A region of an RNA molecule, usually a messenger RNA (mRNA), that can bind a specific metabolite and ‘switch off’ further transcription or translation of the mRNA. It acts as a mechanism in bacterial cells for regulating the concentration of a metabolite by controlling the synthesis of, for example, an enzyme or transport protein involved in synthesis or uptake of the metabolite. When levels of the metabolite are high, it binds to the riboswitch and alters the folding pattern (secondary structure) of the RNA helix such that further transcription of the mRNA is halted; when levels fall, and no metabolite is bound, the riboswitch adopts an alternative structure and transcription is able to proceed.