A stable alpha-helical element in the carboxy-terminal domain of free and chromatin-bound histone H1 from sea urchin sperm.

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The carboxy-terminal domain (residues 121-248) of sea urchin sperm-specific H1 is not random coil but partly alpha-helical, even in 1 mM sodium phosphate, pH 7. The helix resides in a 57 residue proline-free segment which, in the intact histone, immediately abuts the central globular domain. The proline-free region, which is rich in lysine and alanine, is relatively resistant to tryptic digestion when the carboxy-terminal domain is bound to DNA. Two (overlapping) resistant peptides are shown by circular dichroism measurements to be substantially alpha-helical in 1 mM sodium phosphate and to increase in helix content to approximately 70% in 1 M NaCLO4. Tryptic digestion of chromatin gives resistant fragments containing both the globular domain and the contiguous proline-free segment, strongly suggesting that the alpha-helical segment also exists in chromatin, where it would be ideally placed to direct the path of the linker DNA entering or leaving the nucleosome. The linker in sea urchin sperm chromatin is long (approximately 74 bp), and the unusually long alpha-helical segment in the carboxy-terminal tail of sperm H1 which has amphipathic character due to the alanine distribution, and is likely to be curved, may be a special feature tailored to organize it.

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