Structure of Poly (U)•Poly (A)•Poly (U)

Abstract

The molecular structure of poly (U) • poly (A) • poly (U) has been determined and refined using the continuous x-ray intensity data on layer lines in the diffraction pattern obtained from an oriented fiber of the RNA. The final R-value for the preferred structure is 0.24, far lower than that for the plausible alternatives. The polymer forms an 11-fold right-handed triple-helix of pitch 33.5å and each base triplet is stabilized by Crick-Watson-Hoogsteen hydrogen bonds. The ribose rings in the three strands have C3′-endo, C2′-endo and C2′-endo conformations, respectively. The helix derives additional stability through systematic interchain hydrogen bonds involving ribose hydroxyls and uracil bases. The relatively grooveless cylindrical shape of the triple-helix is consistent with the lack of lateral organization.