DNA-induced α-Helical Structure in the NH<inf>2</inf>-terminal Domain of Histone H1

It is important to establish the structural properties of linker histones to understand the role they play in chromatin higher order structure and gene regulation. Here, we use CD, NMR, and IR spectroscopy to study the conformation of the amino-terminal domain of histone H1°, free in solution and bound to the DNA. The NH2-terminal domain has little structure in aqueous solution, but it acquires a substantial amount of α-helical structure in the presence of trifluoroethanol (TFE). As in other H1 subtypes, the basic residues of the NH2-terminal domain of histone H1° are clustered in its COOH-terminal half. According to the NMR results, the helical region comprises the basic cluster (Lys11-Lys20) and extends until Asp23. The fractional helicity of this region in 90% TFE is about 50%. His24 together with Pro25 constitute the joint between the NH2-terminal helix and helix I of the globular domain. Infrared spectroscopy shows that interaction with the DNA induces an amount of α-helical structure equivalent to that observed in TFE. As coulombic interactions are involved in complex formation, it is highly likely in the complexes with DNA that the minimal region with α-helical structure is that containing the basic cluster. In chromatin, the high positive charge density of the inducible NH2-terminal helical element may contribute to the binding stability of the globular domain.