August 26, 2012 | 8
There is more than one type of genetic material within the cell. As well as DNA, which stores the code for making cellular protiens, there is also RNA, which contains similar snatches of code but is less stable and more mobile than DNA. If DNA is a library of books which are not allowed to be removed, then RNA is little buts of paper containing copies of pages that are spread around for people to read.
Given its power to act as an intermediary between DNA and protiens, which are two of the most important macromolecules within the cell, RNA has a huge number of jobs to do. One of those jobs is to regulate which parts of the DNA are making proteins. Not all of the DNA in the cell is being used all the time, and small pieces of RNA have the ability to show the cell which parts of the DNA need to be working at any one time.
In the case of the bacteria Streptococcus pneumoniae, the small RNAs can turn on the parts of the DNA required to make the bacteria virulent. It seems that the bacteria uses very specific RNA fragments to turn on different genes at different stages in its virulence cycle. At each stage, a specific set of small RNAs will be produced in order to control gene expression. A recent paper from PLoS Pathogens (reference below) carried out three sets of experiments to show this. Firstly, they sequenced the entire genome of the Streptococcus in order to find the sections that looked similar to other bacterial small RNAs. Once identified (they found around 89!) they specifically removed the ability of the cells to make the some of the small RNAs to see the effect that had on bacterial virulence. Finally, they looked for the targets of these RNAs, to find out which parts of the DNA expression they were actually affecting.
To explore the effect of removing the small RNA sections they looked at a measurement called the “competitive index” which involved infecting mice with both the wild-type and mutated bacteria (bacteria without the small RNAs) and seeing how well they compared when in competition with each other. A competitive index of one means that equal amounts of wild-type and mutant bacteria were found, less than one means that more wild-type bacteria were found and greater than one means that more of the mutated bacteria were found. As expected, in almost all individual cases the mutants performed worse than the wild-type in infection, some performing significantly badly.
The graph above shows the effect of removing certain sRNA from bacteria infecting the blood. Each point represents a single individual, with the lines showing the average of the strain. The scale is logarithmic, which means that while strain F5 was around 5 times worse than the wild-type on average, strain R12 was over 100 times worse. What’s also interesting is that one individual with the F5 mutation performed better than the wild type, although two of them also performed much, much worse. The paper explored the fate of mutations on virulence in the nose/throat area and the lung, with mutants performing worse in all cases.
Finding the target sites of the small RNAs was more complex, as each one appeared to both up-regulate (turn on the production) and down-regulate a large number of proteins. To explore this, the researchers carried out Northern Blots, which leave a stain for every protein produced inside the cell, for both the mutants and the wild type and then compared the similarities and differences. The graph below shows the huge differences in proteins controlled, and suggests that each small RNA has a large number of effects within the cell, controlling a range of responses including DNA repair, synthesis of nucleotides, and virulence.
Using RNA to send important messages to the genome is an advantageous strategy for bacteria, as it uses less energy than creating proteins to do the job, and requires less DNA to store the information. RNAs can either be very specific, or focus on multiple targets, allowing them to have very defined roles in controlling large genetic changes, such as the onset of bacterial virulence. The main task the researchers have now, is to work out the precise function of all these RNAs, and which genes they tweak to create the pathogenic bacterial cell.
Credit link for image 1
Credit link for image 2 (Link to the CDCs Public Health Image Library)
Reference 1: Mann B, van Opijnen T, Wang J, Obert C, Wang YD, Carter R, McGoldrick DJ, Ridout G, Camilli A, Tuomanen EI, & Rosch JW (2012). Control of Virulence by Small RNAs in Streptococcus pneumoniae. PLoS pathogens, 8 (7) PMID: 22807675