RNA polyadenylation is one of the steps in the processing of the ribonucleic acid (RNA) chain for protein synthesis. The messenger RNA (mRNA) chain undergoes many steps after it is copied from the DNA template till the protein can be synthesized. The mRNA chain contains all the information required for the production of the protein in the form of the nucleic acid chain.
MRNA Transcription and Processing
The mRNA is produced by the enzyme RNA polymerase that reads the genetic code (A,T,G,C sequence) in the DNA and copies it to form the mRNA chain. This mRNA chain needs to be processed and transported to the ribosomes that are the regions of protein synthesis.
Once the mRNA is produced, it is very short-lived. The mRNA undergoes the addition of a 5' end cap where a modified guanine nucleotide is added to the 5' end of the polymer chain. The presence of the 5' cap allows the other enzymes to recognize the starting point of the mRNA and also protects the mRNA chain from the RNAase degradation.
The mRNA chain may also undergo splicing which involves removing stretches of amino-acids that are non coding.
Importance of Polyadenylation
Polyadenylation is the addition of multiple A nucleotide to the mRNA chain at the 3' end. The addition of the poly A chain serves multiple purposes.
(1) The presence of the poly(A) chain signals the end of the coding region of the mRNA
(2) The poly(A) stretch prevents the mRNA from being degraded by exonucleases
(3) The poly(A) chain is important for transport of the mRNA from the nucleus where it is synthesized to the ribosomes where protein is translated.
(4) Over time the A chain gets shortened and when it is short enough the mRNA gets degraded. Thus, the length of the poly(A) chain is an indicator of the longevity of the mRNA chain.
Polyadenylation Signal at the 3' End
Extensive analysis of the pre-mRNA has shown that all eukaryotic mRNA chains have a AAUAAA signal at the 3' end. This sequence is found at about 10 to 35 nucleotides upstream of the polyA chain. When this sequence was mutated, virtually all polyadenylation was lost, suggesting that this sequence is important for the processing of the pre-mRNA and allowing for the polyadenylation to occur.
Preparing for Polyadenylation
Polyadenylation occurs in a series of steps with the aid of many other proteins.
(1) The cleavage and polyadenylation specificity factor (CPSF) binds to the upstream poly(A) signal of the mRNA chain and forms an unstable complex.
(2) Three additional proteins bind to the CPSF-RNA complex. These are the cleavage stimulatory factor (CSF), cleavage factor I and cleavage factor II.
(3) The poly(A) polymerase binds to this multiprotein complex and allows for cleavage to occur.
(4) Now, the free 3' end is rapidly polyadenylated.
Polyadenylation of the free 3' end occurs in two steps. In the first slow phase, about 10 to 12 A nucleotides are added slowly to the free 3' end. Then between 200 to 250 A nucleotides are added rapidly to the 3' end of the mRNA.
A protein called Poly (A) binding protein II (PABP II) binds to the mRNA chain to facilitate the rapid addition of the adenosines to the chain. When the length of the polyA chain reaches between 200 and 250 nucleotides, the PABP II signals to the polyA polymerase (PAP) to terminate the addition of As to the chain.