Strandedness discrimination in peptide-polynucleotide complexes.

Preferential binding to single- or double-stranded nucleic acids is important for the activity of many proteins that process RNA and DNA. We have investigated the mechanism of strandedness discrimination with peptides derived from the putative DNA-binding domain of the RecA protein, a bacterial recombinase that modulates its affinity for single-stranded DNA by means of ATP binding and hydrolysis. Contributions of electrostatic and non-electrostatic interactions to binding of these peptides with polynucleotides were evaluated by fluorescence spectroscopy as a function of salt concentration and peptide charge. Binding of these peptides to single- and double-stranded nucleic acids was dominated by non-electrostatic interactions. Small electrostatic contributions selectively enhanced peptide complexation with single-stranded nucleic acids. Similar results were observed in control experiments carried out with tripeptides containing charged and aromatic amino acid residues. It was possible to modify the strandedness preference of peptide-polynucleotide complexes by changing electrostatic contributions to the binding free energy. These observations suggest a mechanism whereby some proteins that interact with DNA or RNA might determine and regulate their relative affinity for single- and double-stranded nucleic acids.