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.

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1. span>RNA editing is a post-transcriptional process that alters nucleotide sequences in RNA molecules, leading to changes in the size and sequence of pre-mRNAs. There are two primary forms of RNA editing: insertion/deletion editing and substitution editing.
2. span>Insertion/Deletion Editing: This form involves the addition or removal of nucleotides in the RNA sequence. A well-known example is the editing of mitochondrial mRNAs in trypanosomes, where uridines are inserted or deleted. This editing is crucial for the correct translation of mitochondrial proteins, as it can restore open reading frames or create start and stop codons.
3. span>Substitution Editing: This involves the alteration of specific nucleotides within the RNA sequence. A common example is the conversion of cytidine to uridine (C-to-U) in plant mitochondria and chloroplasts. Another example is the editing of apolipoprotein B (apoB) mRNA in mammals, where a C-to-U change creates a stop codon, resulting in a shorter protein product (apoB48) compared to the full-length apoB100.
4. span>Impact on Protein Products: RNA editing can significantly impact the resulting protein products by altering amino acid sequences, creating premature stop codons, or modifying regulatory elements. This can affect protein function, localization, and interactions, ultimately influencing cellular processes and organismal phenotypes.
5. span>In summary, RNA editing is a versatile mechanism that increases the diversity of the proteome by modifying RNA sequences post-transcriptionally. Understanding these processes provides insights into gene regulation and the evolution of genetic complexity.

Key Concepts

RNA Editing

RNA editing is a molecular process that alters the nucleotide sequence of RNA transcripts after they have been synthesized from DNA. This modification can lead to changes in the resulting protein products by either changing the amino acid sequence or affecting the stability and localization of the mRNA. Two primary forms of RNA editing are A-to-I editing and C-to-U editing, which can significantly influence gene expression and protein function.

A-to-I Editing

A-to-I editing involves the conversion of adenosine (A) to inosine (I) in RNA molecules, primarily mediated by the enzyme ADAR (adenosine deaminases acting on RNA). This type of editing can alter codons, potentially changing the amino acid sequence of proteins or creating new splice sites. For example, in the glutamate receptor gene, A-to-I editing can affect receptor function and neuronal signaling, impacting synaptic plasticity.

C-to-U Editing

C-to-U editing refers to the deamination of cytidine (C) to uridine (U) in RNA, primarily catalyzed by the enzyme APOBEC. This form of editing can lead to changes in the coding sequence of mRNAs, affecting protein structure and function. An example is the editing of the apolipoprotein B (APOB) mRNA, which results in the production of two different protein isoforms, influencing lipid metabolism and cardiovascular health.