Topic 20 RNA Processing Study Notes

In eukaryotes, the primary RNA transcript must undergo processing to become a translatable mRNA, contrasting with bacteria where RNA does not require processing for translation.

Definition: Capping involves the addition of a 7-methylguanosine cap at the 5' end of the RNA transcript.
Functions:
- Required for export of RNA from the nucleus.
- Stabilizes mRNA, protecting it from degradation.
- Acts as a signal for translation initiation.
Figure Reference: Figure 21.19 illustrates the capping process.

Definition: Polyadenylation is the addition of a long A-tract (poly-A tail) to the 3' end of the RNA transcript.
Process:
- The RNA transcript is cleaved approximately 30 bases downstream of an AAUAAA sequence, which is located 3' to the coding region.
- A string of A residues (commonly 300 adenines) is then added to form the poly-A tail.
Representation:
- Before cutting: 5' AAUAAA (30 bases)__3'
- After cutting and poly-A addition: 5' AAAAAAAA(300) 3'
Functions of Polyadenylation:
- Enhances mRNA stability by reducing the effects of 3' exonucleases.
- Plays a role in the nuclear export of mRNA.
- Involved in the mRNA translation process.
Figure Reference: Figure 21.20 represents the polyadenylation process.

Definition: Splicing is the process where introns (noncoding sequences) are removed from pre-mRNA, and exons (coding sequences) are joined together.
Key Features:
- Eukaryotic genes can have multiple introns, while bacterial genes do not.
- Exons are the expressed sequences that remain in the mature mRNA.
- The exact origin of introns remains unclear; it's debated whether bacteria lost their introns or if eukaryotes gained them.
Utility: Introns contribute to differential splicing, allowing for the creation of multiple proteins from a single gene.
- This is particularly prominent in scenarios where tissue-specific protein forms are generated.
Splicing Mechanism:
- Involves specific sequences that flank the intron: the 5' junction, branch point, and 3' junction.
- Defects in these sites can lead to splicing errors and diseases such as thalassemia.
The Spliceosome:
- Composed of small nuclear ribonucleoprotein particles (snRNPs or snurps).
- snRNPs are complexes of small nuclear RNA and protein, playing roles in recognizing RNA sequences and facilitating splicing by arranging the RNA ends.
Figure Reference: Figure 21.23 provides an illustration of the splicing process; Figure 21.22 illustrates snRNPs.

By the end of this topic, students should be able to:
- Describe the outlined steps in mRNA maturation.
- Explain why these processing steps are critical.
- Describe alternative splicing and models explaining its mechanisms.
- Discuss how splicing problems can lead to diseases.

Splicing involves two key transesterification reactions:
1. The first reaction cleaves the exon 1-intron boundary, initiated by the attack of the 2’-OH group of the A nucleotide at the branch point, generating a lariat structure through a unique 2'-phosphodiester bond.
2. The second reaction entails the 3'-OH of exon 1 reacting with the 3' splice junction, which removes the intron and joins exon ends.
- The lariat structure formed is then displaced and subsequently degraded.
Figure Reference: Figure 21.24 depicts the transesterification reactions taking place in splicing.

RNA Concentration: The RNA concentration within cells is dictated by rates of RNA synthesis versus degradation.
Degradation Processes:
- RNA degradation can initiate at either end:
- 5’ end: decapping.
- 3’ end: deadenylation of the poly-A tail.
- Both processes are sequence independent.
RNA Interference (RNAi):
- This sequence-specific degradation of RNA requires a small interfering RNA (siRNA) or microRNA (miRNA) that binds complementary to the target mRNA.
- With the help of associated proteins, the mRNA is cleaved and degraded.
Implications: These mechanisms enable cells to regulate gene product synthesis effectively.
Figure Reference: Figures 21.26 and 21.27 illustrate RNA degradation and interference mechanisms.

rRNA and tRNA undergo extensive processing pre-functionalization, including base modifications such as methylation, hydroxylation, or deamination.
- These modifications are essential for their functionality.
Secondary Structure: Although RNAs are typically single-stranded, they can fold into complex secondary structures due to base pairing, with examples including tRNA and ribosomal RNA, which contains double-stranded sections.
Figure Reference: Figures 21.29, 21.30, and 21.31 provide visual representations of RNA modification and structural complexity.

RNA is synthesized in the nucleus and subsequently translated in the cytoplasm.
Export Process: Mature mRNAs are transported through nuclear pores composed of a set of organized proteins in the nuclear membrane.
- Transport is regulated and requires recognition of proteins attached to the poly-A tail, the 5’ cap, and additional sequences within the mRNA.

Errors in RNA processing can lead to improperly formed mRNAs:
- An mRNA lacking the necessary 5' cap or poly-A tail is primed for degradation rather than exportation, reflecting a quality control mechanism.
- Improper splicing may not always be detectable in the nucleus and may result in the translation of mutant proteins if exported.
- Mutations can introduce new splice sites causing errors during splicing, thereby potentially including intron sequences or omitting exon sequences in mature mRNA transcripts.
Consequences: Such aberrations in mRNA can lead to diseases due to the production of malfunctioning proteins.

To reinforce understanding, refer to Textbook Chapter 21 Review Questions: Q 57, 59, 61, 65, 69, 77, 83.