Figure 3.

tRNA subcellular dynamics. tRNAs are drawn in their second cloverleaf structure. The color coding of nucleotides is the same as for Figure 1 and Figure 2 except some nucleosides that occur in the nucleus are orange, whereas representative nucleoside additions that occur in the cytoplasm are brown. Pre-tRNAs are transcribed in the nucleus where leader and trailer sequences (purple) are removed prior to CCA (green) addition. End matured, partially modified intron-containing pre-tRNAs are exported to the cytoplasm by Los1 and at least one unknown exporter. Those pre-tRNAs encoded by genes lacking introns are likely exported by both Los1 and Msn5. Splicing and additional modifications occur after export to the cytoplasm. Cytoplasmic tRNAs constitutively return to the nucleus via retrograde transport. Mtr10 functions in tRNA retrograde import but it is unknown whether its role is direct or indirect. Imported tRNAs accumulate in the nucleus if cells are deprived of nutrients; otherwise they are reexported to the cytoplasm by Los1, Msn5, and at least one unidentified exporter. See text for details.