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Li Xiaoling Office: QQ: M1623 313320773 E-MAIL: 313320773 @qq.com 2017/5/6 Content Chapter 1 Introduction Chapter 2 The Structures of DNA and RNA Chapter 3 DNA Replication Chapter 4 DNA Mutation and Repair Chapter 5 RNA Transcription Chapter 6 RNA Splicing Chapter 7 Translation Chapter 8 The Genetic code Chapter 9 Regulation in prokaryotes Chapter 10 Regulation in Eukaryotes 2017/5/6 HOW TO LEARN THIS COURSE WELL? To learn effectively To preview and review Problem-base learning Making use of class time effectively Active participation Bi-directional question in class Group discussion Concept map Tutorship To call for reading, thingking and discussing of investigative learning EVALUATION (GRADING) SYSTEM Question in-class and attendance : 10 points Group study and attendance: 20 points Final exam: 70 points Bonus Molecular Biology of the Gene, 5/E --- Watson et al. (2004) Part I: Chemistry and Genetics Part II: Maintenance of the Genome Part III: Expression of the Genome Part IV: Regulation Surfing the contents of Part IV --The heart of the frontier biological disciplines 6 •Molecular Biology Course Chapter 10 Gene Regulation in Eukaryotes 7 TOPIC 1 Conserved Mechanisms of Transcriptional Regulation from Yeast to Human. TOPIC 2 Recruitment of Protein Complexes to Genes by Eukaryotic Activators. TOPIC 3 Transcriptional Repressors TOPIC 4 Signal Integration and Combinatorial Control. TOPIC 5 Signal Transduction and the Control of Transcriptional Regulators. TOPIC 6 Gene Silencing by Modification of Histones and DNA. TOPIC 7 Epigenetic Gene Regulation. 8 Principles of Transcription Regulation 1. Gene Expression is Controlled by Regulatory Proteins (调控蛋白) Gene expression is very often controlled by Extracellular Signals, which are communicated to genes by regulatory proteins: Positive regulators or activators INCREASE the transcription Negative regulators or repressors DECREASE or ELIMINATE the transcription 9 Similarity of regulation between eukaryotes and prokaryote 1.Principles are the same: • signals (信号), • activators and repressors (激活蛋白和阻 遏蛋白) • recruitment and allostery, cooperative binding (招募,异构和协同结合) 2. The gene expression steps subjected to regulation are similar, and the initiation of transcription is the most pervasively regulated step. 10 Difference in regulation between eukaryotes and prokaryote 1. 2. 3. 4. Pre-mRNA splicing adds an important step for regulation. (mRNA前体的剪接) The eukaryotic transcriptional machinery is more elaborate than its bacterial counterpart. (真核转录机器更复杂) Nucleosomes and their modifiers influence access to genes. (核小体及其修饰体) Many eukaryotic genes have more regulatory binding sites and are controlled by more regulatory proteins than are bacterial genes. (真核基因有更多结合位点) 11 A lot more regulator bindings sites in multicellular organisms reflects the more extensive signal integration Bacteria Yeast Human 12 Fig. 10-1 Enhancer (激活元件) : a given site binds regulator responsible for activating the gene. Alternative enhancer binds different groups of regulators and control expression of the same gene at different times and places in responsible to different signals. Activation at a distance is much more common in eukaryotes. Insulators (绝缘子) or boundary elements (临 界元件) are regulatory sequences between enhancers and promoters. They block activation of a linked promoter by activator bound at the enhancer, and therefore ensure activators work discriminately. 