An alpha-helical peptide model for electrostatic interactions of proteins with DNA. The N terminus of RecA.

A series of synthetic peptides have been studied as models for non-specific protein-DNA interactions. In an alpha-helical conformation, the charged amino acid residues of the N-terminal 24 residues of RecA protein are asymmetrically distributed; at neutral pH there is a +4 charge on one face of the helix and a -3 charge on the other face. Modeling suggests that the positive face of the helix can bind five DNA phosphate groups by electrostatic interactions. Circular dichroism (c.d.) spectra indicate that the analogous peptide, Rec24 (AIDENKQKALAAALGQIEKQFGKG-amide), is largely unstructured in water but becomes highly helical in the presence of DNA. Peptide titrations of fluorescent etheno-DNA confirm that the changes in the c.d. spectrum of the peptide are associated with binding, although a dependence of the c.d. signal on the degree of DNA saturation is observed, indicating that peptide can be bound in more than one conformation. At saturation the peptide binds to 5.0(+/- 0.5) DNA phosphate groups as predicted and the electrostatic nature of the binding is confirmed by a strong dependence on salt concentration. A "mutant" peptide where an acidic glutamate residue replaces an alanine on the basic face of the Rec24 helix exhibits weaker binding to single-stranded DNA, also consistent with the electrostatic nature of the proposed peptide-DNA interaction. Extending Rec24 by ten amino acid residues, where the additional residues do not participate in the helical motif, does not noticeably affect binding. Thus, we show experimentally that an asymmetric charge distribution on an alpha-helix can represent an important element for binding nucleic acids.