Final answer:
Riboswitches are allosteric RNA leaders that bind small effector molecules, affecting the stability and structure of mRNA, thereby regulating gene expression at the transcriptional and translational levels.
Step-by-step explanation:
Riboswitches are allosteric RNA leaders that bind small effector molecules to control gene expression. These specialized RNA molecules are found in the 5' end of certain mRNA transcripts in prokaryotes. When a small molecule, such as a metabolite, binds to a riboswitch, it can cause the mRNA to fold into specific stem-loop structures. This binding may stabilize certain configurations of the mRNA, which can influence whether transcription continues or is terminated early. Furthermore, in some cases, the binding of the small molecule can also prevent translation of the mRNA, providing an additional level of regulatory control over gene expression.