In eukarya an additional guanosine residue is added post-transcriptionally to the 5′-end of tRNAHis?molecules. The addition occurs opposite a universally conserved adenosine73?and is thus the result of a non-templated 3′-5′ addition reaction. The additional guanosine residue is an important determinant for aminoacylation by EC?6.1.1.21, histidyl-tRNA ligase. The enzyme requires a divalent cation for activity [2]. ATP activation is not required when the substrate contains a 5′-triphosphate (ppp-tRNAHis) [3].
