Post Transcriptional Modifications Polyadenylation

Post Transcriptional modification is a term called for biological processes common to eukaryotic cells which chemically alter the primary RNA transcript from a gene to create a mature functional RNA molecule, which thereafter leaves to the nucleus to perform various other functions. Post-transcriptional modifications are also known as co-transcriptional modifications. These post-transcriptional modifications are categorised into many types which are achieved through multiple molecular mechanisms. Post-transcriptional modifications also have a role in the changes of rRNA, tRNA, snRNA, miRNA, etc.

Post Transcriptional Modifications for production of mature mRNA

The important post transcriptional modifications that take place for the production of mature mRNA are

1. 5’ Capping.
2. 3’ maturation (cleavage and polyadenylation).
3. Splicing.
4. Transport of RNA to Cytoplasm.
5. Stabilization/Destabilization of mRNA.

Capping

The capping of pre-mRNA is carried out by the addition of 7-methylguanosine to the 5’end. This is achieved by the removal of terminal 5’ phosphate that is done with the help of phosphate enzymes. This reaction is accelerated by the enzyme adenosyltransferase, which at last produces diphosphate 5‘end. The 5’ end product, thereafter attacks the alpha phosphorus atom of the GTP molecule. The enzyme called ‘guanine –N’ helps to transfer a methyl group from adenosyl methionine to the guanine ring. This type of ring is called a cap 0 structure.

Some of the functions of the 5’ Cap are as follows:

1. Enhanced translation.
2. Protection from some ribonucleases.
3. Enhanced transport from the nucleus.
4. Enhanced splicing of the first intron for some pre-mRNAs.

3’ maturation (Cleavage and polyadenylation)

When a string of adenine nucleotide (-AAAAAA-3′) is added with a 3’ end of the primary transcript, then it is known as polyadenylation. Proteins are the ingredients required for polyadenylation, new script and cleavage. Proteins required for cleavage of Pre-mRNA are:

1. Cleavage & polyadenylation specificity factor (CPSF).
2. Cleavage stimulation factor (CstF).
3. Cleavage factors I and II (CFI & CFII), these are RNA-binding proteins.
4. PolyA Polymerase (PAP).
5. nRNAP II.

The termination of transcription cannot be carried out by RNA polymerase II at the fixed position. However, they often transcribe beyond the 3’ end of the mature mRNA. The correct 3’ end is generated by the specific endonuclease cleavage, addition of *250 adenosines in the form of a poly (A) tail. This tail helps to confer resistance to 30’ exonucleases and also helps in the efficient translation initiation.

Functions of the Poly A-tail

1. To promote mRNA stability.
2. Promotes recruitment by ribosomes.
3. Synergistic stimulation with Cap.
4. Enhances translation.

The sequence of 3’ Polyadenylation

1. A cleavage sequence CA.
2. Polyadenylation signal sequence. AAUAAA.
3. GU rich sequence present in 20-40 nucleotides, which is downstream to the cleavage site.

Splicing

Introns are the most intervening sequences, which interrupt the sequence of mature mRNA (exons) and these are usually non-coding. However, on the other hand, introns like 1-19 have the characteristics to do code. Introns are highly reactive and get spliced upon the RNA assuming the secondary structure after transcription. These sequences are untranslatable because they remain absent in the mature transcript. For better understanding, splicing is a process of joining the exons by a pre-mRNA sequence as a result the introns are eliminated. Mostly, the Introns are present in eukaryotes and rarely found in prokaryotes. Introns show varied variations in their length and number with an average size of 3.3 KB in human beings. In addition to generating functional mRNAs, splicing assists in the export of mRNAs to the cytoplasm.

Mechanism of Splicing

Splicing of Intron is carried out in the nucleus with the help of a precise and complex arrangement of protein and ribonuclear substances. With the assistance of splicing mature mRNAs to the cytoplasm, this process involves around 70 proteins in higher eukaryotes and 5 fewer ribonucleic protein particles. Thereafter, pre-, RNAs (exons) and introns are processed to form an mRNA containing only exons. In this mechanism, two different types of splicing take place as Self-splicing and spliceosome mediated splicing.

RNA Polyadenylation

Post Transcriptional Modification of RNA plays a great role in various biological processes. Polyadenylation is considered a well-conserved regulatory mechanism in the involvement of eukaryotes and endonucleolytic cleavage along with adenosine monophosphate units on 3’ends resulting in the formation of Poly-A Tail.

Conclusion

Post Transcriptional modification Polyadenylation is an important molecular function performed in the human body, which involves multiple biological processes. The analysis of polyadenylation patterns through varied tissues had helped the scientists and doctors various valuable information which further helped them to cope with the chronic diseases.