A riboswitch (of ribonucleic acid (RNA), which consists of it, and the English switch, 'power button') is an RNA sequence that is part of an mRNA and can bind a specific ligand and, hence, expression of the relevant gene. It is an aptamer that occurs naturally in some bacteria, plants and fungi.
Principle of the mechanism of actionA riboswitch has a complex 3D structure through internal base pairing. When the riboswitch binds its ligand, its structure changes. The internal base pairing goes differently, thus expressing / expressing a gene that was not expressed / less, or vice versa. A riboswitch can work at transcriptional or translational level, and can act actively or suppressively. Riboswitch working at transcription level
During DNA transcription, the mRNA binds a ligand, thereby affecting the transcription termination. In the case of activation, the conformation change of the riboswitch causes a transcription termination sequence to be no longer accessible to the RNA polymerase. For example, a portion of the transcription termination sequence can be linked by the base pair to the riboswitch sequence. As a consequence, the transcription, which would be degraded without ligand binding, is continued. If the riboswitch works suppressively, it is the exact opposite: just because the ligand is bound, the transcription termination sequence is readable and transcription is discontinued. As a result, the mRNA can not be translated into a protein and thus the gene expression is reduced. Riboswitch working at translation level
Binding to the ligand does not affect the transcription. Binding of the formed mRNA to the ligand determines whether the portion of the sequence required to bind to the ribosomes is accessible or not. In the case of an activating riboswitch binding to the ligand causes this sequence to become accessible to the ribosomes. An mRNA that could not be translated in the absence of the ligand now results in a protein. The ligand thus causes increased gene expression. In the case of a suppressive riboswitch, binding to the ligand correctly ensures that the ribosomes can not bind to the mRNA, which causes no protein to be produced. The ligand thus causes reduced gene expression.
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