Recurring RNA motifs: from A-minor to ribose zipper.

The twenty-first century has been called the 'century of RNA'. This name not only reflects the many recently discovered functional roles of RNA in biology but also the many new experimental RNA structures determined in this timeframe, which includes ribosomes, small ribozymes, self-splicing introns, spliceosomes, metabolite-sensing riboswitches, viral RNAs, and ornate large extremophilic RNAs, to name a few. In addition to zipping itself up via base pairing to form a complementary, DNA-like double helix, single-stranded RNA folds back on itself, producing many noncanonical interactions that are integral to its biological activity. Although the number of unique, experimentally determined RNA structures still lags far behind the number of proteins - existing in a so-called "structural dark matter" universe - a sufficient number of structures are known to identify folds and interaction themes that are frequently repeated. Here several common, recurring RNA motifs are presented for the non-specialist. An emphasis is placed on defining sequences or exemplar structures of each motif, close-packing and hydrogen-bonding attributes, classification (Leontis-Westhof) nomenclature, and examples of where the motif is found in nature. This atlas represents a foundation to understand how combinations of motifs lead to elegant hierarchical RNA tertiary and quaternary folds whose diverse forms impart distinct functions.