13 CHAPTER 10 Gene Regulation in eukaryotes 一、真核的转录激活蛋白的结构特征 The structure features of the eukaryotic transcription activators Topic 1: Conserved Mechanisms of Transcriptional Regulation from Yeast (酵母) to Mammals (哺乳动物) 14 The basic features of gene regulation are the same in all eukaryotes, because of the similarity in their transcription and nucleosome structure. Yeast is the most amenable to both genetic and biochemical dissection, and produces much of knowledge of the action of the eukaryotic repressor and activator. The typical eukaryotic activators works in a manner similar to the simplest bacterial case. Repressors work in a variety of ways. 15 1. Eukaryotic activators (真核激活蛋白) have separate DNA binding and activating functions【与原核相似】, which are very often on separate domains of the protein. Fig. 10-2 Gal4 bound to its site on DNA 16 Eukaryotic activators---Example 1: Gal4 Gal4 is the most studied eukaryotic activator Gal4 activates transcription of the galactose genes in the yeast S. cerevisae. Gal4 binds to four sites (UASG) upstream of GAL1, and activates transcription 1,000-fold in the presence of galactose 17 Fig. 10-3 The regulatory sequences of the Yeast GAL1 gene. Eukaryotic activators---Example 1: Gal4 Experimental evidences showing that Gal4 contains separate DNA binding and activating domains. 1. Expression of the N-terminal region (DNAbinding domain) of the activator produces a protein bound to the DNA normally but did not activate transcription. 2. Fusion of the C-terminal region (activation domain) of the activator to the DNA binding domain of a bacterial repressor, LexA activates the transcription of the reporter gene. Domain swap experiment 实验介绍系列1-Experiment introduction series 18 Domain swap experiment Moving domains among proteins, proving that domains can be dissected into separate parts of the proteins. Many similar experiments shows that DNA binding domains and activating regions are 19 separable. Box 1 The two hybrid Assay (酵母双杂交) is used to identify proteins interacting with each other. (实验介绍系列2) Fuse protein A and protein B genes to the DNA binding domain and activating region of Gal4, respectively. Produce fusion proteins 20 2. Eukaryotic regulators use a range of DNA binding domains, but DNA recognition involves the same principles as found in bacteria. Homeodomain proteins Zinc containing DNA-binding domain: zinc finger and zinc cluster Leucine zipper motif Helix-Loop-Helix proteins : basic zipper and HLH proteins 21 Bacterial regulatory proteins • Most use the helix-turn-helix motif to bind DNA target • Most bind as dimers to DNA sequence: each monomer inserts an a helix into the major groove. Eukaryotic regulatory proteins 1. Recognize the DNA using the similar principles, with some variations in detail. 2. In addition to form homodimers (同源二聚 体), some form heterodimers (异源二聚体) to recognize DNA, extending the range of DNA-binding specificity. 22 Homeodomain proteins: The homeodomain (同源结构域) is a class of helix-turn-helix DNA-binding domain and recognizes DNA in essentially the same way as those bacterial proteins. What is the same? Figure 10-5 23 Zinc containing DNA-binding domains (锌指 结构域): Zinc finger proteins (TFIIIA) and Zinc cluster domain (Gal4) Figure 10-6 24 Leucine Zipper Motif (亮氨酸拉链基序) : The Motif combines dimerization and DNAbinding surfaces within a single structural unit. Figure 10-7 25 Dimerization (二聚化) is mediated by hydrophobic interactions between the appropriately-spaced leucine (亮氨酸) to form a coiled coil structure 26 27 Helix-Loop-Helix motif: similar as in leucine zipper motif. Figure 10-8 28 myogenic factor:生肌调节蛋白是 一种转录因子。 29 Because the region of the a-helix that binds DNA contains baisc amino acids residues, Leucine zipper and HLH proteins are often called basic zipper and basic HLH proteins. Both of these proteins use hydrophobic amino acid residues for dimerization. 30 3. Activating regions (激活区域) are not well-defined structures The activating regions are grouped on the basis of amino acids content. Acidic activation region (酸性激活区域): contain both critical acidic amino acids and hydrophobic acids.yeast Gal4 Glutamine-rich region (谷氨酰胺富集区): mammalian activator SP1 Proline-rich region (脯氨酸富集区): mammalian activator CTF1 31 CHAPTER 17 Gene Regulation in eukaryotes 二、真核转录激活蛋白的招募调控方式和远距调控特征 Activation of the eukaryotic transcription by recruitment & Activation at a distance Topic 2: Recruitment of Protein Complexes to Genes by Eukaryotic Activators 32 Eukaryotic activators (真核激活蛋白) also work by recruiting (招募) as in bacteria, but recruit polymerase indirectly in two ways: 1. Interacting with parts of the transcription machinery. 