Illustration of two tetrahydrofolate riboswitch molecules from the bacterium Streptococcus mutans. Riboswitches are segments of bacterial mRNA (messenger ribonucleic acid) that regulate the transcription of the mRNA's protein product. This riboswitch regulates tetrahydrofolate, the biologically active form of folate, which is a building block in many biological molecules, including amino acids and nucleic acids, and a cofactor in many cellular reactions.