Xin Hua, Takahata Shinya, Blanksma Mary, McCullough Laura, Stillman David J, Formosa Tim
Department of Biochemistry, University of Utah School of Medicine, Salt Lake City, 84112, USA.
Mol Cell. 2009 Aug 14;35(3):365-76. doi: 10.1016/j.molcel.2009.06.024.
FACT has been proposed to function by displacing H2A-H2B dimers 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.
有人提出，FACT通过从核小体中置换H2A - H2B二聚体以形成六聚体来发挥作用。然而，这里所描述的酵母FACT（yFACT）的结果表明，核小体被重新组织成一种具有原始组成但结构更松散、更具动态性的形式。首先，yFACT在体外增强了整个核小体上各位点的羟基自由基可及性和核酸内切酶消化作用，而不仅仅是在与H2A - H2B接触的区域。可及性显著增加，但DNA仍受到部分保护。其次，在二聚体未从核小体上置换的情况下，核酸酶敏感性也会增加。第三，在体内转录激活过程中，从GAL1 - 10启动子上移除核小体需要yFACT，但未观察到形成六聚体时预期的二聚体占有率的优先降低。我们提出，yFACT促进了两种核小体形式之间的可逆转变，并且这种活性有助于染色质屏障的建立和维持以及对其的克服。