2. Recruiting nucleosome modifiers that alter chromatin in the vicinity of a gene. 33 1. Activators recruit the transcription machinery to the gene. 34 The eukaryotic transcriptional machinery contains polymerase and numerous proteins being organized to several complexes, such as the Mediator and the TFⅡD complex. Activators interact with one or more of these complexes and recruit them to the gene. Figure 10-9 35 Box 2 Chromatin Immuno-precipitation (ChIP) (染 色质免疫共沉淀) to visualize where a given protein (activator) is bound in the genome of a living cell.) (实验介绍系列3) 36 Activator Bypass Experiment (越过激活子实验)Activation of transcription through direct tethering of mediator to DNA. (实验介绍系列4) Directly fuse the bacterial DNAbinding protein LexA protein to Gal11, a component of the mediator complex to activate GAL1 expression. Figure 10-1037 At most genes, the transcription machinery is not prebound, and appear at the promoter only upon activation. Thus, no allosteric activation of the prebound polymerase has been evident in eukaryotic regulation。 38 2. Activators also recruit modifiers that help the transcription machinery bind at the promoter Two types of Nucleosome modifiers : Those add chemical groups to the tails of histones (在组蛋白尾上加化学基团), such as histone acetyl transferases (HATs) Those remodel the nucleosomes (重塑核 小体), such as the ATP-dependent activity of SWI/SNF. 39 How the nucleosome modification help activate a gene? 1. “Loosen” the chromatin structure by chromosome remodeling (Fig. 10-11b) and histone modification such as acetylation (Fig. 10-11a), which uncover DNA-binding sites that would otherwise remain inaccessible within the nucleosome. 40 uncover DNA-binding sites (组蛋白乙酰化酶) Fig 10-11 Local alterations in chromatin 41 2. Adding acetyl groups to histones helps the binding of the transcriptional machinery. One component of TFIID complex bears bromodomains that specifically bind to the acetyl groups. Therefore, a gene bearing acetylated nucleosomes at its promoter have a higher affinity for the transcriptional machinery than the one with unacetylated nucleosomes. 42 溴区结构域蛋白 One component of TFIID complex bears bromodomains. Figure 10-39 Effect of histone tail 43 3. Action at a distance: loops and insulators Many enkaryotic activators-particularly in higher eukaryotes-work from a distance. How? 1. Some proteins help, for example Chip protein in Drosophila. 2. The compacted chromosome structure help. DNA is wrapped in nucleosomes in eukaryotes. So sites separated by many base pairs may not be as far apart in the cell as thought. 44 Specific cis-acting elements called insulators (绝缘子) control the actions of activators, preventing the activating the non-specific genes 45 Insulators block activation by enhancers Figure 10-12 46 Transcriptional Silencing (转录沉默) Transcriptional Silencing is a specialized form of repression that can spread along chromatin, switching off multiple genes without the need for each to bear binding sites for specific repressor. Insulator elements (绝缘子元件) can block this spreading, so insulators protect genes from both indiscriminate activation and repression.[绝缘子的概念到此才介绍完毕] Application:A gene inserted at random into the mammalian genome is often “silenced”, and placing insulators upstream and downstream of that gene can protect the gene from silencing. 47 4 Appropriate regulation of some groups of genes requires locus control region (LCR). 1. 2. Human and mouse globin genes are clustered in genome and differently expressed at different stages of development A group of regulatory elements collectively called the locus control region (LCR), is found 30-50 kb upstream of the cluster of globin genes. It binds regulatory proteins that cause the chromatin structure to “open up”, allowing access to the array of regulators that control expression of the individual genes in a defined order. 