Lecture 9 2020-2021

Control Points in Gene Expression

  • Various control points are crucial for regulating gene expression.

  • Post-transcriptional control occurs after mRNA is synthesized in the nucleus.

  • Key areas of control include:

    • mRNA degradation

    • Inactive mRNA transport

    • mRNA processing

    • Protein translation

    • Protein activity and localization control.

Alternative mRNA Splicing: The Concept

  • Alternative splicing is a process allowing the production of multiple mRNA variants from a single gene.

  • Exons: coding sequences that remain in the mRNA post-splicing.

  • Introns: non-coding sequences that are removed during splicing.

  • Splice variants: different mRNAs produced from various combinations of exons.

Mechanisms of Alternative Splicing

  • Cells exploit alternative splicing to generate diversity in protein outputs from one gene.

  • Key types of alternative splicing include:

    • Intron retention

    • Mutually exclusive exons

    • Alternative splice sites (5' & 3')

    • Exon inclusion/skipping

    • Tissue-specific regulation

Prevalence in Eukaryotes

  • Alternative splicing is common, with over 90% of genes in mammalian cells subject to splicing variations.

  • It impacts gene product diversity and mRNA properties (e.g., stability).

Alternative Splicing in Disease: Apoptosis

  • Splicing of apoptosis-regulated genes determines cell survival:

    • Anti-apoptotic vs Pro-apoptotic isoforms influence whether a cell undergoes apoptosis.

Example: Drosophila Sex Determination

  • Mechanisms of sex determination via alternative splicing:

    • The sex lethal (Sxl) gene plays a pivotal role in females (X:A ratio).

    • Males splice transformer (tra) gene to create a dysfunctional protein, while females produce a functional Tra protein.

    • Activation of DSX transcription factor occurs differently in males and females due to the presence of Tra.

Example of Human Disease: Myotonic Dystrophy

  • Myotonic Dystrophy is linked to splicing defects:

    • Dominantly inherited muscular dystrophy affecting muscle function.

    • Caused by CTG repeat expansions in the DMPK gene.

    • The expanded RNA sequesters critical splicing regulators, leading to specific disease symptoms.

Mdm2 Splicing in Cancer

  • In cancer, Mdm2 gene undergoes alternative splicing, producing variants that may contribute to tumor diversity and progression.

  • It regulates p53, a crucial protein in protecting against DNA damage.

Regulatory Elements of Splicing

  • Alternative splicing can be influenced by enhancers and silencers:

    • Specific RNA sequences can promote or inhibit splicing site choices.

    • Exonic and Intronic Splicing Enhancers (ESE/ISE) and Splicing Silencers (ESS/ISS) regulate splicing efficiency and choice.

Fate of RNA Binding Proteins

  • The Fox family of RNA binding proteins regulate alternative splicing in specific tissues, promoting the evolution of complex organisms.

  • They recognize specific motifs vital for splicing regulation in metazoans.

Trans-Splicing Reactions

  • Trans-splicing involves joint processing of exons from different pre-mRNA molecules; rare in multicellular organisms but prevalent in some organisms like trypanosomes.

  • Short Sequence Leader RNAs (SL RNA) play roles in this process.

Summary of Regulated Alternative mRNA Splicing

  • Alternative splicing significantly enhances genomic coding potential.

  • Generates diverse mRNA forms, including tissue and developmental specificity.

  • Particularly critical in non-dividing cells, particularly in neuronal and muscle cells.

mRNA Processing in Eukaryotes

  • Three stages of RNA processing:

    • 5' capping

    • RNA splicing: removal of introns and linkage of exons.

    • 3' polyadenylation for mRNA stability.