A RNA.DNA hybrid that can adopt two conformations: an x-ray diffraction study of poly(rA).poly(dT) in concentrated solution or in fibers.

The RNA.DNA hybrid poly(rA).poly(dT) can adopt two conformations, depending upon its degree of hydration. Fibers yield a conventional A' RNA-like pattern at 79% relative humidity. In contrast, under highly solvated conditions this material yields an x-ray diffraction pattern with striking similarities to that of B DNA and clearly different from the RNA-like diffraction patterns previously obtained from fibers of other hybrids. A structural model is proposed for the solution form of the hybrid that has several similarities to models proposed for B DNA. The hybrid model accommodates the 2'-hydroxyl groups of the poly(rA) strand in intrachain hydrogen bonds to the adjacent ribose moieties.