48 Figure 10-13 Please compare LCR with the Lac operon controlled gene expression in bacteria 49 Another group of mouse genes whose expression is regulated in a temporarily and spatially ordered sequence are called HoxD genes. They are controlled by an element called the GCR (global control region) in a manner very like that of LCR. 50 CHAPTER 10 Gene Regulation in eukaryotes 三、真核转录阻遏蛋白(或抑制蛋白)及其调控 Topic 3: Transcriptional Repressor & its regulation In eukaryotes, most repressors do not repress transcription by binding to sites that overlap with the promoter and thus block binding of polymerase. (Bacteria often do so) 51 Commonly, eukaryotic repressors recruit nucleosome modifiers that compact the nucleosome or remove the groups recognized by the transcriptional machinery [contrast to the activator recruited nucleosome modifers, histone deacetylases (组蛋白去乙酰化酶) removing the acetyl groups]. Some modifier adds methyl groups to the histone tails, which frequently repress the transcription. This modification causes transcriptional silencing. 52 Three other ways in which an eukaryotic repressor works include: (1) Competes with the activator for an overlapped binding site. (2) Binds to a site different from that of the activator, but physically interacts with an activator and thus block its activating region. (3) Binds to a site upstream of the promoter, physically interacts with the transcription machinery at the promoter to inhibit transcription initiation. 53 Competes for the activator binding Inhibits the function of the activator. Figure 10-19: Ways in which eukaryotic repressor work 54 Binds to the transcription machinery Recruits nucleosome modifiers (most common) 55 A specific example: Repression of the GAL1 gene in yeast In the presence of glucose, Mig1 binds to a site between the USAG and the GAL1 promoter, and recruits the Tup1 repressing complex. Tup1 recruits histone deacetylases, and also directly interacts with the transcription machinery to repress transcription. 56 CHAPTER 10 Gene Regulation in eukaryotes 四、真核转录调控的特色:信号整合和组合调控 Features of the eukaryotic transcriptional regulation: signal integration and combinatorial control Topic 4: Signal Integration and Combinatorial Control 57 1. Activators work together synergistically (协作地) to integrate signals. 激活蛋白一起协作来整合多种信号 58 Review the Lac operon control in bacteria. Two signals are integrated to control Lac expression Glucose Lactose 59 Figure 10-6 In multicellular organisms, signal integration (信号整合) is used extensively. In some cases, numerous signals are required to switch a gene on. However, each signal is transmitted to the gene by a separate regulator, and therefore, multiple activators often work together, and they do so synergistically (two activators working together is greater than the sum of each of them working alone.) 60 Three strategies of the synergy (协作 的三种方式): S1: Multiple activators recruit a single component of the transcriptional machinery. For example, by touching the different part of the mediator complex (中介体复合物). The combined binding energy has an exponential effect on recruitment. S2: Multiple activators each recruit a different component of the transcriptional machinery. These components binds to the promoter DNA inefficiently without help. 61 S3: Multiple activators help each other bind to their sites upstream of the gene they control. (Figure 10-14) 62 Figure 17-14: Cooperative binding of activators a.“Classical” cooperative binding. b. Both proteins interacting with a third protein. c. The first protein recruit a nucleosome remodeller whose action reveal a binding site for the second protein. d. Binding a protein unwinds the DNA from nucleosome a little, revealing the binding site for another protein. 63 2. Signal integration: the HO gene is controlled by two regulators; one recruits nucleosome modifiers and the other recruits mediator. [S3策略的 example] The HO gene is only expressed in mother cells and only a certain point in the cell cycle, resulting in the budding division feature of yeast S. cerevisiae (啤酒酵母). The mother cell and cell cycle conditions (signals) are communicated to the HO gene (target) by two activators: SWI5 and SBF (communicators). 