Ligand specificity in the glmS riboswitch. The glmS riboswitch is a ribozyme. (a) On binding of its small-molecule metabolite, glucosamine 6-phosphate, the glmS precursor mRNA (pre-mRNA) cleaves itself, promoting its own degradation. (b) The structure of glucosamine 6-phosphate (GlcN6P) and some close structural analogs. (c) Result of agarose gel electrophoresis of ribonuclease-cleaved samples. The glmS RNA was incubated with its known effector (GlcN6P) or with one of the other compounds shown in (b), for the times indicated. The leftmost lanes show untreated pre-mRNA (NR) and pre-mRNA incubated with no effector added (−). The top row of bands shows the full-length pre-mRNA (Pre); the bottom row, the ribonuclease cleavage product (Clv). Only GlcN6P induced cleavage of the pre-mRNA, indicating that the glmS riboswitch is highly specific for this ligand; the results also show that the riboswitch requires Mg²⁺.