The 5 'end of the mRNA is modified in the eukaryotic nucleus (but not in the mitochondria or chloroplasts). Modification reactions are probably common in all eukaryotes. Transcription begins with a nucleoside triphosphate (almost always a purine, A or G). The first nucleotide retains its 5' triphosphate group and forms the usual phosphodiester bond from its 3' position to the 5' position of the next nucleotide.
But when mature mRNA is treated in vitro with enzymes that must degrade it into individual nucleotides, the 5' end does not produce the expected nucleoside triphosphate. Instead, it contains two nucleotides connected by a 5'-5' triphosphate bond and also contains methyl groups. The terminal base is always a guanine that will be added to the original RNA after transcription.
The addition of G at the 5' end is catalyzed by a nuclear enzyme, guanidyl transferase. The reaction occurs as soon as transcription begins and it is not possible to detect more than traces of the original 5' triphosphate end in nuclear RNA. The total reaction can be represented as a condensation between GTP and the original 5' btriphosphate terminal of the RNA.
