Interfering with RNA
A birthday tribute to someone I really admire here on G+, who is an amazing mentor and friend. Happy Birthday +Rajini Rao, hope you have a wonderful year ahead and it’s been a pleasure knowing and working with you on our many projects together on G+!
A few days ago during our ENCODE Hangout on Air, I mentioned microRNAs. I wanted to write a post today about the general mechanism of how a gene can be ‘silenced’ through a process known as RNA interference.
• As we explained during the Hangout, the Central Dogma of Molecular Biology is DNA → RNA → Protein. This means the DNA blueprint makes a sequence-specific copy of RNA, which in turn acts as a blueprint for a specific sequence of amino acids which make up a protein.
• So imagine if you could somehow destroy the RNA blueprint (known as the messenger RNA, or mRNA) for a particular protein. This would prevent the protein from being made – no blueprint, no protein. This is what RNA interference, or gene silencing is.
• It was first observed by plant scientists working on petunias in 1990. They were trying to make the color of the flower darker, so they introduced extra copies of the gene chalcone synthase, a key enzyme involved in the synthesis of pink and violet flower pigment. Their logic makes sense – adding more copies of the gene for chalcone synthase should make more protein, which in turn should make flowers darker. But when they did the experiment, instead of darker flowers they got lighter flowers, or fully or partially white flowers absent of any color (see image above). Clearly, introducing extra copies of the chalcone synthase gene was decreasing the activity of chalcone synthase.
• Similar phenomena were reported in fungi, and also in plant viruses. However, it was not until 1998 when Andrew Fire and Craig Mello formally identified the process as ‘RNA interference’ in their groundbreaking work on the nematode worm Caenorhabditis elegans. They injected sequence specific double-stranded RNA into the worm and then observed the silencing of the target gene (the mRNA blueprint went missing, and therefore so did the protein). This was the first time that double-stranded RNA was identified as the causative agent for the gene silencing phenomenon.
• The discovery completely revolutionized biology; Fire and Mello were awarded the Nobel Prize just eight short years after the publication of their work, which is very unusual (usually Nobel-prizes recognize work that is several decades old!). The work was so important because now we could silence any gene to see the effect it would have on the organism.
• Why is this important? Imagine you have a car, and you want to learn the function of the different parts. If you remove (or silence!) a wheel, the car cannot move. Therefore you can conclude that the wheel is necessary for motion. The same could be done within the organism; silence a gene, and you notice that the animal is now impaired in movement, so you can conclude that the gene may be involved in muscle development or coordination. Silence another gene, and you notice the eggs look strange, and you conclude that the gene was involved in egg development. RNA interference allowed scientists to assign a function to a gene; this has been and will remain an invaluable tool in molecular biology for decades ahead.
Paper references (all open access):
- “Planting the Seeds of a New Paradigm” (and image source) — paper showing the effect of RNAi with chalcone synthase
- “Potent and specific genetic interference by double-stranded RNA in Caenorhabditis elegans”
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