The subcellular localization of tRNA processing events does not seem to be conserved among all eukaryotes. Many localization studies were carried out in the yeast S. cerevisiae which, however, seems to differ in this respect from many other eukaryotes [129] . tRNA splicing is assumed to occur in the cytoplasm in S. cerevisiae, based on the localization of yeast tRNA endonuclease subunits to the outer mitochondrial membrane [130] . Controversial data have been reported concerning the localization of tRNA splicing in plants. On the one hand, some plant tRNA splicing enzymes at least partly localize to the nucleus [131] . On the other hand, impaired nuclear export of tRNA results in accumulation of unspliced pre-tRNA in Arabidopsis thaliana [132] arguing in favor of cytoplasmic tRNA splicing in plants in analogy to the coupling of nuclear export and cytoplasmic tRNA splicing in yeast rna1-1 mutants [32, 133, 134] . In the vertebrate X. laevis, tRNA splicing-preceded by 3 0 -end formation including CCA addition-has been shown to occur in the nucleus by micro-dissection and micro-injection studies carried out with oocytes [135] [136] [137] . In accord with these observations, subunits of the human tRNA endonuclease have been reported to be nuclear proteins [38] .