The organic crystallizing agent 2-methyl-2,4-pentanediol reduces DNA curvature by means of structural changes in A-tracts

Contemporary predictive models for sequence-dependent DNA structure provide a good estimation of overall DNA curvature in most cases, However, the two current models differ fundamentally in their view of the origin of DNA curvature, An earlier model that associates DNA bending primarily, although not exclusively, with stretches of adenines (A-tracts) is based on results of comparative gel retardation, cyclization kinetics, hydroxyl radical cutting, and other solution measurements. It represents an intersection of wedge and junction models, More recently, a non-A-tract bending model has been proposed, built on structural results from x-ray crystallography and molecular modeling, In this view, A-tracts are proposed to be straight and rigid, whereas mixed sequence DNA is bent. Because a key premise of the non-A-tract bending model is the crystallographic observation that A-tracts are straight, we have examined the effect in solution of 2-methyl-2,4-pentanediol (MPD), an organic solvent used in crystal preparation for crystallographic DNA structure determinations, Using cyclization analysis, DNase I cutting, chemical probing, and electron microscopy on DNA oligomers with and without A-tracts, we show that the presence of MPD in solution dramatically affects A-tracts and that the effect is specific to these sequence elements. Combined with the previous observation that MPD affects gel mobility of curved sequences with A-tracts, our findings support the bent A-tract model and call for caution in the interpretation of crystallographic results on DNA structure as these are presently obtained.