 Because the 5' end of the RNA molecule ends in a phosphate group, the bond formed between the RNA and the GTP molecule is an unusual 5'-5' triphosphate linkage, instead of the 3'-5' linkages between the other nucleotides that form an RNA strand.In capping enzymes, a highly conserved lysine residue serves as the catalytic residue that forms a covalent enzyme-GMP complex.The transfer RNA (tRNA) for histidine is unique among eukaryotic tRNAs in requiring the addition of a guanine nucleotide before being aminoacylated by the histidine tRNA synthetase.