Polyadenylation

Polyadenylation is one of the major methods of the processing of eukaryotic pre-mRNA after the capping mechanism.

Introduction to polyadenylation:

Virtually all eukaryotic mRNAs have a series of up to 250 adenosines at their 3′ ends called poly-A tail. A poly-A tail is added to the 3′ end of mRNAs and plays an important role in mRNA stability, nucleocytoplasmic export, and translation.

The poly-A tail is not specified by the DNA and is added to the transcript by a template-independent RNA polymerase called poly-A polymerase (PAP) after endo nucleolytic cleavage of the nascent RNA near the 3′ terminus. These two coupled reactions (cleavage and formation of poly-A tail), are collectively referred to as cleavage and poly-adenylation or, simply, polyadenylation.

The steps of polyadenylation:

A single processing complex undertakes both the cleavage and poly-adenylation and a signal is required.

Step 1:

• In mammals, this process is directed by a signal sequence called poly-A signal in the mRNA, almost invariably 5′-AAUAAA-3′ (upstream element).
• It is located between 10 to 30 nucleotides upstream of the polyadenylation site.
• Almost equal and lesser than 30 nucleotides downstream of the polyadenylation site is a GU-rich (or U-rich) downstream element.
• Both the poly-A signal sequence and the GU rich elements are the binding sites for the multi-subunit protein complexes, which are which are respectively, the cleavage and poly-adenylation specificity factor (CPSF) and the cleavage stimulating factor (CstF).

The diagram, describes the above step 1:

Step 2:

• Generation of poly-A tail at 3′ terminus first requires cleavage factors- CFI and CF II which act as endonucleases to cleave the RNA at poly-adenylation site.

The diagram, describes the above step 2:

• Cleavage is followed by the poly-adenylation in a tightly coupled manner. The poly (A) polymerase (PAP) catalyzes the target reaction in template independent manner. Its substrate is ATP.

Step 3:

The reaction of poly-adenylation passes through two consecutive stages-

• A rather short oligo-A sequence (~10 residues) is added to the 3′ end.
• The oligo-A tail is extended to the full ~200 residue length.

The diagram, describes the above step 3:

Step 4:

• This reaction needs another stimulatory factor that identifies the oligo-A tail.
• This additional factor includes poly-adenylate-binding protein (PABP), which helps the polymerase to add the adenosines.
• PABP influences the length of the poly-A tail that is formed and appears to play a role in the maintenance of the tail after synthesis.

The diagram, describes the above step 4:

The schematic representation of the process of polyadenylation:

Conclusion:

1. This process also occurs in some mRNA in bacteria.
2. In E. coli, this process in some mRNA leads to the formation of poly-A tail of 10-40 nucleotides.
3. It is catalyzed by poly (A) polymerase associated with ribosomes.
4. Here, poly-A tail acts as a binding site for the nucleases (PNPase and RNaseE) responsible for degradation of mRNA.

Other related notes:

1. 5′ capping: https://thebiologyislove.com/5-capping/
2. Alternative poly-adenylation: https://thebiologyislove.com/alternative-polyadenylation/

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