64 SWI5: acts only in the mother cell and binds to multiple sites some distance from the gene unaided, which recruit enzymes to open the SBF binding sites. SBF: only active at the correct stages of the cell cycle, and cannot bind the sites unaided. Alter the nucleosome (组蛋白 乙酰化酶) Figure 10-15 65 Figure 10-14: Cooperative binding of activators. (c) The first protein recruit a nucleosome remodeller whose action reveal a binding site for the second protein. 66 3. Signal integration: Cooperative binding of activators at the human binterferon gene. [S3策略的example] The human b-interferon gene (target gene) is activated in cells upon viral infection (signal). Infection triggers three activators (communicator): NFkB, IRF, and Jun/ATF. Activators bind cooperatively to sites adjacent to one another within an enhancer located about 1 kb upstream of the promoter, which forms a structure called enhanceosome. 67 (人的b干扰素基因) 增强子 增强体 1. Activators interact with each other 2. HMG-I binds within the enhancer and aids the binding of the activators (bends the DNA to promote the activator interaction) Figure 10-16 68 HMG-I is constitutively active in the cells, and play an architectural role in the INF-b gene activation process. 69 Figure 10-14: Cooperative binding of activators. (a)“Classical” cooperative binding through direct interaction between the two proteins. 70 4. Combinatory control (组合调控) lies at the heart of the complexity and diversity of eukaryotes, in which Both activators and repressors work together. 组合调控是真核生物复杂性和多样性的核心。 它属于协作调控的一种具体方式。在组合调 控中,激活蛋白和阻遏蛋白共同起作用。 71 Review Page 499 & Slide 47 in Ch 16 7: Combinatorial Control (组合调 控): CAP controls other genes as well A regulator (CAP) works together with different repressors at different genes, this is an example of Combinatorial Control. In fact, CAP acts at more than 100 genes in E.coli, working with an array of partners. 72 There is extensive combinatorial control in eukaryotes.---A generic picture Four signals Three signals Figure 10-18 In complex multicellular organisms, combinatorial control involves many more regulators and genes than shown above. Both activators and repressors can be involved. 73 5. An example: combinatory control of the mating-type genes from S. cerevisiae (啤酒酵母的交配 型的组合调控) 74 The yeast S. cerevisiae exists in three forms: ---two haploid cells (单倍体) of different mating types- a and a. ---the diploid cells (双倍体) (a/a) formed when an a and an a cell mate and fuse. Cells of the two mating types (a and a) differ because they express different sets of genes: a specific genes and a specific genes. 75 a cells make the regulatory protein a1, a cells make the protein a1 and a2. Both cell types express the fourth regulator protein Mcm1 that is also involved in regulatory the mating-type specific genes. How do these regulators work together to keep a cell in its own type? [Figure 17-19] 76 Figure 17-19: Control of cell-type specific genes in yeast Cooperative binding Cooperative binding 77 CHAPTER 10 Gene Regulation in eukaryotes 五、基于真核转录调控的前沿学科:信号传导 Signal transduction---A life science frontier centered on the eukaryotic transcriptional regulation. Topic 5: Signal Transduction (信号传导) and the Control of Transcriptional Regulators 78 Topic 4: Signal Transduction and the Control of Transcriptional Regulators 1. Signals are often communicated to transcriptional regulators through signal transduction pathway 信号经常通过信号传导途径被运输到转录调节蛋白 79 Environmental Signals/Information (信号) 1. Small molecules such as sugar, histamine (组胺). 2. Proteins released by one cell and received by another. In eukaryotic cells, most signals are communicated to genes through signal transduction pathway (indirect), in which the initiating ligand is detected by a specific cell surface receptor. What about in bacteria? 80 Signal transduction pathway 1. The initial ligand (“signal”) binds to an extracellular domain of a specific cell surface receptor 2. The signal is thus communicated to the intracellular domain of receptor (via an allosteric change or dimerization ) 3*. The signal is then relayed (分程传递) to the relevant transcriptional regulator. 