Question

Discuss transcription termination in eukaryotes

Answer 1

Eukaryotic transcription termination is a complicated and not well-understood process.

But based on findings, the process has been described as follows.

The start site is present a few nucleotides downstream of the promoter region, which is -35 to -10 bp upstream. The start site will be +1.

Downstream there is a termination site. Since mRNA in eukaryotes has both introns and exons, the process becomes a bit more complicated than the prokaryotic termination sites. Because the introns need to be spliced and the exons need to be joined, by the time it reached the end of transcription. It involves the unique addition of multiple adenine residues to the mRNA.

Once the transcription is done, the mRNA transcript should carry:

· Poly A tail at 3’ end

· 5’ CAP with inwardly linked Guanine

Without these two, the lifespan of mRNA is very much reduced.

Poly adenylation is a process that involves various factors and proteins just like how the RNA polymerase II requires multiple transcription factors to escape the Promoter.

When RNA Poly II reaches the termination site, it will produce a stretch of mRNA. The free 5’ of this mRNA should be CAPped.

There is Polyadenylation Identification Sequence in the termination site. This provides information to the RNA polymerase that transcription is going to be terminated. It is a signal that prompts the addition of adenine residues at the 3’ end. This addition is the signal that transcription is going to be terminated.

Transcription termination:

Once this signal is received, RNA polymerase keeps moving forward. Two proteins get attached to the C- terminal tail of RNA during the elongation process. The proteins are CPSF (Cleavage and Polyadenylation Specificity Factor) and CStF (Cleavage Stimulating Factor).

Whenever these proteins come across the Polyadenylation Identification Sequence, that sequence is transcribed. RNA polymerase moves continuously. RNA chain lengthens. Since the polymerase moves forward, the identification sequence stays back, and transcription is done for that region. Now the CPSF and CStf will be transferred from C-terminal to the Identification Sequence, which is their binding site in the RNA.

Additional cleavage factors are a type of endonucleases. These cleave right after the initiation sequence and remove the CStf. After cleavage, the mRNA is now free. Now only the CPSF is attached to the identification sequence of the free mRNA.

Another protein called the Poly A Polymerase (PAP) comes into action. This enzyme can attach many “A” residues at the 3’ end of the free mRNA, without the use of any template. It utilizes the adenine from ATP.

Next, another protein, Poly A Binding protein (PBP), a single-stranded binding protein is recruited. This PBP binds to the Poly A chain, to prevent secondary structure formation.

RNA Polymerase II dissociation:

These are hypotheses in the study, but the more acceptable situation is the first situation explained below.

1. The RNA Polymerase does not dissociate immediately after the RNA is cleaved. It will continue synthesizing a few unnecessary nucleotides. They need to receive a dissociation signal. Once the mRNA gets cleaved from the polymerase, a conformational change is triggered in the Polymerase, and hence it dissociated from the DNA.
2. The newly formed mRNA does not have 5’CAP. This is sensed by the RNA Polymerase. It recognizes that there is no need for synthesis because there is no 5’ CAP, and gets dissociated.

After this maturation occurs where introns will be removed and exons will be joined.