Studies modifying the substrates used for in vitro processing reactions revealed that splice site recognition by archaeal tRNA splicing endonucleases relies on the presence of the bulge-helix-bulge motif [14] . Archaeal tRNA endonucleases consist of two dimers, each composed of a catalytic and a structural subunit. These dimers combine in an antiparallel fashion and interact with the pseudosymmetric substrate. Each exon-intron boundary is cleaved by one of the diagonally juxtaposed catalytic subunits, which together form the two composite active sites [40] . Despite the conservation of the recognition motif, four types of oligomeric organization of archaeal splicing endonucleases have co-evolved with minor changes of their substrates [41] giving rise to a 4 (homotetramers, found in Euryarchaeota), a 2 (homodimers, found in Euryarchaeota), (ab) 2 (dimers of heterodimers found in some Crenarchaeota such as e.g., Nanoarchaeum equitans), and abcd (heterotetramers, also found in Crenarchaeota) enzymes [39, [42] [43] [44] . In the case of the homotetrameric endonucleases, two of the four identical chains function as structural subunits whereas the two remaining polypeptides serve as catalytic subunits [45] . The monomers of the homodimeric splicing endonucleases might have arisen from an in-frame gene duplication and thus encode for a fusion of a structural and a catalytic domain [42] .