STABILIZATION OF INTRAMOLECULAR TRIPLE SINGLE-STRAND STRUCTURE BY CATIONIC PEPTIDES/

For better comprehension of possible physiological roles of triple-hel ical DNA structures, it is important to understand if the proteins can stabilize intramolecular tripler (H-DNA). One plausible mode of stabi lization is through the neutralization of electrostatic repulsion of n egatively charged phosphates in the three DNA strands by positively ch arged arginine and lysine residues of a bound protein. To gain an insi ght into interactions between H-DNA and cationic protein domains, we e xamined the effect of Lys- and Arg-rich oligopeptides on the B-DNA to H-DNA transition. These oligopeptides as well as another type of polyc ation, spermine, shifted the equilibrium toward H-DNA, These polycatio ns introduced Little change in DNA superhelicity, so that an increase in torsional stress was not responsible for facilitated H-DNA formatio n. Competing influences of polycations and monovalent cations suggest a significant involvement of electrostatic interactions in H-DNA stabi lization. The Arg-rich peptides are more effective in H-DNA stabilizat ion than the Lye-rich ones. However, as inferred from experiments on i ntermolecular complexes, this is not due to a better stabilization of triple helix or destabilization of double helix. It is possible that A rg-rich peptides interact with the unpaired single strand in H-DNA and stabilize its unpaired conformation.