The mammalian rapid tRNA decay pathway is critical for N⁷-methylguanosine-hypomodified tRNA degradation under physiological conditions

Chemical modifications of transfer RNAs (tRNAs) are integral to their stability and to translation. Loss of N ⁷ -methylguanosine (m ⁷ G) on specific tRNAs reduces their steady-state abundance and impairs translation in mammals, but whether these decreases reflect active degradation under physiological growth conditions is unresolved. Here, using human HCT116 cells, we show that knockdown of the 5′→3′ exonuclease, XRN2, restores tRNA levels diminished by METTL1 depletion. Leveraging conditional protein knockdown, we performed time-resolved measurements of mature tRNA levels and directly quantified decay kinetics. We show that in the absence of heat stress, m⁷G-hypomodified tRNAs undergo XRN2-dependent accelerated decay. Finally, partial loss of the Drosophila XRN2 ortholog, Rat1, genetically rescues male sterility of mettl1 mutants, demonstrating organismal relevance. These findings define a conserved constitutive rapid tRNA decay pathway in mammals and indicate inhibition of tRNA decay as a potential therapeutic strategy in disorders caused by tRNA hypomodification.