Guanylyltransferases catalyse the transfer of a guanylyl group to an acceptor. Many eukaryotic guanylyl transferases are capping enzymes that catalyze the formation of the 5' cap in messenger RNAs. Others specifically add a guanine nucleotide to the transfer RNA for histidine, which is required for it to be aminoacylated by the histidine tRNA synthetase. Guanylyl transferases also exist for transferring guanosine nucleotides to sugar molecules, such as mannose and fucose. (Adapted from https://en.wikipedia.org/wiki/Guanylyltransferase.)