遗传学chapter16gene regulation in eukaryotes样式编辑母版文本样式

1、Chapter 16 Gene Regulation in Eukaryotes Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-1 Outline of Chapter 16 How we use genetics to study gene regulation ? Using mutations to identify cis - acting elements and trans - ? acting proteins How genes are reg

2、ulated at the initiation of ? transcription Three polymerases recognize three classes of promoters. ? Trans - acting proteins control class II promoters. ? Chromatin structure affects gene expression. ? Genomic imprinting and DNA methylation ? How genes are regulated after transcription ? RNA splici

3、ng ? Micro - RNA ? Protein modification and Inc. Permission required to reproduce or display stability ? Copyright ? The McGraw-Hill Companies, 18-2 Differences of gene regulation in eukaryotes compared with prokaryotes Eukaryotic genomes carry far more DNA, challenging ? for proteins to locate bind

4、ing sequences. Chromatin structure makes DNA unavailable to ? transcription machinery. Additional RNA processing events occur. ? Transcription and translation are spatially separated. ? Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-3 Gene expression in eu

5、karyotes Fig. 16.1 Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-4 Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-5 In eukaryotes, three RNA polymerases transcribe different sets of genes RNA polymerase I ? tran

6、scribes rRNA . rRNAs are made ? of tandem repeats on one or more chromosomes. RNA polymerase I ? transcribes one primary rRNA transcript which is further processed to 28S, 5.8S, and 18S rRNA . Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-6 RNA polymerase

7、 III transcribes tRNAs and other ? small RNAs (5S rRNA , snRNAs ). Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-7 RNA polymerase II transcribes all protein - and micro - RNA - ? encoding genes. RNA pol . II recognizes cis - acting regulatory regions comp

8、osed ? of promoters and enhancers. Promoter contains initiation site and TATA box. ? Enhancers are distant from target gene. ? Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-8 cis - acting elements Promoter DNA sequences near the beginning of genes that ?

9、signal RNA polymerase where to begin transcription. Enhancer DNA sequences that regulate transcription from ? nearby genes. Can lie far way from gene ? Can be reversed ? Augment or repress basal levels of transcription ? Fig. 16.2 Copyright ? The McGraw-Hill Companies, Inc. Permission required to re

10、produce or display 18-9 Reporter constructs are a tool for identifying promoters and enhancers Promoters and enhancers under investigation are ? fused to the coding region of reporter genes, such as lacZ or GFP. Fig. 16.3 Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce o

11、r display 18-10 Reporter ? constructs can help identify promoters and enhancers. Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-11 trans - acting elements Basal factors that ? bind to the promoter. Transcriptional ? activators and repressors that bind to t

12、he enhancers. Fig. 16.4 Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-12 trans - acting proteins control transcription from class II promoters Basal factors bind ? to the promoter. TBP TATA box - ? binding protein TAF TBP ? associated factors RNA polymera

13、se ? II binds to basal factors. Fig. 16.6 Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-13 Activator proteins Interact with other proteins to activate and increase ? transcription above basal levels. Also called transcription factors. ? Two structural dom

14、ains mediate these functions: ? DNA - binding domain, bind to enhancer DNA in specific ? ways. Transcription - activating domain ? Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-14 Transcriptional ? activators bind to specific enhancers at specific times t

15、o increase transcriptional levels. Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-15 Fig. 16.7 The locus control region (LCR) is a cis - acting regulatory sequence that operates sequentially on a cluster of related genes Human ? - globin gene cluster conta

16、ins five genes that ? can all be regulated by a distant LCR (locus control region), 50 kb upstream of ? - globin gene. Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-16 LCR is needed for activation of ? - globin gene Copyright ? The McGraw-Hill Companies,

17、Inc. Permission required to reproduce or display 18-17 LCR could effect activation of distant globin genes through DNA looping Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-18 Common DNA - binding motifs of transcription factors Helix - loop - helix ? Hel

18、ix - turn - helix ? Zinc - finger: found mostly in eukaryotes. ? C C Helix-turn-helix Zn C C Zinc finger Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-19 Fig. 16.8 Some activators have a third domain that is responsive to specific signals from the environ

