What is a riboswitch? a small molecule that regulates the translation of specific mRNAs a small piece of noncoding RNA that can regulate translation of mRNA molecules in bacteria a gene regulatory protein that turns on the expression of ribosomal proteins an mRNA that can regulate its own transcription and translation a ribosome that participates in attenuation
The correct answer and explanation is:
Correct Answer:
An mRNA that can regulate its own transcription and translation.
Explanation (≈300 words):
A riboswitch is a regulatory segment of an mRNA molecule that can bind a small target molecule (like a metabolite) and cause a change in the expression of the genes encoded by that mRNA. What makes riboswitches unique is that they are part of the mRNA itself—they do not require proteins or other RNAs to function. Instead, the mRNA has a specific structure that can sense environmental cues and respond by altering its own transcription or translation.
Riboswitches are typically found in bacteria, though examples have been found in plants and fungi. They are usually located in the 5′ untranslated region (5′ UTR) of the mRNA. Structurally, a riboswitch consists of two parts:
- Aptamer domain: This is the sensing component. It binds specifically to a small molecule such as a vitamin, amino acid, or nucleotide.
- Expression platform: This domain undergoes structural changes depending on whether the ligand is bound to the aptamer. These changes determine whether transcription continues or translation occurs.
Mechanisms of Regulation:
- Transcriptional control: In some riboswitches, ligand binding causes the formation of a transcription terminator hairpin in the mRNA, causing RNA polymerase to stop transcription prematurely.
- Translational control: In other cases, the binding of the ligand blocks the ribosome-binding site (Shine-Dalgarno sequence), preventing translation from beginning.
For example, when a metabolite (like thiamine pyrophosphate, TPP) is abundant, it binds the riboswitch, which causes a conformational change that halts transcription or translation, thereby preventing unnecessary synthesis of the metabolite.
In conclusion, riboswitches are cis-acting elements—they are part of the mRNA and directly control their own expression in response to cellular metabolite levels, making them a sophisticated form of gene regulation without the need for proteins.