Endonuclease-based phylogeny of pre-tRNA introns splicing motifs and related features. The homotetrameric α4 endonuclease splicing at characteristic intron–exon junctions of archaeal pre-tRNAs (present in most Crenarchaeota and a few Euryarchaeota) is assumed to have evolved divergently towards a homodimeric β2 enzyme in Euryarchaeota and into a heterotetrameric α,β,δ,γ enzyme in Eukarya (see text). The monomer of the homotetrameric ancestral splicing endonuclease is designated by “α”; that of the homodimeric enzyme is designated by “β” and the four different subunits of the eukaryotic heterotetrameric enzyme by “α, β, γ, and δ”. Note that the size of the β subunit is about twice that of the α subunit and probably results from gene duplication and subsequent fusion of an ancestral α-like subunit gene (Lykke-Andersen and Garrett 1997; Bujnicki and Rychlewski 2000; Li and Abelson 2000).
