For 10 points, name these elements that respond to ligand binding by forming alternative structures that regulate the  expression of mRNA.
One family of these structures is characterized by the presence of a three-way helical junction, while the downstream  peptide motif is hypothesized to form one of these structure because it can conform to a shape that resembles a  pseudoknot. Another class of these structures, which contain natural aptamers, works by forming a stem-loop structure  followed by several (*) uridine residues, and that structure acts as a rho- independent terminator. These structures are generally  located in 5-prime untranslated regions, and some of these structures work by blocking and unblocking Shine-Dalgarno  sequences. For 10 points, name these elements that respond to ligand binding by forming alternative structures that regulate the  expression of mRNA.
One family of these structures is characterized by the presence of a three-way helical junction, while the downstream  peptide motif is hypothesized to form one of these structure because it can conform to a shape that resembles a  pseudoknot.
riboswitches