HMG1 Domains: The Victims of the Circumstances

The method of circular dichroism (CD) was used to compare DNA behavior during its interaction with linker histone H1 and with nonhistone chromosomal protein HMG1 at different ionic strength and at different protein content in the system. The role of the negatively charged C-terminal segment of HMG1 was analyzed using recombinant protein HMG1-(A+B), which lacks the C-terminal amino acid sequence. The ψ-type CD spectra were common for DNA interaction with histone H1, but no spectra of this type were observed in HMG1–DNA systems even at high ionic strength. The CD spectrum of the truncated recombinant protein at high salt concentration somewhat resembled the ψ+-type spectrum. Two very intense positive bands were located near 215 nm and near 272 nm, and the whole CD spectrum was positive. The role of the C-terminal part of HMG1 in the formation of ordered DNA–protein complexes is discussed.