RNA_Processing

RNA Processing in Eukaryotes

Overview of Transcription

• Prokaryotes:

  • Transcription occurs directly in the cytoplasm.

  • RNA polymerase binds to the promoter, transcribes mRNA from the template strand starting at the transcription start site and continuing to the terminator.

  • Resulting mRNA includes:

    • 5' UTR (Untranslated Region)

    • Coding sequence (between the start and stop codons)

    • 3' UTR after the stop codon.

  • Prokaryotic mRNA is immediately translated into protein.

• Eukaryotes:

  • Transcription occurs within the nucleus.

  • RNA polymerase binds to the promoter, beginning transcription similarly to prokaryotes.

  • Produces a pre-mRNA / primary transcript that includes:

    • 5' UTR

    • Introns (non-coding sequences)

    • Exons (coding sequences)

    • Stop codon

    • 3' UTR.

  • Pre-mRNA must undergo processing before translation.

Steps of RNA Processing in Eukaryotes

1. Addition of 5' Cap:

   • Involves the addition of a 7-methylguanylate (7 mG) cap at the 5' end.

   • This cap protects the mRNA from degradation by exonucleases and is crucial for ribosome binding during translation.

2. Splicing:

   • Removal of introns and joining of exons via a complex known as spliceosome.

   • Spliceosome includes snRNPs (small nuclear ribonucleoproteins) which recognize splice sites (GU-AG consensus sequence).

   • The process creates a lariat structure as introns are cut out, and exons are joined together.

3. Addition of Poly A Tail:

   • Involves adding 80 to 250 adenines to the 3' end by polyadenylate polymerase (PAP).

   • Poly A binding proteins protect the tail from degradation, contributing to stability.

Role of Untranslated Regions (UTRs)

• UTRs are important for:

  • Stability and transport of mRNA from the nucleus to the cytoplasm.

  • Binding of ribosomes for translation initiation.

  • Containing cis-regulatory elements that interact with trans-acting factors influencing translation and localization of mRNA.

Mechanism of Splicing

• Key Sites in the Intron:

  • Donor Splice Site: Begins with a GU sequence (5' end of intron).

  • Acceptor Splice Site: Ends with an AG sequence (3' end of intron).

  • Branch Point: Contains an adenine nucleotide critical for the splicing process.

• Splicing process includes:

  • U1 snRNP recognizes the donor site, recruiting U2 snRNP at the branch point, leading to a conformational change and recruitment of additional snRNPs (U4, U5, U6).

  • Formation of a lariat structure when the branch point adenine attacks the donor splice site.

  • Exons are ligated together, and the lariat (intron) is degraded after splicing.

Alternative Splicing

• Produces multiple mRNA variants from a single gene, allowing for:

  • Exon exclusion: Certain exons can be skipped.

  • Intron retention: Some introns may remain.

  • Partial exon exclusion: Portions of an exon may be excluded.

  • Partial intron retention: Parts of introns may be retained.

• Result: Different mature mRNAs can lead to the production of proteins with diverse functions from the same genetic sequence.

Summary of Processing Coordination

• The capping, splicing, and polyadenylation happen co-transcriptionally as RNA polymerase synthesizes the pre-mRNA, leading directly to the mature mRNA ready for translation.