9.10:
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.
9.10:
Pre-mRNA Processing: Modification of pre-mRNA Ends
In eukaryotic cells, transcripts made by RNA polymerase are modified and processed before exiting the nucleus. Unprocessed RNA is called precursor mRNA or pre-mRNA to distinguish it from mature mRNA.
Once about 20-40 ribonucleotides have been joined together by RNA polymerase, a group of enzymes adds a cap to the 5' end of the growing transcript. In this process, a 5' phosphate is replaced by modified guanosine that has a methyl group attached (7-methyl guanosine). This 5' cap helps the cell distinguish mRNA from other types of RNA and plays a role in the subsequent translation.
During or shortly after transcription, a large complex called the spliceosome cuts out various parts of the pre-mRNA transcript and rejoins the remaining sequences. RNA sequences that remain in the transcript are called exons (expressed sequences), while portions removed are called introns. Interestingly, a single RNA segment can be an exon in one cell type and an intron in another. Similarly, a single cell can contain multiple variants of a gene transcript that has been alternatively spliced, enabling the production of multiple proteins from a single gene.
When transcription is completed, an enzyme adds approximately 30-200 adenine nucleotides to the 3' end of the pre-mRNA molecule. This poly-A tail protects the mRNA from degradation in the cytoplasm. The mature mRNA then exits the nucleus for translation.