A study of the localization of high mobility group proteins in chromatin.

High mobility group (HMG) proteins from fetal calf thymus and mouse brain chromatin were purified and compared electrophoretically. The four major HMG proteins characteristic of fetal calf thymus chromatin (HMG's 1, 2, 14, and 17) were also found to be present in mouse brain chromatin. Nuclei from these two eucaryotic tissues were digested with DNase I and micrococcal nuclease and the acid-soluble proteins solubilized by the two nucleases in both tissues were analyzed on starch gels. Limited digestion of fetal calf thymus nuclei with DNase I led to the solubilization of a substantial fraction of proteins HMG-1 and HMG-2 together with smaller amounts of H1. In addition, limited digestion with micrococcal nuclease released approximately 70% of HMG's 1 and 2 and variable amount of H1 into the soluble fraction. The observation that HMG proteins 1 and 2 are selectively solubilized under conditions in which active genes have been shown to be preferentially digested in various other cell types suggests their selective association with chromatin regions which are transcriptionally competent.