A high mobility group protein binds to long CAG repeat tracts and establishes their chromatin organization in Saccharomyces cerevisiae.

Long CAG repeat tracts cause human hereditary neurodegenerative diseases and have a propensity to expand during parental passage. Unusual physical properties of CAG repeat tracts are thought to contribute to their instability. We investigated whether their unusual properties alter the organization of CAG repeat tract chromatin. We report that CAG repeat tracts, embedded in yeast chromosomes, have a noncanonical chromatin organization. Digestion of chromatin with the restriction enzyme Fnu4HI reveals hypersensitive sites occurring approximately 125 bp apart in the repeat tract. To determine whether a non-histone protein establishes this pattern, we performed a yeast one-hybrid screen using CAG repeat tracts embedded in front of two reporter genes. The screen identified the high mobility group box protein Hmo1. Chromatin immunoprecipitation of epitope-tagged Hmo1 selectively precipitates CAG repeat tracts DNAs that range from 26 to 126 repeat units. Moreover, deletion of HMO1 drastically alters the Fnu4HI digestion pattern of CAG repeat chromatin. These results show that Hmo1 binds to CAG repeat tracts in vivo and establish the basis of their novel chromatin organization.