Guanylyl transferase catalyzes the transfer of a GMP (guanosine monophosphate) moiety to the 5' end of the mRNA. This reaction is highly specific and involves a multi-step process: 1. Cleavage of the terminal phosphate group from the nascent RNA. 2. Transfer of a GMP group from GTP to the 5' end of the RNA. 3. Methylation of the added guanine base, resulting in the formation of a 5' cap structure.
