Retrograde nuclear import of tRNA precursors is required for modified base biogenesis in yeast

The retrograde movement of tRNAs from the cytoplasm to the nucleus occurs constitutively in eukaryotic cells but its functional significance remains unclear. We show evidence suggesting that in Saccharomyces cerevisiae, a spliced tRNA precursor must be imported into the nucleus before the biogenesis of a modified base can occur. Wybutosine (yW) is a modified base adjacent to the anticodon of tRNAPhe and is required for accurate decoding. Glucose starvation or overexpression of the nuclear tRNA binding protein Trz1p both caused nuclear retention of cytoplasmic tRNAs, impaired the yW synthesis, and induced the accumulation of its intermediate, N1-methylgunanosine (m1G), showing that the postspliced tRNAPhe is imported to the nucleus, where m1G is formed by Trm5p, after which it is reexported to the cytoplasm, where the yW synthesis is completed by cytoplasmic enzymes.