Coexistence of A- and B-form DNA in a single crystal lattice

Abstract

It is well known that DNA can exist in a variety of conformations which can be interconverted by relatively mild changes in conditions. The in vivo conformation of DNA is usually thought to be the B form, but there is recent evidence that other conforma-tions may be important in DNA–protein recognition. Different fragments of DNA crystallized under virtually identical conditions can form A, B or Z helices¹. A fragment that adopted an A conformation in a crystal was found in the B conformation in solution², whereas NMR spectroscopy of A-DNA films³ revealed the presence of a substantial amount of disordered B-DNA. Until now, however, a DNA fragment of a given sequence has not been crystallized in more than one global conformation^4,5. We report here an X-ray diffraction study of crystals of the DNA octamer dGGBrUABrUACC. In addition to a 'framework' of A-DNA, which gives discrete X-ray reflections, there are partially disordered B-DNA helices, recognized by their diffuse scattering features.