Fig. 2.

Three distinct RNA ligase families. Three structurally characterized clades of RNA ligases evolved separately by fusion of unique C-terminal domain modules to a common conserved nucleotidyltransferase domain. The fold of an ancestral NTase is suggested to resemble that of T4 Rnl2 as shown (from PDB ID 1S68). The structurally diverse C-terminal domains are shown for T4 Rnl1 (PDB ID 2C5U), T4 Rnl2 (PDB ID 2HVQ), and Pyrococccus abyssi RNA ligase (PDB ID 2VUG), the prototypes of the Rnl1, Rnl2, and Rnl3 families.