Structure and function of pseudoknots involved in gene expression control.

Natural RNA molecules can have a high degree of structural complexity but even the most complexly folded RNAs are assembled from simple structural building blocks. Among the simplest RNA elements are double-stranded helices that participate in the formation of different folding topologies and constitute the major fraction of RNA structures. One common folding motif of RNA is a pseudoknot, defined as a bipartite helical structure formed by base-pairing of the apical loop in the stem-loop structure with an outside sequence. Pseudoknots constitute integral parts of the RNA structures essential for various cellular activities. Among many functions of pseudoknotted RNAs is feedback regulation of gene expression, carried out through specific recognition of various molecules. Pseudoknotted RNAs autoregulate ribosomal and phage protein genes in response to downstream encoded proteins, while many metabolic and transport genes are controlled by cellular metabolites interacting with pseudoknotted RNA elements from the riboswitch family. Modulation of some genes also depends on metabolite-induced messenger RNA (mRNA) cleavage performed by pseudoknotted ribozymes. Several regulatory pseudoknots have been characterized biochemically and structurally in great detail. These studies have demonstrated a plethora of pseudoknot-based folds and have begun uncovering diverse molecular principles of the ligand-dependent gene expression control. The pseudoknot-mediated mechanisms of gene control and many unexpected and interesting features of the regulatory pseudoknots have significantly advanced our understanding of the genetic circuits and laid the foundation for modulation of their outcomes.