Despite the importance of local structural detail to a mechanistic understanding of RNA catalysis and binding functions, RNA backbone conformation has been quite recalcitrant to analysis. There are too many variable torsion angles per residue, and their raw empirical distributions are poorly clustered. This study applies quality-filtering techniques (using resolution, crystallographic B factor, and all-atom steric clashes) to the backbone torsion angle distributions from an 8,636-residue RNA database. With noise levels greatly reduced, clear signal appears for the underlying angle preferences. Half-residue torsion angle distributions for ␣--␥ and for ␦--are plotted and contoured in 3D; each shows about a dozen distinct peaks, which can then be combined in pairs to define complete RNA backbone conformers. Traditional nucleic acid residues are defined from phosphate to phosphate, but here we use a base-to-base (or sugar-to-sugar) division into ''suites'' to parse the RNA backbone repeats, both because most backbone steric clashes are within suites and because the relationship of successive bases is both reliably determined and conformationally important. A suite conformer has seven variables, with sugar pucker specified at both ends. Potential suite conformers were omitted if not represented by at least a small cluster of convincing data points after application of quality filters. The final result is a small library of 42 RNA backbone conformers, which should provide valid conformations for nearly all RNA backbone encountered in experimental structures.RNA structure ͉ RNA conformation ͉ backbone conformers ͉ quality filtering ͉ all-atom contacts R NA has long been known to play a central role in the storage, and especially in the communication, of biological information and to be well-suited for specific and regulated molecularbinding interactions. More recently, it has also been shown to perform enzymatic catalysis (1, 2) and is therefore quite likely to have been critical in the first development of living systems (3). The size, complexity, and specific detail of RNA 3D structure are essential to its various functions, and in that respect RNA is more like protein than like DNA. The determination, analysis, and modification of RNA structure have become an important aspect of biology, with major contributions from NMR (4), electron microscopy (5), and crystallography (6). Progress accelerated recently with the tour-de-force x-ray structures of the ribosomal 50S and 30S (7-9) subunits, which expanded the database of known RNA structures enough to make statistical analysis feasible.Despite all the new information, determining and analyzing RNA structure are both still very difficult tasks. Large RNA structures can typically be determined only to resolutions of Ϸ2.5 Å or lower, where the phosphates and base planes can be located quite reliably, but the sugars and especially the rest of the backbone are not seen well at all. As shown in Fig. 1, there are six rotatable torsion angles per residue along the RNA backbon...