Structure and function of preQ riboswitches.

PreQ riboswitches help regulate the biosynthesis and transport of preQ (7-aminomethyl-7-deazaguanine), a precursor of the hypermodified guanine nucleotide queuosine (Q), in a number of Firmicutes, Proteobacteria, and Fusobacteria. Queuosine is almost universally found at the wobble position of the anticodon in asparaginyl, tyrosyl, histidyl and aspartyl tRNAs, where it contributes to translational fidelity. Two classes of preQ riboswitches have been identified (preQ-I and preQ-II), and structures of examples from both classes have been determined. Both classes form H-type pseudoknots upon preQ binding, each of which has distinct unusual features and modes of preQ recognition. These features include an unusually long loop 2 in preQ-I pseudoknots and an embedded hairpin in loop 3 in preQ-II pseudoknots. PreQ-I riboswitches are also notable for their unusually small aptamer domain, which has been extensively investigated by NMR, X-ray crystallography, FRET, and other biophysical methods. Here we review the discovery, structural biology, ligand specificity, cation interactions, folding, dynamics, and applications to biotechnology of preQ riboswitches. This article is part of a Special Issue entitled: Riboswitches.