Eukaryotic Transcription and mRNA Processing

Eukaryotic Transcription and mRNA Processing

Overview of Eukaryotic mRNA Processing

In eukaryotic cells, the initial RNA transcript (pre-mRNA) produced by transcription undergoes several processing steps before becoming mature messenger RNA (mRNA) capable of being translated into protein. These modifications are essential for mRNA stability, export from the nucleus, and efficient translation.

• Pre-mRNA to mRNA processing includes:

  • Addition of a 7-methylguanylate cap to the 5' end (5' capping)

  • Removal of introns (splicing)

  • Addition of a PolyA tail to the 3' end (polyadenylation)

• These modifications remove introns and increase mRNA stability, enabling export from the nucleus.

• Processing is coupled with transcription and occurs before the mRNA leaves the nucleus.

5' Capping of mRNA

The 5' cap is a modified guanine nucleotide added to the 5' end of the pre-mRNA soon after transcription begins. This cap is essential for mRNA stability and translation initiation.

• Enzyme involved: Guanylyl transferase catalyzes the addition of the cap.

• Structure: The cap consists of a 7-methylguanosine linked via a 5'-5' triphosphate bridge to the first nucleotide of the mRNA.

• Functions:

  • Protects mRNA from 5' to 3' exonucleases

  • Facilitates export from the nucleus

  • Promotes binding to the ribosome for translation initiation

  • Regulates splicing of the first intron

Example: The cap structure can be represented as:

$m^7G(5')ppp(5')N$

where m7G is 7-methylguanosine and N is the first nucleotide of the mRNA.

Splicing: Removal of Introns

Splicing is the process by which non-coding sequences (introns) are removed from the pre-mRNA, and coding sequences (exons) are joined together to form mature mRNA.

• Introns: Non-coding regions that interrupt the coding sequence of genes.

• Exons: Coding regions that remain in the mature mRNA.

• Spliceosome: A large ribonucleoprotein complex (about 3.3 MDa) composed of small nuclear RNAs (snRNAs) and proteins, responsible for catalyzing splicing.

• Consensus sequences: Conserved sequences at the exon-intron boundaries and branch point are essential for accurate splicing.

Key consensus sequences:

• 5' splice site (donor): GU sequence at the 5' end of the intron

• 3' splice site (acceptor): AG sequence at the 3' end of the intron

• Branch point: Conserved A nucleotide, usually within a sequence such as UACUAAC (yeast) or a similar motif in animals

Splicing mechanism:

1. The 2' OH of the branch point A attacks the 5' splice site, forming a lariat structure.

2. The free 3' OH of exon 1 attacks the 3' splice site, joining exons and releasing the intron lariat.

Biological consequences of splicing errors:

• Intron retention or exon loss can lead to frame shifts, premature stop codons, or altered protein products.

• Mutations in splicing signals can disrupt gene expression and cause disease.

Polyadenylation: Addition of the PolyA Tail

Polyadenylation is the addition of a stretch of adenine nucleotides (PolyA tail) to the 3' end of the pre-mRNA. This modification is important for mRNA stability, export, and translation.

• Enzyme involved: Poly(A) polymerase

• Function:

  • Protects mRNA from degradation

  • Facilitates export from the nucleus

  • Enhances translation efficiency

Comparison: Eukaryotic vs. Prokaryotic mRNA Processing

There are significant differences between eukaryotic and prokaryotic mRNA processing and translation.

Feature	Eukaryotes	Prokaryotes
Introns	Present (require splicing)	Absent (rare exceptions)
5' Cap	Present	Absent
PolyA Tail	Present	Absent
Transcription & Translation	Spatially and temporally separated (nucleus vs. cytoplasm)	Coupled (occur simultaneously in cytoplasm)
mRNA Stability	More stable due to processing	Less stable

Summary of mRNA Processing Steps

1. 5' Capping: Addition of 7-methylguanosine cap to the 5' end

2. Splicing: Removal of introns and joining of exons

3. Polyadenylation: Addition of PolyA tail to the 3' end

These steps are essential for producing mature, translatable mRNA in eukaryotic cells.

Additional info: The notes reference the role of the carboxy terminal domain (CTD) of RNA polymerase II in recruiting processing factors, and the importance of consensus sequences in splicing. Errors in splicing can have significant biological consequences, including disease.