Biogenesis and functions of aminocarboxypropyluridine in tRNA

Transfer (t)RNAs contain a wide variety of post-transcriptional modifications, which play critical roles in tRNA stability and functions. 3-(3-amino-3-carboxypropyl)uridine (acp³U) is a highly conserved modification found in variable- and D-loops of tRNAs. Biogenesis and functions of acp³U have not been extensively investigated. Using a reverse-genetic approach supported by comparative genomics, we find here that the Escherichia coli yfiP gene, which we rename tapT (tRNA aminocarboxypropyltransferase), is responsible for acp³U formation in tRNA. Recombinant TapT synthesizes acp³U at position 47 of tRNAs in the presence of S-adenosylmethionine. Biochemical experiments reveal that acp³U47 confers thermal stability on tRNA. Curiously, the ΔtapT strain exhibits genome instability under continuous heat stress. We also find that the human homologs of tapT, DTWD1 and DTWD2, are responsible for acp³U formation at positions 20 and 20a of tRNAs, respectively. Double knockout cells of DTWD1 and DTWD2 exhibit growth retardation, indicating that acp³U is physiologically important in mammals.