chemical reaction during 5'capping.

An overview of polyadenylation of 3’ end: post transcriptional modification, prior to the nuclear export.

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The final RNA processing event, i.e. the polyadenylation of the 3’ end of the mRNA is linked with the termination of the transcription.

The CTD tail of the polymerase is involved in the recruiting of the enzymes necessary for the polyadenylation.
The DNA contains a poly-A signal sequence. Once the polymerase has reached the sequence and transcribed the poly-A signal sequence into the RNA, the poly-A signal sequence (these sequences once transcribed into RNA, triggers transfer of the CPSF and CstF from the CTD tail of the polymerase to the RNA, described in the text later) in the RNA triggers the transfer of polyadenylation enzymes to RNA leading to three events.
1. Cleavage
2. Addition of the “A” residues.
3. Termination of transcription.
The CTD tail of the polymerase carries two protein complexes as it reaches the end of the gene.
1. CPSF (cleavage and polyadenylation specificity factor)
2. CstF (cleavage stimulation factor)

The binding of the CPSF (cleavage and polyadenylation specificity factor) and CstF (cleavage stimulation factor) is followed by the recruitment of other proteins as well. This leads to RNA cleavage and then polyadenylation.
The poly-A polymerase mediates the process of polyadenylation and adds about 200 adenines to the RNA’s 3’ end produced by the cleavage.
The poly-A polymerase uses ATP (adenosine triphosphate) as a precursor and adds the nucleotides using the same chemistry as that of RNA polymerase, but here in the absence of a template. Thus long “A” tail is only present in RNA (and not in the DNA)
After polyadenylation (the final step in post transcriptional modification, prior to the nuclear mRNA transport) the mature mRNA is then transported from the nucleus to the cytosol.
At present, not more is known about the relation of the polyadenylation with the termination of the transcription. Two basic models have been proposed to explain the link between polyadenylation and termination.
· First: the transfer of the enzymes responsible for the polyadenylation from the CTD tail of the polymerase to the RNA triggers a conformational change in the polymerase that reduces the processivity of enzymes leading to spontaneous termination soon afterwards.
· Second: after cleavage of the RNA transcript the polymerase keeps on transcribing the DNA short while, the polymerase senses the absence of the 5’ cap on the second RNA molecule; as a result, the RNA recognizes the transcript as improper and terminates the transcription.