Guanylyl transferases areenzymes that transfer aguanosine mono phosphate group, usually fromGTP to another molecule, releasingpyrophosphate. Manyeukaryotic guanylyl transferases arecapping enzymes that catalyze the formation of the5' cap in theco-transcriptional modification ofmessenger RNA. Because the5' end of theRNA molecule ends in aphosphate group, the bond formed between the RNA and the GTP molecule is an unusual 5'-5' triphosphate linkage, instead of the3'-5' linkages between the other nucleotides that form an RNA strand. In capping enzymes, a highly conservedlysine residue serves as thecatalytic residue that forms acovalent enzyme-GMP complex.[1]
Thetransfer RNA (tRNA) forhistidine is unique among eukaryotic tRNAs in requiring the addition of a guanine nucleotide before beingaminoacylated by the histidinetRNA synthetase. Theyeast guanylyl transferase specific to tRNAHis is unique in being the only known non-tRNA synthetase enzyme that specifically recognizes the tRNAanticodon.[2]
Guanylyl transferases also exist for transferring guanosine nucleotides to sugar molecules, such asmannose andfucose.[citation needed]
