X-ray fibre diffraction study of an elevated temperature structure of poly(dA).poly(dT).

A reversible conformational transition between two discrete double helical forms of poly(dA).poly(dT) has been put into evidence by X-ray fibre diffraction. We observed that the transition between the well known B' conformation and a new helical structure (B*) occurs at a relative humidity near 80%, when the temperature is raised above 30 degrees C. It appears that the B* conformation is not just a distorted B' form of poly(dA).poly(dT) but rather a stable (up to a least 70 degrees C) distinct double helical structure of that polynucleotide. Analysis of X-ray patterns allowed us to present the geometrical parameters of a molecular model of this new double helix. It consists of 11.4 nucleotide pairs per turn in a pitch length of about 36.7 A. The proposed high-temperature right handed helical structure of poly(dA).poly(dT) is a member of the B-DNA family since the duplex has C1'-exo furanoses in both antiparallel but geometrically identical sugar-phosphate strands. The present finding may shed light on interpretations of results obtained from premelting or nucleosome formation processes involving (dA.dT) tracts in synthetic or natural DNA polymers.