Several bacterial species have evolved to sacrifice individual members of their populations upon phage infection by activating various suicide response pathways [61] . One of these suicide response mechanisms entails the activation of the latent anticodon nuclease PrrC in phage-infected E. coli CTr5x. The nuclease-kept in an inactive state in absence of phage infection-cleaves the host's tRNA Lys in the anticodon loop to shut down protein synthesis and thus impair phage propagation. Cleavage by PrrC yields 2 0 ,3 0 -cyclic phosphate and 5 0 -hydroxyl termini resembling the products of tRNA splicing endonuclease reactions. Bacteriophage T4, however, has evolved to cope with this defense mechanism. The phage pnk and rli genes encode for proteins capable of restoring cleaved tRNA Lys [62] . It has been demonstrated that T4 Pnkp (the protein encoded by the pnk gene) acts both as phosphatase [63] and polynucleotide kinase [64] thus converting the 2 0 ,3 0 -cyclic phosphate and 5 0 -hydroxyl of tRNA halves into 2 0 ,3 0 -cis diol and 5 0 -phosphate termini (Fig. 2B , left box, left branch). T4 Rnl1 (encoded by the rli gene) subsequently joins these tRNA halves by 5 0 -3 0 RNA ligation thus restores a functional tRNA [65] . In addition to T4 Rnl1, another RNA ligase, T4 Rnl2, has been identified in the genome of bacteriophage T4 [66] . T4 Rnl1 and T4 Rnl2 constitute distinct and only distantly related protein families sharing characteristic features (Fig. 3) . Members of the Rnl2 family have been detected and examined in viral, bacterial, and archaeal genomes, suggesting a common phylogenetic origin and possibly a function in RNA repair [66] [67] [68] [69] [70] . Potentially, this type of ligases might also be involved in archaeal tRNA splicing. In trypanosomatids, 5 0 -3 0 RNA ligases of the Rnl2 type are involved in RNA-guided editing of mitochondrial pre-mRNAs by nucleolytic cleavage and ligation [71] . The finding that tRNA splicing in yeast occurs as a twostep reaction, an ATP-independent endonuclease reaction, and an ATP-dependent ligase reaction constituted the first piece of evidence for the existence of a eukaryotic tRNA ligase [35] . Later, an RNA ligase activity was characterized in wheat germ extracts [55, 72] by using a nucleolytic fragment-obtained by RNase T1 digestion of tobacco mosaic virus RNA [73] -as an artificial ligase substrate.