Circular dichroism, thermal denaturation, and deoxyribonuclease I digestion studies of nucleosomes highly enriched in high mobility group proteins HMG 1 and HMG 2.

Salt-soluble (S) nucleosomes that contain near equimolar high mobility group nonhistone chromosomal proteins HMG 1 and HMG 2 and lack histone H1 were isolated from mouse myeloma nuclei. Comparisons of the sedimentation, near-UV circular dichroism, thermal denaturation, and pattern of DNase I digestion of S nucleosomes with these properties of nucleosome cores or "typical" nucleosomes containing H1 did not detect significant differences. These results indicate that HMG 1 and 2 do not affect the conformation and stability of nucleosomes or nucleosomal DNA and are consistent with the proposal that major functions of HMG 1 and 2 are to replace H1 and maintain the (micro) solubility and accessibility of local chromatin regions. In contrast to these similarities, the initial rate of DNase I digestion of S nucleosomes was approximately 3 times that of chromatin depleted in S nucleosomes. This is consistent with a relation of S nucleosomes to transcription and suggests that subtle factors (not necessarily HMG 1 and 2) determine DNase I susceptibility.