The bacterial glycine riboswitch is an RNA element that can bind the amino acid glycine. Glycine riboswitches usually consist of two metabolite-binding aptamer domains with similar structures in tandem. The aptamers cooperatively bind glycine to regulate the expression of downstream genes. In Bacillus subtilis, this riboswitch is found upstream of the gcvT operon which controls glycine degradation. It is thought that when glycine is in excess it will bind to both aptamers to activate these genes and facilitate glycine degradation.
A truncated version of the glycine riboswitch exhibits sigmoidal binding curves with Hill coefficients greater than one, which lead to the idea of positive cooperativity between the two aptamer domains. Data in 2012 shows that cooperative binding does not occur in the switch with its extended 5' leader, though the purpose of the dual aptamer switch is still uncertain.