Taken together, apart from HSPC117, which appears to be conserved among all domains of life, the additional components of the HSPC117 complex do not show a comparably broad taxonomic coverage. Orthologues of DDX1, FAM98B, and CGI-99 are spread more widely than ASW, which seems to be restricted to fewer species (Fig. 5A ). Another RNA ligase activity leading to the unusual 2 0 ,5 0 -phosphodiester bond has been shown to act in extracts prepared from various bacterial species (Fig. 2B , right box) [57] . Its biochemical purification led to the identification of the ligase gene ligT, conserved not only in bacteria but also in euryarchaeota and crenarchaeota [59, 78] . The apparent dependence of its activity on the presence of modifications in its tRNA substrates suggests that it is involved in repair or processing of tRNA in its host [10, 59] . LigT from E. coli catalyses an ATP-independent equilibrium reaction between 2 0 ,3 0 -cyclic phosphate and 5 0 -hydroxyl termini and 2 0 ,5 0 -phosphodiester bonds [59] . More recently, a homologous archaeal 2 0 -5 0 RNA ligase has been characterized in the euryarchaeon Pyrococcus furiosus confirming the broad phyletic occurrence of LigT proteins [60] . Although the high degree of conservation suggests an evolutionarily important role for 2 0 -5 0 ligase genes, the exact biological function of LigT proteins and 2 0 ,5 0 -phosphodiester bond formation are still unknown [59, 60, 78] . Interestingly, a few eukaryotic genomes also encode homologs of bacterial and archaeal LigT proteins (Figs. 5B, 6) . The activities and biological functions of these proteins have not yet been addressed.