Basic concept:
- DNA is like a recipe book coding for proteins made from amino acids.
- Genes (recipes) are transcribed into mRNA, which ribosomes “read” to build proteins.
- mRNA letters (A, U, G, C) correspond to amino acids; “A” stands for adenosine.
Relevance : GS 3(Science and Technology)
What is A-to-I mRNA editing?
- ADAR enzymes convert adenosine (A) in mRNA to inosine (I).
- Ribosomes read inosine as guanine (G), causing changes in the protein sequence.
- This editing can alter protein function by changing amino acids.
Why is it puzzling?
- DNA could directly encode G instead of A, but it doesn’t—mRNA editing adds complexity.
- For example, editing can convert stop codons (UAG, UGA) into a codon for tryptophan (UGG), allowing proteins to be longer.
- The purpose of this complicated mechanism is unclear.
Recent study insights from Fusarium graminearum (a fungus):
- No A-to-I editing during vegetative (growth) stage on infected plants.
- Massive A-to-I editing (over 26,000 sites) during sexual reproduction stage.
- Focused on 71 genes with premature stop codons (PSC genes) “rescued” by editing.
- Deleting PSC genes affected fungus only during sexual stage, proving editing’s developmental role.
Functional implications:
- Unedited versions of some PSC genes help resist environmental stress during vegetative growth, so direct DNA mutation (A→G) would be disadvantageous early on.
- Suggests evolutionary advantage in delaying editing until necessary for development.
Evolutionary perspective:
- A-to-I editing may be a transitional evolutionary mechanism.
- Over time, more genes might depend on editing, making ADAR essential for gene expression.
- This could eventually lead to accumulation of G-to-A mutations in DNA “masked” by editing.
Scientific challenge:
- Understanding the net evolutionary benefit of A-to-I editing is more complex than discovering its function.
- The mechanism adds a regulatory layer that seems unnecessarily complicated.
Broader significance:
- mRNA editing adds flexibility to gene expression without permanent DNA changes.
- Can help organisms adapt protein function dynamically to developmental or environmental cues.
- Raises fundamental questions about genetic information processing and evolution.
