Certain small metabolic intermediates influence translation initiation of mRNA molecules by stabilizing or destabilizing the way that a specific region, the riboswitch, in the 5'-untranslated region folds. In the absence of metabolite, the riboswitch folds into a structure that permits the SD in the mRNA to interact with the ASD on the 16S rRNA. In the presence of metabolite (M), the riboswitch folds into a structure in which the SD is part of a helical structure and so cannot interact with the ASD. The remarkable feature of the riboswitch is that specificity for the metabolite is determined entirely by the RNA molecule and does not involve protein; that is, the RNA folds into a structure that binds а sресifiс mеtаbоlitе аnd по оthеrs. Fоr instаnсе, thе ribоswitсh that is part of an mRNA that codes for an enzyme required for thiamine (vitamin B1) synthesis binds thiamine pyrophosphate.
Genes controlled by riboswitches often code for proteins that participate in the formation or transport of the metabolite that is sensed by the riboswitch. Other metabolites that have been shown to regulate translation initiation by their interaction with a riboswitch include vitamin B12, flavin mononucleotide, S-adenosylmethionine, guanine, lysine, and glycine. Riboswitches that influence translation initiation have been demonstrated in bacteria and archaeons. In many cases, the binding of a metabolite to a riboswitch also triggers premature transcription termination because the resultant RNA folding produces a transcription terminator. Plants and fungi also appear to influence splicing rather than translation initiation. In view of the widespread distribution of riboswitches in nature, it would be surprising if riboswitches were not eventually shown to contribute to gene regulation in animals.