A new pathway of pre-tRNA surveillance in yeast
During tRNA maturation in yeast, aberrant pre-tRNAs are targeted for 3-5 degradation by the nuclear surveillance pathway, and mature tRNAs are targeted for 5-3 degradation by the rapid tRNA decay (RTD) pathway, due to lack of certain body modifications or to destabilizing mutations. Here we show that the RTD pathway also targets pre-tRNAs through an unknown, but distinct, mechanism that occurs after nuclear export. Anticodon stem RTD variants of both tRNATyr and tRNASer(CGA) are substrates for pre-tRNA RTD, triggered by the accumulation of end-matured unspliced pre-tRNA due to altered secondary structure of the region comprising the anticodon stem-loop and the intron. Furthermore, increased nuclear availability of a pre-tRNA RTD substrate can provoke decay by nuclear surveillance. We interpret these results in terms of a model of opportunistic tRNA decay, wherein tRNAs are degraded due to a combination of structural instability and increased availability to decay pathways.