Ryan M P, Jones R, Morse R H
Molecular Genetics Program, Wadsworth Center, New York State Department of Health, and State University of New York School of Public Health, Albany 12201-2002, USA.
Mol Cell Biol. 1998 Apr;18(4):1774-82. doi: 10.1128/MCB.18.4.1774.
The SWI-SNF complex in yeast and related complexes in higher eukaryotes have been implicated in assisting gene activation by overcoming the repressive effects of chromatin. We show that the ability of the transcriptional activator GAL4 to bind to a site in a positioned nucleosome is not appreciably impaired in swi mutant yeast cells. However, chromatin remodeling that depends on a transcriptional activation domain shows a considerable, although not complete, SWI-SNF dependence, suggesting that the SWI-SNF complex exerts its major effect at a step subsequent to activator binding. We tested this idea further by comparing the SWI-SNF dependence of a reporter gene based on the GAL10 promoter, which has an accessible upstream activating sequence and a nucleosomal TATA element, with that of a CYC1-lacZ reporter, which has a relatively accessible TATA element. We found that the GAL10-based reporter gene showed a much stronger SWI-SNF dependence than did the CYC1-lacZ reporter with several different activators. Remarkably, transcription of the GAL10-based reporter by a GAL4-GAL11 fusion protein showed a nearly complete requirement for the SWI-SNF complex, strongly suggesting that SWI-SNF is needed to allow access of TFIID or the RNA polymerase II holoenzyme. Taken together, our results demonstrate that chromatin remodeling in vivo can occur by both SWI-SNF-dependent and -independent avenues and suggest that the SWI-SNF complex exerts its major effect in transcriptional activation at a step subsequent to transcriptional activator-promoter recognition.
酵母中的SWI-SNF复合物以及高等真核生物中的相关复合物，被认为可通过克服染色质的抑制作用来辅助基因激活。我们发现，转录激活因子GAL4在定位核小体中与位点结合的能力，在swi突变酵母细胞中并未受到明显损害。然而，依赖转录激活结构域的染色质重塑显示出相当程度（尽管并不完全）的SWI-SNF依赖性，这表明SWI-SNF复合物在激活因子结合后的步骤中发挥其主要作用。我们通过比较基于GAL10启动子的报告基因（其具有可及的上游激活序列和核小体TATA元件）与CYC1-lacZ报告基因（其具有相对可及的TATA元件）对SWI-SNF的依赖性，进一步验证了这一观点。我们发现，对于几种不同的激活因子，基于GAL10的报告基因比CYC1-lacZ报告基因表现出更强的SWI-SNF依赖性。值得注意的是，由GAL4-GAL11融合蛋白对基于GAL10的报告基因进行转录，显示出对SWI-SNF复合物几乎完全的需求，这强烈表明需要SWI-SNF来允许TFIID或RNA聚合酶II全酶的进入。综上所述，我们的结果表明，体内染色质重塑可通过SWI-SNF依赖性和非依赖性途径发生，并表明SWI-SNF复合物在转录激活因子与启动子识别后的步骤中，在转录激活中发挥其主要作用。
