FLEXIBILITY OF RNA

One of the fundamental properties of the RNA helix is its intrinsic resistance to bend- or twist-deformations. Results of a variety of physical measurements point to a persistence length of 700–800 Å for double-stranded RNA in the presence of magnesium cations, approximately 1.5–2.0-fold larger than the corresponding value for DNA. Although helix flexibility represents an important, quantifiable measure of the forces of interaction within the helix, it must also be considered in describing conformational variation of nonhelix elements (e.g. internal loops, branches), since the latter always reflect the properties of the flanking helices; that is, such elements are never completely rigid. For one important element of tertiary structure, namely, the core of yeast tRNA^Phe, the above consideration has led to the conclusion that the core is not substantially more flexible than an equivalent length of pure helix.