enzymes in taxa that have no fungal-type RNA ligase raises the prospect that Tpt1 might catalyze reactions other than RNA 2'-phosphate removal. A survey of Tpt1 enzymes from diverse sources reveals that whereas all of the Tpt1 enzymes are capable of NAD+-dependent conversion of an internal RNA 2'-PO4 to a 2'-OH (the canonical Tpt1 reaction), a subset of Tpt1 enzymes also catalyzed NAD+-dependent ADP-ribosylation of an RNA or DNA 5'-monophosphate terminus. Aeropyrum pernix Tpt1 (ApeTpt1) is particularly adept in this respect. One-step synthesis of a 5'-phospho-ADP-ribosylated cap structure by ApeTpt1 (with no subsequent 5'-phosphotransferase step) extends the repertoire of the Tpt1 enzyme family and the catalogue of ADP-ribosylation reactions involving nucleic acid acceptors.