英汉对照分子生物学导论 教学课件 ppt 作者 [美]西尔维恩 W.勒潘 王勇 编著Chapter 7.ppt

The fundamental structure of each histone is three α helices connected by two short loops. Its N-terminus is long and has no regular shape. This is called the histone tail. 每种组蛋白的基本结构是由两个短环连接起来的三个α螺旋，其N端延伸出来而且没有固定的形状，它们被称为组蛋白尾。 Different kinds of modifications can take place on the histone tails. Chemical groups such as acetyl groups, methyl groups, and phosphates can be covalently attached to various places on histone tails. This kind of covalent modification has very important effects on transcription. 组蛋白尾上可以发生不同种类的修饰作用。乙酰基、甲基和磷酸等化学基团可以被共价连接到组蛋白尾的不同位置上，这种共价修饰对转录有重要的影响。 For example, lysine acetylation on the histone tails can increase transcription. It is because the addition of an acetyl group to lysine removes the positive charge of lysine. Therefore, DNA becomes less attractive to the histone, and less tightly bound. Now, the DNA is available for transcription. 例如，对组蛋白尾的赖氨酸乙酰化能够增强转录，这是因为当乙酰基加到赖氨酸的R基团上时，赖氨酸失去正电荷从而变成中性。结果，组蛋白对DNA的吸引力下降，因此相互之间的结合也变得不那么紧密。这样，DNA就可以用来转录了。 Generally, the effect of histone modification is not so simple. One type of modification can often have different effects depending on which amino acid is modified. For example, methylation of H3-K9 (the 9th lysine of histone H3) usually represses transcription. But methylation of H3-K4 (the 4th lysine of histone H3) usually activates transcription. 一般来说，组蛋白共价修饰对转录的影响并不是像上面讲的那么简单。依赖于被修饰的是哪个氨基酸，同一种类型的修饰作用也常常会有不同的效果。例如，在H3-K9（组蛋白H3上的第9位赖氨酸）上加上一个甲基通常对转录有阻遏作用。相反，H3-K4的甲基化通常会激活转录。 Nevertheless, the effects of covalent modifications also depend on which other modifications are present. For example, the combination of H3-H10 phosphorylation with H4-K8 and H3-K14 acetylation activates transcription much more than any one of these modifications alone. Here, the functional combinations of modifications at a histone are called the histone code. 共价修饰作用效果的好坏还与是否有其它修饰作用同时发生有关。例如，H3-H10的磷酸化与H4-K8和H3-K14的乙酰化比它们中任何一种的单独修饰对转录的激活作用都要强。这种对组蛋白的功能性修饰组合称为组蛋白密码。 Histone code is similar to a coded lock. On