Folding of terminal histone-H1 peptides in the presence of the oligonucleotide 5'-(AT)6-3'.

Peptides H1(1-16) H1(204-218) of human histone H1, comprising the terminal parts of the N- and C-domain, and H1(120-210), comprising the entire C-domain of calf thymus H1, were studied using CD spectroscopy in the presence of trifluoroethanol (TFE) and the oligonucleotide 5'-(AT)6-3'. TFE induces a strong negative ellipticity at 220 nm, showing that the H1 fragments are capable of helical folding. The CD spectrum of free (AT)6 shows strong negative and positive absorptions in the 200-300 nm region resembling the psi-spectrum of DNA. Free (AT)6 showed no helix-coil transition and remained single stranded at room temperature. Combinations of the H1 peptides with increasing concentrations of (AT)6 in low-ionic-strength phosphate buffer developed a strong negative ellipticity at 235 nm. This ellipticity increased with rising (AT)6 concentration and diminished when the (AT)6 concentration exceeded the 1:1 molar ratio in H1(1-16) and H1(204-218) and the 2:1 molar ratio in H1(120-210). The 235 nm ellipticity is attributed to a complex of the H1 peptide with (AT)6 in which the protein is helical. Interaction between histone peptide and (AT)6 is also indicated by UV-absorption spectra which show that the 260 nm absorption is decreased and the 280 nm absorption is increased as compared to free (AT)6. The free peptides show no absorption in this window. The altered 260 and 280 nm absorption suggests that the single-stranded (AT)6 assumes a left-handed pitch and this is confirmed by the displacement of the 270 nm positive ellipticity of free (AT)6 towards 260 nm. Implications of a left-handed linker DNA for chromatin function are discussed.