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 term inal parts of the N- and C-domain, and Hl(120-210), comprising the ent ire 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 t hat the H1 fragments are capable of helical folding. The CD spectrum o f free (AT), shows strong negative and positive absorptions in the 200 -300 nm region resembling the psi-spectrum of DNA. Free (AT)6 showed n o helix-coil transition and remained single stranded at room temperatu re. Combinations of the H1 peptides with increasing concentrations of (AT)6 in low-ionic-strength phosphate buffer developed a strong negati ve ellipticity at 235 nm. This ellipticity increased with rising (AT)6 Concentration and diminished when the (AT)6 concentration exceeded th e 1:1 molar ratio in H1(1-16) and Hl(204-218) and the 2:1 molar ratio in Hl(120-210). The 235 nm ellipticity is attributed to a complex of t he H1 peptide with (AT)6 in which the protein is helical. Interaction between histone peptide and (AT)6 is also indicated by UV-absorption s pectra which show that the 260 nm absorption is decreased and the 280 nm absorption is increased as compared to free (AT)6. The free peptide s show no absorption in this window. The altered 260 and 280 nm absorp tion suggests that the single-stranded (AT)6 assumes a left-handed pit ch and this is confirmed by the displacement of the 270 nm positive el lipticity of free (AT)6 towards 260 nm. Implications of a left-handed linker DNA for chromatin function are discussed.