19、ment Coactivator Proteins or other molecules that play a ? role in transcriptional activation without binding directly to DNA. Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-20 Fig. 16.9 Most transcription regulators are multimeric proteins Homomers multim

20、eric proteins composed of ? identical subunits. Jun - Jun homodimer ? Heteromers multimeric proteins composed of ? nonidentical subunits Jun - Fos heterodimer ? Fig. 16.10, 16.11 Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-21 Repressors Repressors : Tra

21、nscription factors that suppress the ? activation of transcription caused by activator proteins. Activator - repressor competition: binds to DNA. ? Quenching ( corepressors ): do not bind to DNA. ? Fig. 16.12 a Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 1

22、8-22 Fig. 16.12 b Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-23 Most repressors do not affect basal level of ? transcription. A few repressors can shut down basal level of ? transcription. Binding directly to promoter, blocking RNA polymerase ? from bi

23、nding. Bind to DNA sequences farther from promoter, contact ? basal factor complex at promoter by bending DNA causing a loop where RNA polymerase can not access the promoter. Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-24 Transcription factors may act a

24、s activators or repressors, or have no effect Action of transcription factor depends on: ? Cell type, the presence of interacting proteins. ? Gene that it is regulating, promoter sequence. ? Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-25 Yeast ? 2 repre

25、ssor determines mating type of ? cell. Haploid ? 2 factor ? silences the set of “ a ” genes. Diploid ? 2 factor ? dimerizes with a1 factor and silences haploid - specific genes. Fig. 16.13 Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-26 Other mechanisms

26、of gene regulation Chromatin structure ? Reduces basal transcription. ? Hypercondensation stops transcription. ? Genomic imprinting ? Silences transcription selectively if inherited from one parent. ? Some genes are regulated after transcription ? RNA splicing can regulate expression. ? RNA stabilit

27、y controls amount of gene product. ? Noncoding sequences in mRNA can modulate translation. ? Micro - RNA ? Protein modification after translation can control gene ? function. Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-27 Chromatin structure reduces bas

28、al transcription Nucleosomes sit atop the promoters of most inactive genes. Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-28 Fig. 16.17 Remodeling of chromatin mediates the activation of transcription Copyright ? The McGraw-Hill Companies, Inc. Permission

29、 required to reproduce or display 18-29 Fig. 16.19 Modification of histones have been closely linked to ? transcriptional regulation and are required for many biological processes. Histone H3 and H4. ? Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-30 Hype

30、rcondensation over chromatin domains causes transcriptional silencing Heterochromatic regions on the chromosome are characterized ? by modified histone H3 and methylated CpG dinucleotide in DNA. Fig. 16.21 Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-31

31、Methylation on DNA is associated with transcription ? silencing. Methylation occurs at C5 position of cytosine in CpG ? dinucleotide pair. Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-32 Determine the methylation state of CCGG using restriction enzymes M

32、sp I cleaves CCGG and CC*GG; Hpa II cleaves CCGG but not ? CC*GG. Fig. 16.18 Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-33 The yeast SIR complex silences the transcription of HML and HMR genes Fig. 16.22 Copyright ? The McGraw-Hill Companies, Inc. Perm

33、ission required to reproduce or display 18-34 Genomic imprinting 1. In the 1980s, in vitro fertilization experiments demonstrated: Embryos with one maternal pronucleus and one paternal ? pronucleus can develop into normal, viable, and fertile adult mice. Embryos with pronuclei from two females or tw

34、o males ? could not develop normally. Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-35 2. Mice inheriting Igf2 deletion from father were weighing 40% below average whereas mice inheriting deletion from mother were normal. P- paternal chromosome M-maternal

35、 chromosome Fig. 16.23 a Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-36 Genomic imprinting : the phenomenon in which a ? gene s expression depends on the parent that transmits it. Epigenetic A state of gene functionality that is not ? encoded within the

36、 DNA sequence but that is still heritable from one generation to the next. Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-37 DNA methylation is involved in genomic imprinting Methylation of cytosine in CG dinucleotides within ? imprinted region. Methylated

