Surface histidine residue of archaeal histone affects DNA compaction and thermostability

Archaeal histone, which possesses only the core domain part of eukaryal histone, induced DNA compaction by binding to DNA. Based on structural modeling, tetramer formation by dimer-dimer interaction is considered to require two intermolecular ion pairs formed between histidine and aspartate. To examine the role of the ion pairs on DNA compaction, mutant histones were constructed and analyzed using HpkB from Thermococcus kodakaraensis KOD1 as a model protein. The mutant histones, HpkB-H50A, HpkB-H50V, and HpkB-H50G were constructed by replacing conserved surface His50 with Ala, Val, and Gly, respectively. Circular dichroism analysis indicated no significant difference between wild-type and mutants in their structures. Gel mobility shift assays showed that all mutants possessed DNA binding ability, like wild-type HpkB, however all mutants compacted DNA less efficiently than the wild-type. Moreover, all mutants could not maintain the nucleosome-like structure (compacted form of DNA) above 80 degrees C. These results suggest that surface ion pairs between His and Asp play an important role in maintenance of nucleosome structure and DNA stabilization at high temperature.