Pre-mRNA Processing: Modification of pre-mRNA Ends

In eukaryotic cells, the newly-transcribed mRNA is called precursor mRNA, or pre-mRNA. Each pre-mRNA receives two important modifications. One at the five-prime end of the molecule, called the cap, and another at the three-prime end of the molecule, called the tail.

The five-prime cap is composed of a single 7-methylguanosine, a modified guanine nucleotide, that is attached to the first nucleotide of the pre-mRNA by a triphosphate linkage.

A specific set of nucleotides toward the three-prime end of the pre-mRNA transcript, usually A, A, U, A, A, A, is called a polyadenylation signal. It recruits the three-prime end processing machinery and directs an endoribonuclease to cut the transcript at the three-prime cleavage site.

Another enzyme, polyadenylate polymerase, or PAP, then adds a long string of adenine nucleotides, as many as 200, to the three-prime end of the transcript.

The five-prime cap and the three-prime poly-A tail protect the ends of the transcript from degradation by exonucleases. The three-prime tail also signals to transport molecules that the mRNA transcript is ready to leave the nucleus. Outside of the nucleus, the five-prime cap helps the ribosome attach to the transcript so it can begin translation.