Alpha-helix in the carboxy-terminal domains of histones H1 and H5.

Although the carboxy-terminal domains of histones H1 and H5 exist as random-coil in aqueous solution, secondary structure prediction suggests that this region has a high potential for alpha-helix formation. We have measured CD spectra in various conditions known to stabilize alpha-helices, to determine whether this potential can be realized in an appropriate environment. Trifluoroethanol increases the helix contents of H1, H5 and their carboxy-terminal fragments, presumably through promotion of axial hydrogen bonding. Sodium perchlorate is also effective and better than sodium chloride, suggesting stabilization by binding of bulky perchlorate ions rather than simple charge screening. Extrapolating from these measurements in solution, and taking into account the occurrence of proline residues throughout the carboxy-terminal domain, we propose that binding to DNA stabilizes helical segments in the carboxy-terminal domains of histones H1 and H5, and that it is this structured form of the domain that is functionally important in chromatin.