The THI-box riboswitch, or how RNA binds thiamin pyrophosphate.

Riboswitches are genetic control elements present mainly in the 5' untranslated regions of messenger RNAs that, upon binding of a small metabolite (like some vitamins, amino acids, and nucleobases), undergo conformational changes, affecting the expression of downstream genes. Structural studies of riboswitches are important for understanding how they recognize their ligands with high specificity and affinity. The thiamin pyrophosphate binding riboswitch (THI- box) is widely distributed in the three kingdoms of life and is involved in very distinct modes of gene regulation. Three recent THI-box structural analyses revealed how polyanionic RNA is able to bind a molecule with a negatively charged pyrophosphate group like thiamin pyrophosphate (TPP) and how it can discriminate between TPP and monophosphorylated analog molecules. These studies give insight into the genetic regulatory mechanisms in which the THI-box is involved.