In both archaea and eukarya, the tRNA endonuclease catalyses the cleavage of pre-tRNA at the two exon-intron boundaries and generates tRNA exon halves displaying a 2 0 ,3 0 -cyclic phosphate at the 3 0 -end of the 5 0 -exon and a 5 0hydroxyl at the 5 0 -end of the 3 0 -exon (Fig. 2B , box at the top). In addition, the reaction yields a linear intron with 2 0 ,3 0 -cyclic phosphate and 5 0 -hydroxyl termini ( Fig. 2A) . Although being mechanistically and evolutionarily related [39] , it has been shown that archaeal and eukaryal tRNA splicing endonucleases differ with respect to substrate recognition [9] .