What a riboswitch is and how it works
A riboswitch is a region on the mRNA that can act like a switch to turn gene expression on or off. In the presence of a specific ligand, a riboswitch changes shape uncoupling the polymerase of the DNA. Polymerase is no longer able to transcribe thus terminating gene expression. Riboswitches are useful for gene regulation.
The mRNA has the ability to fold and base pair with itself forming different structures, such as those found on the regions that make up a riboswitch. On a basic riboswitch, there are two main regions. One region is more sensitive to ligands, also known as an aptamer. The second region is also known as a terminator hairpin and its existence is dependent on whether or not a ligand is bound to the aptamer. When a ligand binds to an aptamer it causes the aptamer to become more stable. Because a more stable aptamer region forms, the mRNA downstream from the aptamer undergoes a shape change and a terminating hairpin forms via mRNA base pairing with itself. In the event that a ligand has bound to an aptamer and a hairpin has formed, transcription is terminated by the uncoupling of the DNA polymerase. mRNA is transcribed until the polymerase reaches a polyuracil region down stream of the riboswitch. The polyuracil region is also known as a pause site because the polymerase tends to slow down as it passes through. The polymerase is knocked of the DNA by the hairpin as it reaches the polyuracil region and uncouples thus terminating transcription. If a ligand does not bind to the aptamer region, the polymerase will simply continue transcribing down the DNA strand and through the pause site.
A little history
Riboswitches were first discovered around 2002. Metabolic pathways and such were noticed to change in the presence of certain chemistry such as vitamin B but it was not understood why this occurred and how it changed gene expression. It was discovered that vitamin B acts as a ligand and binds to the aptamer on the cobalamin riboswitch. With this discovery more naturally occurring riboswitches were discovered. The discovery of riboswitches allowed for a better understanding of how the DNA is able to regulate its own gene expression.
Types of riboswitches and surrounding conditions
Naturally occurring riboswitches usually terminate gene expression when bound by a ligand and have only two domains: an aptamer and a terminating hairpin. There are riboswitches that can turn gene expression on when bound by a ligand such.
Some riboswitches are not dependent on a ligand but rather temperature. An example is a magnesium transport protein riboswitch found in Salmonella enterica. When a certain temperature is reached, the cell experiences heat shock and transcription of the mgtA gene is turned on. Increased transport of magnesium is thought to have thermoprotective functions by diffusion of energy and oxygen thus protecting the cells membrane and proteins from denaturing.
Like other molecules, riboswitches are sensitive to pH levels, salt concentration, and temperature.
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