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DNA-bridging by a palindromic alpha-helix.
1Medical Research Council Laboratory of Molecular Biology, Cambridge, United Kingdom.
Summary
Sea urchin sperm chromatin has a long DNA repeat and a large histone H1. This histone's unique sequence may stabilize DNA structure within the chromatin fiber.
Area of Science:
- Chromatin structure
- Molecular biology
- Biochemistry
Background:
- Sea urchin sperm chromatin exhibits an unusually long nucleosomal DNA repeat (240 bp).
- This chromatin also contains a larger-than-average histone H1 molecule.
- The solenoidal model describes the 300-Å diameter chromatin fiber.
Purpose of the Study:
- To correlate the long DNA repeat and large histone H1 in sea urchin sperm chromatin.
- To explain how these features fit into the solenoidal model of chromatin structure.
- To propose a mechanism for histone H1's role in DNA stabilization.
Main Methods:
- Sequence comparison of sea urchin spermatogenous H1 with other H1 sequences.
- Analysis of a 55-amino acid insert within the H1 sequence.
- Structural modeling based on sequence characteristics.
Main Results:
- A 55-amino acid insert (residues 122-176) was identified in sea urchin H1.
- A 37-residue palindromic sequence (residues 140-176) within the insert was found.
- This palindromic sequence likely forms two alpha-helical DNA-binding units.
Conclusions:
- The H1 insert's palindromic alpha-helices stabilize an extra superhelical turn in linker DNA.
- This supports models where linker DNA forms a right-handed superhelix within the solenoid.
- The mechanism resembles the 'scissors-grip' interaction seen in leucine-zipper proteins.