Describe the role of two forms of RNA editing that lead to changes in the size and sequence of pre-mRNAs. Briefly describe several examples of each form of editing, including their impact on respective protein products.

RNA editing refers to the process by which specific nucleotides within RNA molecules are altered after transcription but before translation. This process can change the size or sequence of a pre-mRNA molecule, affecting its coding capacity and ultimately the protein product synthesized. There are two main forms of RNA editing: insertion/deletion and substitution.

Insertion/deletion RNA editing involves the addition or removal of nucleotides within the pre-mRNA molecule, resulting in changes to the overall size and sequence of the transcript. This can lead to altered reading frames, creating new functional protein isoforms or preventing protein synthesis altogether. Example 1: Guide RNA-mediated editing in trypanosomes: In this process, guide RNAs (gRNAs) with complementary sequences to target mRNA molecules direct the insertion or deletion of uracil residues in the mRNA, changing the nucleotide sequence. These changes can affect the reading frame and generate different protein isoforms. Example 2: Editing of Apolipoprotein B (ApoB) mRNA in mammals: In ApoB mRNA editing, cytidine is deaminated to form uracil within the coding region. This creates a premature stop codon, leading to a truncated protein product, ApoB48, which performs different functions in lipid metabolism compared to the full-length ApoB100 protein.

Substitution RNA editing involves the modification of specific nucleotide bases within the pre-mRNA molecule, altering its sequence without affecting its overall size. This can change individual amino acids within the resulting protein or introduce new stop codons, influencing protein function. Example 1: Adenosine-to-Inosine (A-to-I) editing: In this process, adenosine residues are deaminated to yield inosine within the pre-mRNA sequence. Since inosine behaves like guanosine during translation, this editing leads to the substitution of amino acids in the protein product. The GluR2 gene is an example where an Adenosine-to-Inosine editing transforms a glutamine codon into an arginine codon, affecting calcium permeability properties of the resulting AMPA receptor. Example 2: Cytidine-to-Uridine (C-to-U) editing in plant organelles: In this form of editing, cytidine residues are deaminated to generate uracil, resulting in altered codons and changes in the amino acid sequence of the protein product. In plant mitochondria and chloroplasts, this type of RNA editing frequently occurs to correct conserved start and stop codons, ensuring proper protein synthesis. In conclusion, RNA editing through insertion/deletion or substitution results in changes to the size and sequence of pre-mRNAs, influencing the function of protein products. By understanding these two forms of editing and their examples, one can appreciate the diverse mechanisms employed by cells to regulate gene expression and protein function.