Gene Expression: RNA Processing
Last updated
Last updated
In eukaryotes, the transcription of a gene by RNA polymerase II produces pre-mRNA which has to undergo several processing steps to become a fully functional mRNA. These steps include modifications of the transcript's 5' and 3' ends and the excision of some internal parts of the RNA molecule.
Soon after the start of transcription by RNA polymerase, a modified guanine nucleotide is added to the 5' end of the growing RNA molecule.
This modification, known as a 5' cap, serves several functions:
Protection from degradation: the 5' cap protects the pre-mRNA from degradation by nucleases.
Nuclear export signal: the presence of the 5' cap serves as one of the molecular signals indicating that the RNA is mature and ready to be exported from the nucleus to the cytoplasm. This ensures that only fully processed mRNA molecules are transported to the site of protein synthesis.
Initiation of translation: in eukaryotes, the 5' cap is essential for the ribosome to bind to the mRNA and initiate protein synthesis.
Most eukaryotic protein-coding genes contain two major types of segments: coding segments called exons and non-coding sequences called introns. During transcription by RNA polymerase II, both exons and introns are included in the pre-mRNA transcript.
However, prior to translation, introns must be removed from the pre-mRNA and exons must be joined together in a process called RNA splicing. Splicing is catalysed by a large molecular complex known as the spliceosome, which consists of several proteins and small nuclear RNAs (snRNAs). The spliceosome recognises specific sequences at the boundaries between exons and introns, called splice sites. Splicing results in the production of mature mRNA molecules containing only the sequences necessary for protein synthesis.
While the precise role of introns and splicing in eukaryotic organisms is still not fully understood, there are several potential advantages associated with splicing:
Regulatory elements in introns: some introns contain regulatory elements that can modulate gene expression.
Exon-intron structure and protein evolution: the exon-intron structure of genes may facilitate the faster evolution of new proteins. It has been observed that many protein domains, which are functional units of proteins capable of folding into stable tertiary structures, are encoded by single exons. Additionally, proteins often consist of similar sets of domains. The alternation of exons with introns allows for exon shuffling through recombination events. This process can generate new combinations of exons, giving rise to novel genes that encode functionally active proteins, thus contributing to protein evolution and diversification.
Alternative splicing for protein diversity: RNA splicing enables the generation of diverse protein isoforms from a single gene through a mechanism known as alternative splicing. Different exons can be included or excluded from mature mRNA transcripts, leading to the production of multiple protein isoforms with distinct functions.
As RNA polymerase II progresses along the gene during transcription, it eventually encounters a specific sequence known as the polyadenylation signal, which is transcribed into the growing pre-mRNA. The canonical polyadenylation signal sequence is AAUAAA.
The polyadenylation signal is recognised by cleavage proteins that cut the transcript a short distance downstream from the signal. This cleavage event results in the release of the pre-mRNA.
An enzyme called poly-A polymerase (PAP) adds a string of adenine nucleotides, known as the poly-A tail, to the 3' end of the pre-mRNA. Typically, about 200 adenine nucleotides are added to create the poly-A tail.
Similar to the 5' cap, the poly-A tail protects mRNA from degradation and is required for mRNA export from the nucleus.
The figure below summarises RNA processing in eukaryotic cells.
More details on RNA splicing can be found in the following videos.
