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 mRN…
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.
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Q1: What is the 5' cap and why is it added to pre-mRNA?
The 5' cap is a 7-methylguanosine, a modified guanine nucleotide attached to the first nucleotide of pre-mRNA via a triphosphate linkage. It protects the transcript from degradation by exonucleases and helps the ribosome attach during translation. The cap also helps cells distinguish mRNA from other RNA types.
Q2: How does the poly-A tail protect mRNA and signal its readiness for translation?
Polyadenylate polymerase adds approximately 200 adenine nucleotides to the 3' end of pre-mRNA, forming the poly-A tail. This tail protects mRNA from degradation by exonucleases in the cytoplasm and signals to transport molecules that the transcript is ready to exit the nucleus for translation.
Q3: What is a polyadenylation signal and how does it direct 3' end processing?
A polyadenylation signal is a specific sequence of nucleotides, typically A, A, U, A, A, A, located toward the 3' end of pre-mRNA. It recruits the 3' end processing machinery and directs an endoribonuclease to cut the transcript at the 3' cleavage site, allowing polyadenylate polymerase to add the poly-A tail.
Q4: What is the difference between exons and introns in pre-mRNA?
Exons are RNA sequences that remain in the mature transcript and are expressed as proteins. Introns are portions removed during splicing by the spliceosome. Interestingly, a single RNA segment can function as an exon in one cell type and an intron in another, enabling alternative rna splicing to produce multiple proteins from one gene.
Q5: When does the 5' cap get added during transcription?
The 5' cap is added early during transcription, once approximately 20-40 ribonucleotides have been joined together by RNA polymerase. A group of enzymes replaces the 5' phosphate with modified 7-methyl guanosine, which helps the cell recognize mRNA and prepares it for subsequent translation.
Q6: How do the 5' cap and 3' poly-A tail work together to protect mRNA?
Both the 5' cap and 3' poly-A tail protect mRNA ends from degradation by exonucleases. The 5' cap marks the transcript as mRNA and facilitates ribosome binding, while the 3' tail signals nuclear export and protects the transcript in the cytoplasm, ensuring the mRNA remains stable for translation.
Q7: What happens to pre-mRNA after the 5' cap and poly-A tail are added?
After both modifications are complete, the mature mRNA exits the nucleus for translation. The 5' cap and 3' poly-A tail signal to transport molecules that the transcript is ready to leave the nucleus. Outside the nucleus, the 5' cap helps the ribosome attach to initiate protein synthesis.