37、 C s silence genes in the region by ? preventing RNA polymerase and other transcriptional factors from binding to DNA. Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-38 Methylation can be maintained across generations by ? methylases that recognize methyl

38、groups on one strand and respond by methylating the opposite strand. Fig. 16.23 b Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-39 Role of DNA methylation on genomic imprinting of Igf2 and H19 Separated by 70 kb. An enhancer downstream of H19 ? interacts

39、with the promoters of both genes. An insulator bound to CTCF blocks the enhancer to ? interact with Igf2 promoter. Insulator: Transcriptional regulation element that stops ? communication between enhancers on one side of it with promoters on the other side. Copyright ? The McGraw-Hill Companies, Inc

40、. Permission required to reproduce or display 18-40 Fig. 16.23 c On the maternal chromosome, CTCF binds ? unmethylated insulator elements between Igf2 and H19 . H19 is turned on. Igf2 is off. On the paternal chromosome, the insulator elements ? and H19 promoter are methylated . H19 is turned off. Ig

41、f2 is turned on. Fig. 16.23 c Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-41 Genomic imprinting selectively silences genes inherited from one parent Fig. 16.23 d Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-

42、42 Genomic imprinting and human disease For sex - linked genetic traits that are imprinted, the sex of the ? individual inheriting a mutant allele does not matter. The parental origin of the mutant allele matters. Incomplete penetrance may reflect genomic imprinting. ? Some individuals inherit the m

43、utation but do not develop into disease. Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-43 Fig. 16.23 e Genomic imprinting is unique to mammals Approx. 1% of the mammalian genome, 500 genes , ? are imprinted. Among vertebrate animals, only mammals imprint

44、? their genomes. Many of the imprinted genes uncovered to date are ? involved in regulating embryonic growth. Most genes imprinted in one mammalian species are ? also imprinted in other mammal species. The biological role is controversial. ? Copyright ? The McGraw-Hill Companies, Inc. Permission req

45、uired to reproduce or display 18-44 The Haig hypothesis In 1991, David Haig and colleagues. ? To explain how genomic imprinting ? get to evolve in mammals. Why in mammals? ? Embryos develop inside mother s body. ? Conflicting interests in transferring nutrients from mother to embryos. Polygamous ? A

46、 female mammal can mate with several males ? and generate multiple embryos fathered by different males. Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-45 The incentive: From the father side : The embryos sired by him ? should grow faster than other embryos

47、 even at the expense of the mother. From the mother side : Keep embryonic growth ? under control and balance her nutrients to all the embryos. Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-46 The hypothesis: The genes that a fetus receives from its mother

48、 are ? most interested in mom s survival and reproduction in the future. Maternal genes that normally enhance fetal growth would ? be down - regulated. Genes that a fetus receives from its father are most ? interested in getting more resources from its mother. Paternal genes that normally promote em

49、bryonic growth ? should be active to help a fetus to outgrow its half - sibling fetus. Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-47 Post - transcriptional gene regulation Alternative RNA splicing ? RNA interference ? Protein modification ? Copyright ?

50、 The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-48 RNA splicing helps regulate gene expression Sxl protein is essential to the female-specific developmental program. Fig. 16.24 a Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 1

51、8-49 Later in development Alternative splicing: production of different mature mRNAs from the same primary transcript by joining different combinations of exons. Fig. 16.24 b Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-50 Micro - RNAs mediate RNA interf

52、erence RNA interference : Trans - acting single - stranded micro - RNAs prevent the expression of specific genes through complementary base pairing. 2000 - 2005 micro - RNAs identified and characterized, 21 - 24 ? nucleotides in length. Micro - RNAs are products processed from longer transcripts. ?

53、? from introns of protein coding transcripts ? ? from products of primary transcripts devoid of ORFs ? Primary miRNA transcripts have 5 - GTP caps and 3 - poly A ? tails. Number of miRNAs may exceed the number of protein coding ? genes. Copyright ? The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-51 miRNA - containing transcripts carry 60 - 120 ? ribonucleotide - long segments of reverse sequence complementarity can snap back spontaneously into hairpin ste