<p>The mRNA capping enzyme in yeasts is composed of two separate chains, alpha a mRNAguanyltransferase and beta an RNA 5'-triphosphate. X-ray crystallography reveals a large conformational change during guanyl transfer by mRNA capping enzymes [<cite idref="PUB00000947"/>].Binding of the enzyme to nucleotides is specific to the GMP moiety of GTP. The viral mRNA capping enzyme is a monomer that transfers a GMP cap onto the end of mRNA that terminates with a 5'-diphosphate tail.</p> mRNA capping enzyme