Polynucleotide ligases constitute a class of enzymes which catalyse the formation of phosphodiester bonds between proximal 5’-PO4 and 3’-OH groups in DNA, RNA and DNA/RNA-hybrid substrates. The reaction proceeds through three steps, where steps 1 and 2 prepare the substrate for ligation in step 3. The focus of this thesis is on ssRNA ligases of the RNA ligase (Rnl) 3 family. RNA ligases can repair ...