14.6:
pre-mRNA Processing
In eukaryotic cells, 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 a 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 sequence of nucleotides toward the end of the pre-mRNA transcript, usually A, A, U, A, A, A, called a polyadenylation signal, recruits an RNA binding protein, and directs an enzyme, an endonuclease, to cut the transcript at the three-prime end of the signal sequence. A different enzyme, polyadenylate polymerase, 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. 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.
The last major change to an RNA transcript is the removal of non-coding sequences called introns. A complex of proteins and RNA, called the spliceosome, searches for markers at the ends of introns, cuts the introns out of the transcript, and attaches together the remaining exons, which are sequences that code for proteins.
14.6:
pre-mRNA Processing
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 to it (7-Methyl guanosine). This 5’ cap helps the cell distinguish mRNA from other types of RNA and plays a role in subsequent translation.
During or shortly after transcription, a large complex called the spliceosome cuts out various parts of the pre-mRNA transcript, rejoining 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.
Suggested Reading
Darnell, James E. "Reflections on the history of pre-mRNA processing and highlights of current knowledge: a unified picture." RNA 19, no. 4 (2013): 443-460. [Source]
Ramanathan, Anand, G. Brett Robb, and Siu-Hong Chan. "mRNA capping: biological functions and applications." Nucleic Acids Research 44, no. 16 (2016): 7511-7526. [Source]
Semlow, Daniel R., and Jonathan P. Staley. "Staying on message: ensuring fidelity in pre-mRNA splicing." Trends in Biochemical Sciences 37, no. 7 (2012): 263-273. [Source]