Mutational analysis of differences in thermostability between histones from mesophilic and hyperthermophilic archaea

Amino acid residues responsible for the large difference in thermostability between HMf3 and HFoB, archaeal histones from the hyperthermophile Methano thermus fervidus and the mesophile Methanobacterium formicicum, respectivel y, have been identified by site-specific mutagenesis, The thermal denaturat ion of similar to 70 archaeal histone variants has been monitored by circul ar dichroism, and the data generated were fit to a two-state unfolding mode l (dimer-->two random coil monomers) to obtain a standard-state (1M) meltin g temperature for each variant dimer. The results of single-, double-, and triple-residue substitutions reveal that the much higher stability of rHMfB dimers, relative to rHFoB dimers, is conferred predominantly by improved i ntermolecular hydrophobic interactions near the center of the histone dimer core and by additional favorable ion pairs on the dimer surface.