yFACT Induces Global Accessibility of Nucleosomal DNA without H2A-H2B Displacement

FACT has been proposed to function by displacing H2A-H2B dinners from nucleosomes to form hexasomes. Results described here with yeast FACT (yFACT) suggest instead that nucleosomes are reorganized to a form with the original composition but a looser, more dynamic structure. First, yFACT enhances hydroxyl radical accessibility and endonuclease digestion in vitro at sites throughout the nucleosome, not just in regions contacted by H2A-H2B. Accessibility increases dramatically, but the DNA remains partially protected. Second, increased nuclease sensitivity can occur without displacement of dimers from the nucleosome. Third, yFACT is required for eviction of nucleosomes from the GAL1-10 promoter during transcriptional activation in vivo, but the preferential reduction in dimer occupancy expected for hexasome formation is not observed. We propose that yFACT promotes a reversible transition between two nucleosomal forms, and that this activity contributes to the establishment and maintenance of the chromatin barrier as well as to overcoming it.