RNA Processing in Eukaryotes

By Meet Patel

Introduction

Because the eukaryotes are pretty much more complex organisms than prokaryotes. The gene expression in eukaryotes is more complex than in prokaryotes. In prokaryotes, like bacteria, the newly transcribed mRNAs are included directly in the action of gene expression. The prokaryotic RNA does not need to modify before the translation process. The most common reason is that the prokaryotic genome is present in the cytoplasm and hence the RNA will be transcribed in the cytoplasm and which will be used directly in the further process, Translation, of the gene expression.

While the Eukaryotes, the transcription process is done under the nucleus and is known as pre-RNA or precursor RNA. The pre-RNA needs to modify itself and transport through the nuclear membrane into the cytoplasm.

There are  three events that occur during RNA processing:

1. 5' capping of mRNA
2. 3' Polyadenylation
3. RNA splicing

Here we discuss two processes and their functions,

5' capping of mRNA:

• The enzyme, triphosphatase cleave one phosphate (gamma phosphate) from the last nucleotide at 5'-end.
• The second step is the addition of the guanine residual at the 5'-end. The reaction is catalyzed by an enzyme specific for guanine, guanylyltransferase.
• In the third step, guanylyl-7-methyl transferase adds methyl group at the 7' position of the sugar molecule of the guanine residual.
• Another enzyme named 2'-o-methyltransferase (a similar function of the guanylyl-7-methyltransferase) adds another methyl group at the adjutant nucleotide.

The function of 5' cap: 5'-cap protects mRNA against RNase activity.

3' Polyadenylation

• 3'-end of the pre-mRNA contain cleave site approx. 11 to 30 nucleotides away from the specific sequence AAUAAA on the 3' side.
• The specific endonuclease (CPSF) cleaves the extra nucleotide sequence downstream of the cleave site.
• The enzyme, Poly adenylate polymerase adds 50 to 300 Adenine residues at the 3'-end.

The function of the 3'- Poly A tail: 3- Poly A tail provides stability and facilitates the exit of mRNA from the nucleus.