Post-Transcriptional Modification of mRNA

The post-transcriptional modification of mRNA involves various phases of processes which are all necessary for the proper translation of it.

The newly synthesised mRNA consists of non-coding regions called introns. Introns are the non-coding interruptive codon sequences in eukaryotic mRNA. They are often found in the coding sequences which will affect the continuity of translation.  While the coding sequences called exons are 100-200 base pairs long, the introns are short. They must be removed right after transcription.

This modification process is called maturation of mRNA and it happens in the nucleus. Such modifications are crucial for the proper funcitoning of RNA and the various biological processes.

Different Phases of Post-Transcriptional Modification of mRNA

Post-transcriptional modification of mRNA involves various types of chemical processes such as acetylation, deamination, hydroxylation, isomerization, methylation, selenylation, reduction, etc. The maturation of mRNA happens in three phases: mRNA capping, polyadenylation of mRNA, and RNA splicing. 

mRNA Capping

Capping is the process of condensing the 5’ end of the newly synthesised mRNA with guanylate residue, to stabilize it and to protect it from exonucleases. The factors involved in this process include,

• Guanyl transferase
• GTP and guanyl residue
• Methyl transferase for methylation

Methylation of the mRNA happens at the 5’ end of the capped mRNA. Capping promotes the binding of the new mRNA to ribosomes as the cap-binding proteins facilitate the binding.

Polyadenylation of mRNA

Polyadenylation of mRNA is the addition of an adenylate residue at the 3’ end. This results in the formation of a long adenyl residue having 200-300 nucleotides known as a poly(A) tail. Formation of the poly (A) tail is facilitated by poly A synthetase or polymerase. It takes place right after transcription.  

However,  mRNA for some histone proteins does not undergo polyadenylation. Polyadenylation is a crucial process after transcription that stabilizes the mRNA and protects it from exonucleases. It also helps the mRNA to separate from the other cellular RNAs. 

RNA Splicing

The third phase of mRNA maturation is splicing, or the removal of introns. Removal of introns or splicing happens in the nucleus before the mRNA is transported to the cytoplasm. 

There are two methods of RNA splicing- through the Lariat and by the spliceosome. 

Removal of Introns Through Lariat

• RNA splicing through Lariat will have a cut at the 5’ splice site that separates and moves the exon to the left end and the intron to the right.
• While the exon on the left stays as a linear molecule, the intron on the right side forms a loop or Lariat. Lariat is formed by joining the 5’ end with the A base to form the branch site.
• In the next step, a cut is made at the 3’ end of the intron in the lariat. Once the intron is removed, the left and right exons join together to form a complete mRNA.
• Later, the removed intron is debranched into a linear sequence and is gradually degraded.

Removal of Introns by Spliceosome

The removal of introns by the spliceosome is executed with the help of the spliceosome apparatus, which has proteins and RNA. These two components come together to form ribonucleoproteins (rnSnRNPs) that splice the mRNA at U1, U2, U4, U5, and U6. 

The spliceosome recognises a consensus sequence and assembles there to cleave the intron and ligate the exons.

Conclusion

The post-transcriptional modification of mRNA is important as it It is helps with the structural stabilization of substrate RNA, plays a crucial role in modulating the function of the ribosome, etc.

Reference

• Agarwal, P. V. |. V. (2004). Cell Biology, Genetics, Molecular Biology, Evolution, and Ecology: Evolution and Ecology. S. Chand Publishing.
• Stojković, V., & Fujimori, D. G. (2014). Radical SAM-Mediated Methylation of Ribosomal RNA. Methods in Enzymology, 560, 355-376. https://doi.org/10.1016/bs.mie.2015.03.002

Additional Topics

• Explain the Process of Transcription in Prokaryotes
• Transcription Mechanism in Eukaryotes