DNA-induced alpha-helical structure in the NH2-terminal domain of histone H1

It is important to establish the structural properties of linker histones t o understand the role they play in chromatin higher order structure and gen e regulation. Here, we use CD, NAM, and IR spectroscopy to study the confor mation of the amino-terminal domain of histone H1 degrees, free in solution and bound to the DNA. The NH2-terminal domain has little structure in aque ous solution, but it acquires a substantial amount of a-helical structure i n the presence of trifluoroethanol (TFE). As in other H1 subtypes, the basi c residues of the NH2-terminal domain of histone H1 degrees are clustered i n its COOH-terminal half. According to the NMR results, the helical region comprises the basic cluster (Lys(11)-Lys(20)) and extends until Asp(23). Th e fractional helicity of this region in 90% TFE is about 50%. His(24) toget her with Pro(25) constitute the joint between the NH2-terminal helix and he lix I of the globular domain. Infrared spectroscopy shows that interaction with the DNA induces an amount of a-helical structure equivalent to that ob served in TFE. As coulombic interactions are involved in complex formation, it is highly likely in the complexes with DNA that the minimal region with a-helical structure is that containing the basic cluster. In chromatin, th e high positive charge density of the inducible NH2-terminal helical elemen t may contribute to the binding stability of the globular domain.