The enzyme is a GTP- and Mn2+-dependent 3′-5′ nucleic acid ligase with the ability to join RNA with 3′-phosphate or 2′,3′-cyclic-phosphate ends to RNA with 5′-hydroxy ends. It can also join DNA with 3′-phosphate ends to DNA with 5′-hydroxy ends, provided the DNA termini are unpaired [6]. The enzyme is found in members of all three kingdoms of life, and is essential in metazoa for the splicing of intron-containing tRNAs. The reaction follows a three-step mechanism with initial activation of the enzyme by GTP hydrolysis, forming a phosphoramide bond between the guanylate and a histidine residue. The guanylate group is transferred to the 3′-phosphate terminus of the substrate, forming the capped structure [DNA/RNA]-3′-(5′-diphosphoguanosine). When a suitable 5′-OH end is available, the enzyme catalyses an attack of the 5′-OH on the capped end to form a 3′-5′ phosphodiester splice junction, releasing the guanylate. When acting on an RNA 2′,3′-cyclic-phosphate, the enzyme cayalyses an additional reaction, hydrolysing the cyclic phosphate to a 3′-phosphate [9]. The metazoan enzyme requires activating cofactors in order to achieve multiple turnover catalysis [8].