4. The transcriptional regulator control the target gene expression (topic 2-4). 81 a. The STAT pathway b. The MAP kinase pathway Figure 10-22:Signal transduction pathway 82 Topic 4: Signal Transduction and the Control of Transcriptional Regulators 2. Signals control the activities of eukaryotic transcriptional regulators in a variety of ways---The mechanisms of signal transduction. 信号传导的机制:信号经常通过不同方式控制转 录调节蛋白的活性 In eukaryotes, a signal can be communicated, directly or indirectly, to a transcriptional regulator. 83 Mechanism 1: unmasking an activating region (Topic 2 & 3): (1) (2) (3) A conformational change to reveal the previously buried activating region. Releasing of the previously bound masking protein. Example: the activator Gal4 is controlled by the masking Gal80) (Fig.17-23). Some masking proteins not only block the activating region of an activator but also recruit a deacetylase enzyme to repress the target genes. Example: Rb represses the function of the mammalian transcription activator E2F in this way. Phosphorylation of Rb releases E2F to activate the target gene expression. 84 Activator Gal4 is regulated by a masking protein Gal80 Gal4 Figure 10-23 85 Mechanism 2: Transport into and out of the nucleus (Fig.10-21) : When not active, many activators and repressors are held in the cytoplasm. The signaling ligand causes them to move into the nucleus where they activate transcription (Fig.10-4b). 86 Other Mechanisms #1: A cascade of kinases that ultimately cause the phosphorylation of regulator in nucleus (new) (Fig.19-4a). 87 Other Mechanisms #2: The activated receptor is cleaved by cellular proteases (蛋白酶), and the c-terminal portion of the receptor enters the nuclease and activates the regulator (new):(Fig.10-4c). 88 CHAPTER 10 Gene Regulation in eukaryotes Topic 6: Gene “Silencing” by Modification of Histones and DNA 890 Transcriptional silencing is a position effect. (1) A gene is silenced because of where it is located, not in response to a specific environmental signal. (2) Silencing can spread over large stretches of DNA, switching off multiple genes, even those quite distant from the initiating event. The most common form of silencing is associated with a dense form of chromatin called “heterochromatin”. Heterochromatin is frequently associated with particular regions of the chromosome, notably the telomeres, and the centromeres. In mammalian cells, about 50% of the genome is estimated to be in some form of heterochromatin. Transcriptional silencing is associated with Modification of nucleosomes that alters the accessibility of a gene to the transcriptional machinery and other regulatory proteins. The modification enzymes for silencing include deacetylases, DNA methylases. Topic 6: Gene “Silencing” by Modification of Histones and DNA 6-1. Silencing in yeast is mediated by deacetylation and methylation of the histones 在酵母中的沉默是通过对组蛋白的 去乙酰化和甲基化介导的 93 The telomeres, the silent mating-type locus, and the rDNA genes are all “silent” regions in S. cerevisiae. Three genes encoding regulators of silencing, SIR2, 3, and 4 have been found (SIR stands for Silent Information Regulator). Rap1 recruits Sir complex to the temomere. Sir2 deacetylates nearby nucleosome. Fig. 10-24. Silencing at the yeast telomere Silencing specificity is determined by Rap1, the telomere DNA-binding protein. It can also be determined by RNA molecules using RNAi machinery (Chapter 18). The spreading of silencing is restricted/controlled by insulators and other kind of histone modifications that block binding of the Sir2 proteins. 95 Transcription can also be silenced by methylation of DNA by histone methyltransferase (H3 and H4, Chapter 7). This enzyme have been recently found in yeast, but is common in mammalian cells. Its function is better understood in higher eukaryotes. In higher eukaryotes, silencing is typically associated with chromatin containing histones that both deacetylated and methylated. Topic 6: Gene “Silencing” by Modification of Histones and DNA 6-2. In Drosophila, HP1 recognizes Methylated Histones and Condense Chromatin. 