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.
Figure 1.1: The 5' Capping Reaction
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.
