A mechanistic study of the histone H1-DNA complex dissociation by sodium dodecyl sulfate

The effect of sodium dodecyl sulfate (SDS) on H1-DNA complex is studied by different methods. The histone HI fluorescence emission increases on SDS addition. It is may be due to the transfer of the only tyrosine residue of H1 to a more hydrophobic environment. Circular dichroism (CD) shows an increase of 17.1% in alpha-helix content of H1 in the presence of SDS. The CD spectrum of DNA in the presence of SDS also indicates the transfer of DNA from the B-form to the A-form both in the absence and presence of H1. The isothermal titration calorimetric investigation indicates that H1-SDS and DNA-SDS interactions are both exothermic. Spectrophotometric study of the percent of precipitation of DNA by H1 has shown that the complex formation is highly cooperative and it is completely dissociates by SDS > 0.5 mM. The results signify two important reasons for H1-DNA complex dissociation: (1) the addition of SDS reduces the ability of H1 to neutralize the negative charges of DNA, and (2) concurrently the hydrophobic chains of SDS induce the DNA conformational changes. Furthermore, the precipitation curve in the presence of lower concentrations of SDS (in the range of 0.25-0.5 mM) shows two domains. The calorimetric titration curve of DNA by H1 or the folded H1 (by 0.4 mM SDS) also reveal two domains. The results indicate that the C-terminal domain of intact H1 possibly is the first domain that is dissociated from DNA and then the separation of the globular domain occurs in the presence of SDS. (C) 2002 Elsevier Science B.V. All rights reserved.