We have used triplet anisotropy decay techniques to study the flexibility of synthetic DNA fragments with different base pair compositions. We have found major differences in the torsional and bending stiffness of poly(dG) ·poly(dC), poly(dA)·poly(dT) and poly(dA-dC)·poly(dT-dG). Poly(dG)·poly(dC) has a torsional modulus more than 40 times larger than poly(dA-dC)·poly (dT-dG), and approximately 20 times larger than poly(dA)·poly(dT). These differences imply that the torsional stiffness of DNA can vary greatly with base composition. The Young's modulus (bending stiffness) we have measured for poly(dG)·poly(dC) is at least twice that of poly(dA-dC)·poly(dT-dG) or random sequence DNA, and is at least threefold greater than that of poly(dA)·poly(dT). This implies that the bending stiffness of DNA is also strongly dependent on base composition. In light of this dramatic base composition dependence, we suggest here that such stiffness variation may lead to local variations in the stability of chromatin or other protein complexes that require bending or twisting of the DNA helix.
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