Yu Qun, Elizondo Susan, Bi Xin
Department of Biology University of Rochester Rochester, NY 14627, USA.
J Mol Biol. 2006 Mar 10;356(5):1082-92. doi: 10.1016/j.jmb.2005.11.089. Epub 2005 Dec 20.
In Saccharomyces cerevisiae, transcriptional silencing of the cryptic mating loci HML and HMR is established by the combined actions of cis-acting silencers and trans-acting proteins, including Sir2p, Sir3p and Sir4p. The Sir proteins serve as an integral part of a special silent chromatin at the HM loci. Deletion of any of the SIR2-SIR4 genes leads to a complete loss of silencing. However, the SUM1-1 mutation can restore silencing at the HM loci. Recently, it has been shown that Sum1-1p is directed to the silencers and internal regions of the HM loci, and interacts with the Hst1p histone deacetylase that is a paralog of the Sir2p histone deacetylase. Like Sir-dependent silent chromatin, Sum1-1p-dependent chromatin is hypoacetylated. These suggest that Sum1-1p and Hst1p play roles similar to those of the Sir proteins in promoting transcriptional silencing. Here, we examine whether Sum1-1p-dependent chromatin is similar to Sir-dependent silent chromatin, which is characterized by densely and precisely positioned nucleosomes. We demonstrate that Sum1-1p-dependent primary chromatin structure at HMR largely resembles, but is not identical with, Sir-dependent silent chromatin, whereas Sum1-1p-dependent HML chromatin largely resembles, but is not identical with, derepressed chromatin found in a sir- background. This correlates with the previous finding that SUM1-1 restores silencing more efficiently at HMR than at HML. We show also that DNA in Sum1-1p-dependent silent chromatin assumes a distinct topology. Moreover, we present evidence indicating that Sum1-1p can increase the stability of Sir-dependent silent chromatin, thereby providing an explanation for the finding that SUM1-1 enhances HML/HMR silencing in a SIR+ background.
在酿酒酵母中，隐蔽交配型基因座HML和HMR的转录沉默是由顺式作用沉默子和反式作用蛋白共同作用建立的，这些蛋白包括Sir2p、Sir3p和Sir4p。Sir蛋白是HM基因座处特殊沉默染色质的重要组成部分。缺失任何一个SIR2 - SIR4基因都会导致沉默完全丧失。然而，SUM1 - 1突变可以恢复HM基因座的沉默。最近的研究表明，Sum1 - 1p定位于HM基因座的沉默子和内部区域，并与Sir2p组蛋白去乙酰化酶的旁系同源物Hst1p组蛋白去乙酰化酶相互作用。与Sir依赖的沉默染色质一样，Sum1 - 1p依赖的染色质也是低乙酰化的。这些表明Sum1 - 1p和Hst1p在促进转录沉默方面发挥着与Sir蛋白类似的作用。在此，我们研究Sum1 - 1p依赖的染色质是否与以密集且精确排列的核小体为特征的Sir依赖的沉默染色质相似。我们证明，HMR处Sum1 - 1p依赖的初级染色质结构在很大程度上类似于但不完全等同于Sir依赖的沉默染色质，而Sum1 - 1p依赖的HML染色质在很大程度上类似于但不完全等同于在sir - 背景中发现的去抑制染色质。这与之前的发现一致，即SUM1 - 1在HMR处比在HML处更有效地恢复沉默。我们还表明，Sum1 - 1p依赖的沉默染色质中的DNA呈现出独特的拓扑结构。此外，我们提供的证据表明Sum1 - 1p可以增加Sir依赖的沉默染色质的稳定性，从而解释了SUM1 - 1在SIR + 背景下增强HML / HMR沉默这一发现。
