DNA is often bent when complexed with proteins. Understanding the forces responsible for DNA bending would be of fundamental value in exploring the interplay of these macromolecules. A series of experiments was devised to test the hypothesis that proteins with cationic surfaces can induce substantial DNA bending by neutralizing phosphates on one DNA face. Repulsions between phosphates in the remaining anionic helix are predicted to result in an unbalanced compression force acting to deform the DNA toward the protein. This hypothesis is supported by the results of electrophoretic experiments in which DNA spontaneously bends when one helical face is partially modified by incorporation of neutral phosphate analogs. Phasing with respect to a site of intrinsic DNA curvature (hexadeoxyadenylate tract) permits estimation of the electrostatic bend angle, and demonstrates that such modified DNAs are deformed toward the neutralized surface, as predicted. Similar model systems may be useful in exploring the extent to which phosphate neutralization can account for DNA bending by particular proteins.
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