Post-Transcriptional RNA Processing
The 5' Cap and the Poly A Tail
5' End Capping
Post-transcriptional processing of the 5' end of the RNA product of DNA transcription comes in the form of a process called the 5' cap. At the end of transcription, the 5' end of the RNA transcript contains a free triphosphate group since it was the first incorporated nucleotide in the chain. The capping process replaces the triphosphate group with another structure called the "cap". The cap is added by the enzyme guanyl transferase. This enzyme catalyzes the reaction between the 5' end of the RNA transcript and a guanine triphosphate (GTP) molecule.
The figure above simply illustrates the reaction between the 5' end of the RNA transcript and the GTP molecule. In the reaction, the beta phosphate of the RNA transcript displaces a pyrophosphate group at the 5' position of the GTP molecule. The cap is formed through a 5'-5' linkage between the two substrates such that the GTP molecule is oriented in the opposite direction as the other nucleotides in the RNA transcript chain.
Once in place, the cap plays a role in the ribosomal recognition of messenger RNA during translation into a protein. Prokaryotes do not have a similar cap because they use other signals for recognition by the ribosome.
The Poly A Tail
Post-transcriptional RNA processing at the opposite end of the transcript comes in the form of a string of adenine bases attached to the end of the synthesized RNA chain. This string of adenine is called the "poly A tail". The addition of the adenines is catalyzed by the enzyme poly (A) polymerase, which recognizes the sequence AAUAAA as a signal for the addition. The reaction proceeds through mechanism similar to that used for the addition of nucleotides during transcription. The poly A tail is found on most, but not all, eukaryotic RNA transcripts. Its significance remains unknown.