The diphosphate 5'-end of the mRNA is joined to the GMP, releasing it from the enzyme. At this time, it is unclear how the RNA diphosphate end is transferred from the active site of the triphosphatase to the guanylyltransferase site. The covalent enzyme-GMP complex can form in the absence of RNA.
Guanylyltransferase (GT) catalyzed second reaction can be represented as:ppN(pN)n + GTP -> GpppN(pN)n + PPi
(Yamada-Okabe et al. 1998).