Don't Shoot the Messenger
Until recently, RNA was only of interest in its role as an intermediary in transcription and translation. However, that has now all changed forever! The discovery that RNA has an important role in controlling gene expression through RNA interference (or RNAi) has led scientists to reassess the significance of RNA and even to ask the question how important is DNA?
As with many major breakthroughs in science, the role of RNAi was discovered by chance through studying petunia genetics. However, it wasn’t until the late 1990s, when researchers Craig C. Mello and Andrew Fire worked on the nematode worm C. elegans, that RNAi was first understood.
Mello and Fire found that RNA is sometimes used to switch off or silence genes through the production of short complimentary RNA fragments (called small interfering RNA strands or siRNA) that specifically bind to messenger RNA from specific genes. These double stranded RNAs are degraded by cellular enzymes. It provides a completely new level of complexity in a cell’s ability to control its gene expression.
The biological role of RNAi is thought to be very ancient – at least one billion years old! As with restriction enzymes in bacteria, RNAi acts as a viral defence mechanism– still used in plants to protect them against double stranded RNA viruses.
RNAi also reminds us of the importance of RNA in the evolution of life on Earth. The so-called RNA World Hypothesis proposes that RNA was the original way life stored genetic information (with DNA being a later addition). Given this, perhaps it is not so surprising that RNA plays such a crucial role in the control of genetic information rather than just as a passive messenger.
As a research tool, RNAi has become something of a craze! RNAi has replaced hugely expensive & laborious knock-out technologies for deleting a gene’s activity to see what it does. This highly selective “gene silencing” reveals what genes are doing so that both where and when the gene is stopped can be controlled more easily. Perhaps this is because, unlike other mechanisms for switching off genes, RNAi mimics a natural process.
It is expected that RNAi will have a major impact in how genetic research is done and also in how we are able to selectively treat diseases in the future. For this reason, the Nobel Prize was jointly awarded to Craig C. Mello and Andrew Fire in 2006.