Five Prime Cap - Capping Process

Capping Process

The starting point is the unaltered 5′ end of an RNA molecule. This features a final nucleotide followed by three phosphate groups attached to the 5′ carbon.

1. One of the terminal phosphate groups is removed (by RNA terminal phosphatase), leaving two terminal phosphates.
2. GTP is added to the terminal phosphates (by a guanylyl transferase), losing two phosphate groups (from the GTP) in the process. This results in the 5′–5′ triphosphate linkage.
3. The 7-nitrogen of guanine is methylated (by a methyl transferase).
4. If the second base from the terminal is adenine, it can be methylated; and the third base from the terminal is generally methylated 10–15% of the time.