Effect of magnesium ions and temperature on the sequence-dependent curvature of DNA restriction fragments

Transient electric birefringence has been used to quantify the curvature of two DNA restriction fragments, a 199-base-pair fragment taken from the origin of replication of the M13 bacteriophage and a 207-base-pair fragment taken from the VP1 gene in the SV40 minichromosome. Stable curvature in the SV40 and M13 restriction fragments is due to a series of closely spaced A tracts, runs of 4-6 contiguous adenine residues located within 40 or 60 base pair 'curvature modules' near the center of each fragment. The M13 and SV40 restriction fragments exhibit bends of similar to 45 degrees. in solutions containing monovalent cations and similar to 60 degrees. in solutions containing Mg2+ ions. The curvature is not localized at a single site but is distributed over the various A tracts in the curvature modules. Thermal denaturation studies indicate that the curvature in the M13 and SV40 restriction fragments remains constant up to 30 degrees C in solutions containing monovalent cations, and up to 40 degrees C in solutions containing Mg2+ ions, before beginning to decrease slowly with increasing temperature. Hence, stable curvature in these DNA restriction fragments exists at the biologically important temperature of 37 degrees C.