Post-transcriptional Modification

In eukaryotes, a synthesized RNA transcript may undergo a number of levels of processing known as post-transcriptional modifications. In the case of messenger RNA, modification is necessary to convert pre-mRNA into a mature mRNA that is ready for protein translation.

One such modification is splicing. The RNA contains a mixture of introns and exons when it is first transcribed. Splicing serves to remove introns, and it occurs either by the action of a spliceosome, or by self-splicing. Alternative splicing shuffles the composition of exons in a transcript, meaning that numerous different transcripts can be made from the same gene.

In mRNA, the 3' end is modified by the addition of multiple adenine residues to produce a structure called a poly-adenylated (poly A) tail. The length of a poly-A tail is highly variable, but over the lifetime of the mRNA these adenine residues are slowly removed. Thus the length of a poly-A tail determines the half-life of mRNA: a longer tail means a longer half-life. The poly-A tail also makes it easier for us to purify eukaryotic mRNA in the lab by annealment to a complementary poly-thymine (poly-T) primer.

In mRNA, the 5' end is modified by the addition of a 7-methylguanosine cap (a methylated guanine residue). The cap is attached the 'wrong way round' forming a 5'-5' pyrophosphate bond, rather than the usual 3'-5' phosphodiester bond. This has two key roles: it enables ribosomal recognition, and thus greater efficiency of protein translation, as well as protecting the mRNA from degradation by 5'-3' exonuclease enzymes.

Note that the 5' cap is the 'front' end of the mRNA and the 3' poly-A tail is the 'back' end. The coding sequence of the transcript lies in between.

After the 5' cap, but before the initation AUG codon, is a 5' untranslated region (UTR) or leader sequence. Equally, after the termination codon, but before the 3' untranslated region (UTR) or trailer sequence is a 3' untranslated region (UTR) or trailer sequence. These are not protein-coding but may have regulatory roles; for instance, the 3' UTR is known to act as a poly-adenylation signal for the addition of the poly-A tail by various enzymes.