在果蝇中,HP1蛋白识别甲基化的 组蛋白和浓缩的染色质。 97 HP1 protein is a component of heterochromatin in Drosophila that binds to methylated residue in histone H3. Such a histone modification is produced by Su(Var)3-9. Different types of modification at the histones can be involved in distinct geene regulation. What will happen when multiple forms of modification occur? ---A “Histone Code” hypothesis. Box 17-4 Is there a histone code? According to this idea, different patterns of modification on histone tails can be “read” to mean different things. The “meaning” would be the result of the direct effects of these modifications on chromatin density and form. But in addition, the particular pattern of modifications at any given location would recruit specific proteins. Topic 6: Gene “Silencing” by Modification of Histones and DNA 6-3. DNA Methylation Is Associated with Silenced Genes in Mammlian cells. DNA甲基化与哺乳动物细胞的沉默 基因相关联。【注DNA甲基化不是 组蛋白甲基化】 Some mammalian genes are kept silent by methylation of nearby DNA sequences. 100 Large regions of mammalian genome are marked by methylation of DNA sequences, which is often seen in heterochromatic regions. [Why?] The methylated DNA sequences are often recognized by DNA-binding proteins (such as MeCP2) that recruit histone decetylases and histone methylases, which then modify nearby chromatin. Thus, methylation of DNA can mark sites where heterochromatin subsequently forms (Fig. 17-25). Fig. 10-25 Switching a gene off through DNA methylation and the subsequent histone modification DNA methylation lies at the heart of Imprinting Imprinting- in a diploid cell, one copy of a gene from the father or mother is expressed while the other copy is silenced. Two well-studied examples: human H19 and insulin-like growth factor 2 (Igf2) genes. Enhancer: activate both gene transcription ICR: an insulator binds CTCF protein and blocks the activity of the enhancer on Igf2.Methylation of ICR allows the enhancer to activate Igf2. H19 repression is mediated by DNA methylation and the subsequent MeCP2 binding to the methylated ICR Figure 10-26 Imprinting 104 CHAPTER 10 Gene Regulation in eukaryotes Topic 7: Epigenetic Regulation 105 Patterns of gene expression must sometimes be inherited. After the expression of specific genes in a set of cells are switched on by a signal, these genes may have to remain switched on for many cell generations, even if the signal that induced them is present only fleetingly (飞快地). The inheritance of gene expression patterns, in the absence of both mutation and the initiating signal, is called epigenetic regulation. Topic 7: Epigenetic Regulation 7-1. Some States of Gene Expression Are Inherited through Cell Division Even When the Initiating Signal Is No Longer Present 有些基因表达状态在起始信号不存 在时仍然在细胞分裂中遗传。 The maintenance of a bacteriophage l lysogen is an example of epigenetic regulation. [Figure 17-27] 107 Establishment of lysogeny: synthesis of the essential lysogenic l repressor is established by transcription from one promoter and then maintained by transcription from another one. DNA methylation provides a mechanism of epigenetic regulation. DNA methylation is reliably inherited throughout cell division [Fig. 17-28]. Certain DNA methylases can methylate, at low frequency, previously unmodified DNA; but far more efficiently, the socalled maintenance methylases modify hemimethylated DNA-the very substrate provided by replication of fully methylated DNA. [A link with reprogramming of somatic cells into Embryonic Stem-like cells. Box 17-6] Figure 10-28 Patterns of DNA methylation can be maintained through cell division Key points of the chapter 1. The structure features of the eukaryotic transcription activators. 2. Activation of the eukaryotic transcription by recruitment & Activation at a distance. 3. Transcriptional repressor & its regulation 4. Signal integration and combinatorial control 5. Signal transduction: communicating the signals to transcriptional regulators. 6. Gene silencing and Epigenetic regulation. 7. 4 experimental methods introduced (yeast two-hybrid and ChIP). 111