Tales from the RNA Underworld - A Blog from the Dark Side of the cell

Bacteria have RNA too!

Up to now most of what I have explained has been from eukayotes  which are species whose DNA resides in a nucleus (like us, plants and fungi). Since I have been helping my colleague Patrick with his book chapter on prokaryotic small RNAs I thought that might be a good topic for this week since I am finding them so fascinating. Prokaryote is the collective name for bacteria and archaea (archaea are the fun species that grow at high temperatures, high salt and can have some very unusual chemistries and we are still very much in the learning stage as to how they operate in extreme environments).

Non coding RNA in bacteria are sometimes called small RNAs. Just like in humans we have RNAs involved in viral defence (called CRISPRs) that take bits of virus to use against them, and just like in humans the viruses try to work around that. There are also many small RNAs involved in responding to cell stresses ( examples are temperature too hot, temperature too cold, too much sugar).

One sRNA I am becoming fond of is called transfer-messenger RNA (tmRNA) which is part a transfer RNA (tRNA) and part a messenger RNA (mRNA). Normally the mRNA threads through the ribosome and tRNAs bring amino acids to the ribosome to build up the coded protein. However, sometimes the ribosomes get stuck, usually because something has gone wrong with the mRNA and the code is just unreadable. This can tie up valuable resources in the cell so in bacteria, tmRNA detects a stuck ribosome, uses the tRNA part to get into the heart of the machinery, then uses its mRNA part to code a rescue sequence to unstick  the ribosome and release the faulty mRNA. The faulty mRNA is degraded and the ribosome can get on with the next mRNA. This whole processes is called trans-translation.

As you can see below the way the tmRNA folds is very complicated as you would expect for an RNA that is doing the job of two.  We are still in the early days of discovering how RNA helps the bacterial cell as it was not too many years ago that this was thought impossible. I guess RNA really did take over the world.