Repeat peptide motifs which contain beta-turns and modulate DNA condensation in chromatin

In order to understand better the roles of repeating basic peptide motifs in modifying DNA structure, we have synthesized typical repeats found in the C-terminal domain of histone H1 (KTPKKAKKP)2 and in the N-terminal domain of nucleolin (ATPAKKAA)2. By using circular dichroism in conjunction with Raman and Fourier-transform infrared spectroscopies, we demonstrate that the abilities of the two peptides to affect DNA conformation are dramatically different. Whilst the binding of the nucleolin repeat to DNA does not significantly alter its conformation, the binding of H1 repeat induces a very marked DNA condensation, giving rise to a psi(-)-type circular dichroic spectrum. The H1 repeat thus adopts a more rigid beta-turn-containing structure which probably binds to the DNA minor groove as assessed by competition with the drug Hoechst 33258. Unexpectedly, the DNA condensation induced by the H1 repeat is enhanced by the nucleolin repeat which by itself does not promote any alteration in DNA conformation.