Lecture 7

• RNA Editing

  The molecular processes by which the information content in an RNA molecule is altered through chemical changes in the base makeup
• Types of RNA Editing

  • C to U Editing
  • A to I Editing
• C to U Editing

  Common in mammalian nuclear RNAs, enzymatically catalyzed by APOBEC which deaminates a cytosine (C) to uracil (U), affecting protein function by altering the encoded amino acids
• A to I Editing

  Occurs in the nucleus of higher eukaryotes, involves the deamination of adenosine (A) to inosine (I) by ADAR, inosine is interpreted as guanosine (G) by the cellular machinery, potentially altering codons and splice sites, which can change protein function and gene expression
• ADAR editing

  • Can create or remove splice sites, adding another layer of regulation to gene expression
• Biological Significance of RNA Editing

  • Functional Diversity: RNA editing can lead to the production of multiple proteins from a single gene, increasing protein diversity without altering the underlying DNA
  • Regulatory Mechanisms: RNA editing is a post-transcriptional modification that provides a means to dynamically regulate gene expression in response to environmental stimuli or developmental cues
• RNA Editing in Trypanosomes

  In Trypanosoma brucei, extensive RNA editing is required for the expression of mitochondrial genes, involving the insertion and deletion of uridine residues guided by small guide RNAs (gRNAs), essential for the creation of functional mRNAs from cryptic or incomplete genomic templates
• Evolutionary Perspective of RNA Editing

  RNA editing is considered a primitive genetic regulation mechanism that might resemble ancestral RNA-based life forms (RNA world hypothesis), the capability of RNA molecules to catalyze reactions (ribozymes) and modify their own structure suggests an evolutionary advantage by adding flexibility to genetic control mechanisms
• Clinical and Therapeutic Implications of RNA Editing

  Understanding RNA editing mechanisms is crucial for developing therapies for diseases where editing is abnormal, such as certain cancers and neurological disorders, manipulation of RNA editing pathways offers potential methods for correcting genetic mutations post-transcriptionally in genetic diseases
• Nuclear Export of mRNP

  The process by which messenger ribonucleoprotein particles (mRNPs) are transported from the nucleus to the cytoplasm, crucial for the mRNA to be translated into proteins by ribosomes in the cytoplasm
• mRNP Components

  • mRNA
  • Proteins: exon-junction complexes (EJC), heterogeneous nuclear ribonucleoproteins (hnRNP), poly(A) binding proteins (PABP), and others that bind during or after transcription and processing
• Function of mRNP Components

  Protect the mRNA, assist in its export, and later guide the translation process
• Nuclear Pore Complex (NPC)

  Large protein complexes that span the nuclear envelope, acting as gates for the transport of mRNPs and other molecules between the nucleus and cytoplasm, allow passive diffusion of small molecules and actively transport larger complexes like mRNPs through a highly regulated mechanism
• Mechanism of mRNP Export

  1. mRNP Preparation: mRNPs are prepared for export through the binding of various proteins, which help in folding the mRNA into a compact, export-ready structure

  2. Interaction with Export Receptors: Export Receptors - Proteins that recognize specific signals on the mRNP, facilitating its interaction with the NPC, NES (Nuclear Export Signals) - Sequences within the mRNP components recognized by export receptors

  3. Export Process: Initial Contact - mRNP makes initial contact with the cytoplasmic filaments of the NPC, Translocation - The mRNP is then actively transported through the nuclear pore, Release into Cytoplasm - Once through the pore, the mRNP is released into the cytoplasm where the export receptors may be recycled back to the nucleus
• Regulatory Aspects of Nuclear Export

  • Integration with Transcription and Processing: Nuclear export is tightly integrated with transcription and RNA processing, ensuring only fully processed and competent mRNPs are exported
  • Quality Control: The cell has mechanisms to retain improperly processed mRNPs within the nucleus, preventing the translation of faulty proteins
• Clinical and Biotechnological Implications of Nuclear Export

  Defects in the nuclear export process can lead to diseases such as cancer and viral infections, where regulatory mechanisms are hijacked or malfunctioning, components of the nuclear export process are potential targets for therapeutic interventions, particularly in treating diseases that involve the misregulation of nuclear-cytoplasmic transport
• RNAi

  A biological process in which RNA molecules inhibit gene expression or translation by neutralizing targeted mRNA molecules, a crucial mechanism for regulating gene expression and maintaining genomic stability
• Key Components of RNAi

  • MicroRNAs (miRNAs): Small non-coding RNA molecules, typically about 22 nucleotides in length, that play a role in post-transcriptional regulation of gene expression
  • Short Interfering RNAs (siRNAs): Similar to miRNAs but typically derived from longer, double-stranded RNA precursors and are often involved in response to viral infection and in gene silencing
• Biogenesis of miRNAs


  1. Transcription: miRNAs are transcribed as primary miRNAs (pri-miRNAs) which are then processed in the nucleus into precursor miRNAs (pre-miRNAs) by Drosha, an RNase III enzyme

  2. Processing: Pre-miRNAs are transported to the cytoplasm where they are further processed by Dicer, another RNase III enzyme, into mature miRNA duplexes
• Mechanism of Action of RNAi

  1. RISC Loading: The mature miRNA strand is incorporated into the RNA-induced silencing complex (RISC), while the passenger strand is degraded

  2. Target Recognition: The miRNA within RISC pairs with complementary sequences on target mRNAs, leading to mRNA degradation or translational repression depending on the degree of complementarity
• Functions and Implications of RNAi

  • Gene Regulation: miRNAs are involved in the fine-tuning of gene expression during development, differentiation, and response to environmental stimuli
  • Defense Mechanism: In plants and some animals, siRNAs are part of an adaptive immune response against viruses
  • Technological and Therapeutic Applications: RNAi is used in research to selectively silence genes and is being developed as a therapeutic tool for treating diseases such as cancer, viral infections, and genetic disorders
• Clinical and Research Highlights of RNAi


  • RNAi in Medicine: Therapies based on RNAi mechanisms, such as siRNA drugs, are being developed to target and silence disease-causing genes
  • Research Tools: RNAi is a powerful tool in molecular biology used to study gene function by knocking down the expression of specific genes in cell lines and animal models
• Challenges and Future Directions of RNAi

  • Delivery and Stability: Effective delivery of RNAi therapies to target tissues and protection against degradation in the biological environment are major challenges
  • Off-target Effects: Ensuring specificity without unintended effects on other genes is critical for the safe application of RNAi in clinical settings