The HMGB1 protein (High Mobility Group protein 1) participates in the formation of functionally significant DNA-protein complexes. HMGB1 protein contains two DNA-binding domains and negatively charged C-terminal region. The latter consists of continuous sequence of dicarboxylic amino acids residues. Structural changes in DNA-protein complexes were studied by circular dichroism spectroscopy (CD) and atomic force microscopy (AFM). Natural HMGB1 and recombinant protein HMGB1(A + B) lacked negatively charged C-terminal region were used. The DNA–HMGB1(A + B) complexes demonstrate an unusually high optical activity in 150 mM NaCl solutions. AFM of the latter complexes shows, that at the low concentration of HMGB1 in the complex the protein is distributed along DNA in a random way. Increase of HMGB1 content leads to cooperative interaction and a redistribution of the bound protein molecules on DNA is observed. Based on the data obtained we conclude that protein–protein interactions play a key role in the formation of highly ordered DNA–HMGB1 complexes. It was shown that C-terminal domain modulate the interactions of DNA with HMGB1 protein. We suggest that the C-terminal domain of HMGB1 also modulates the “packing” of HMGB1 molecules on the DNA.