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Gene regulation in eukaryotes 2

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VI. Gene silencing by modification of histones and DNA Activators and repressors bind near a gene and switch it on or off. The effects are local, and their actions are often controlled by specific extracellular signals.

Gene silencing is a position effect—a gene is silenced because of where it is located, not in response to a specific environment signal. Also, silencing can spread over large stretches of DNA, switching off multiple genes.

The most common form of silencing is associated with heterochromatin. In mammalin cells,-50% of genome is estimated to be in heterochromatin, including telomeres, centromeres.

真核生物转录调控以正调控为主,负调控为辅。

原核以负调控为主。原核的负调控作用多是局部的效应。并且是对特殊的环境作出应答,以适应环境。

而真核细胞所特有的的负调控方式-gene silencing(特点:)1、是一种位置效应,因为某个基因在染色质中处于一定的位置(常染区或者异染区)因而被抑制,而不是对环境信号作出的应答。2、可以沿染色质扩散到一个较大的区域,使大面积基因被抑制。如异染色质区占大半。 Both activation and repression of transcription often involve modification of nucleosomes to alter accessibility of gene. Heterochromatic silencing can be induced by DNA methylation. This kind of silencing isn’t found in yeast but is common in mammals.

DNA methylation can inhibit binding of proteins, such as transcription machinery, or can be recognized by repressors recruiting histine deacetylase.

核小体的修饰一方面使组蛋白被共价修饰,另一方面与DNA的甲基化有关(都不涉及DNA序列的改变,但是都可以抑制基因表达)。

DNA甲基化可以抑制基因转录的原因是:

1、DNA甲基化可以提供一种识别信号,被特异的蛋白质结合将启动子封闭,使转录因子不能结合。

2、被repressor识别并结合后,repressor募集组蛋白去乙酰化酶,使组蛋白去乙酰化结合得更紧密。

Silencing at yeast telomere: deacetylation and histone

Gene regulation in eukaryotes 2

methylation

Rap1 recognize and bind telomere repeats, recruit Sir (silent information regulator) complex. Sir2 is histone deacetylase.

Deacetylation spreads along chromatin, producing extended

region of heterochromatin.

How is spreads limited to telomere and other silent regions?

Histone methylation transferases attach methyl groups to Lys in tails of histones H3 and H4. H3 methylation blocks Sir2 binding, and stops spreading. 酵母端粒区高凝集状态的形成: 在端粒区有大量得重复序列,处于高度凝集得状态,成因:有识别端粒区重复序列得蛋白质叫做Rap1,Rap1与端粒重复序列结合后可以募集Sir2 (3,4)蛋白(silent information regulator)Sir2是一个组蛋白去乙酰化酶将附近低甲基化状态的组蛋白去乙酰化。去乙酰化得组蛋白结合得更紧密,使端粒处于高凝集状态,另一方面,去乙酰化的组蛋白可以被Sir2直接识别,从而直接募集Sir2,这使得去乙酰化作用可以沿DNA扩散,使异染色质区扩大。但是扩散的长度是受限制的,组蛋白甲基转移酶对此起作用,甲基转移酶可以使H3,H4的N tail甲基化,甲基化的H3可以阻止Sir2的扩散。

Gene regulation in eukaryotes 2

Silencing mechanism of yeast telomere

Multiple copies of RAP1 bind to a simple repeated sequence at each telomere region, which lacks nucleosomes (top). This nucleates the assembly of a multiprotein complex (bottom) through protein-protein interactions between RAP1, SIR2, SIR3, SIR4, and the hypoacetylated N-terminal tails of histones H3 and H4 of nearby nucleosomes. SIR2 deacetylates the histone tails. The heterochromatin structure

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