Sekinger E A, Gross D S
Department of Biochemistry and Molecular Biology, Louisiana State University Health Sciences Center, Shreveport, LA 71130, USA.
Cell. 2001 May 4;105(3):403-14. doi: 10.1016/s0092-8674(01)00329-4.
Chromatin is thought to repress transcription by limiting access of the DNA to transcription factors. Using a yeast heat shock gene flanked by mating-type silencers as a model system, we find that repressive, SIR-generated heterochromatin is permissive to the constitutive binding of an activator, HSF, and two components of the preinitiation complex (PIC), TBP and Pol II. These factors cohabitate the promoter with Sir silencing proteins and deacetylated nucleosomal histones. The heterochromatic HMRa1 promoter is also occupied by TBP and Pol II, suggesting that SIR regulates gene expression not by restricting factor access to DNA but rather by blocking a step downstream of PIC recruitment. Interestingly, activation of silent promoter chromatin occurs in the absence of histone displacement and without change in histone acetylation state.
染色质被认为通过限制DNA与转录因子的接触来抑制转录。我们使用一个两侧带有交配型沉默子的酵母热休克基因作为模型系统，发现由SIR产生的抑制性异染色质允许激活因子HSF以及起始前复合物（PIC）的两个组分TBP和Pol II的组成型结合。这些因子与Sir沉默蛋白和去乙酰化核小体组蛋白共同占据启动子。异染色质的HMRa1启动子也被TBP和Pol II占据，这表明SIR调节基因表达不是通过限制因子与DNA的接触，而是通过阻断PIC募集下游的一个步骤。有趣的是，沉默启动子染色质的激活在没有组蛋白置换且组蛋白乙酰化状态没有改变的情况下发生。
