This entry represents KptA, a putative RNA 2'-phosphotransferase. KptA, a functional Tpt1 protein homologue from Escherichia coli is strikingly similar to yeast Tpt1 in its kinetic parameters, although E. coli is not known to have a 2'-phosphorylated RNA substrate [, ].
This entry includes Tpt1 and its homologues from all domains of life. Tpt1 was first discovered as an essential component of the fungal tRNA splicing pathway, which characteristically generates a 2'-PO4, 3'-5' phosphodiester splice junction during the tRNA ligation reaction []. It is an enzyme that catalyzes the transfer of an internal RNA 2'-monophosphate (2'-PO4) to NAD+ to form a 2'-OH RNA, ADP-ribose 1"-2"cyclic phosphate, and nicotinamide []. Interestingly, many Tpt1 homologues have no obvious need for an RNA 2'-phosphotransferase because: (i) they lack tRNA introns; and/or (ii) they are not known to have an enzymatic pathway that generates 2'-PO4, 3'-5' phosphodiester RNA structures. There are more evidence showing that Tpt1 homologues are not limited to the canonical activity of Tpt1 healing the 2'-PO4, 3'-5' phosphodiester RNA splice junction formed during fungal and plant tRNA splicing []. Instead, the ADP-ribosyl transfer to a phosphorylated substrate is the unifying mechanistic feature of Tpt1-catalyzed reactions. For instance, some Tpt1 homologues have been shown to catalyze the transfer of ADP-ribose from NAD+ to a 5'-monophosphate end of RNA or DNA to install a 5'-phospho-ADP-ribose cap structure [].
