Global warfare in the Cell

Unlike the rest of us who love RNA, the general cell (bacteria, humans alike) does not like RNA at all. Why you ask (or the two of you who read my blog). This is because RNA is typically a signature of viruses which are at open war with the cell. Some common RNA-based viruses are Influenza, HIV, SARs, Polio, Rabies and Measles just to mention a few. Now any virus needs a cell to reproduce, it just cannot reproduce lots of ‘baby’ viruses in any other way except taking over the cell as its own private nursery. The cell responds by attacking any stray RNA it can find, and by protecting its own RNAs by marking them with proteins or chemical modifications. RNAi (RNA interference) is also used to protect the cell from viruses.  A system well studied in plants (Virus Induced Gene Silencing, VIGS) has the cell taking the RNA strands as soon as they are detected and using them as siRNAs (small RNA strands from the last blog), so the virus-produced siRNAs find other virus RNAs, pair up with them forming double stranded RNA allowing the Dicer protein to come in and chop up the virus.  However, this is a war. The virus fights back by trying to disable the RNAi system in the cell. It is a good thing for us, and for plants that we have multiple lines of defence. In mammals, long double stranded RNAs can trigger an interferon response to boost our natural immune systems.

Bacteria have an RNA-like mechanism to tackle their viruses (also known as bacteriophages). This is called the CRISPR system, where parts of viruses are used as templates for an RNAi-type search and destroy mission. With these RNA-based defence mechanisms found throughout all life it is likely that RNA-viral defence mechanisms are very ancient (like billions of years old!) Viruses must therefore be just as ancient because nature does not evolve ahead of time, it does not look forward. A viral defence mechanism evolved because there were viruses and those single celled ancestors of ours had to have some defence or they didn’t have progeny.  So RNA is used in defence against RNA viruses even today, showing us in plain sight perhaps a molecular fossil of the ancient RNA world.

  

Bacteriophage http://thet2phage.com/thesongs.html  Influenza http://science.thomsonreuters.com/pharma/h1n1/