Presentation on theme: "Welcome Each of You to My Molecular Biology Class"— Presentation transcript:
1 Welcome Each of You to My Molecular Biology Class
2 Molecular Biology of the Gene, 5/E --- Watson et al. (2004) Part I: Chemistry and GeneticsPart II: Maintenance of the GenomePart III: Expression of the GenomePart IV: RegulationPart V: Methods
3 Part IV Regulation Ch 16: Transcriptional regulation in prokaryotes Ch 17: Transcriptional regulation in eukaryotesCh18: Regulatory RNAsCh 19: Gene regulation in development and evolutionCh 20: Genome Analysis and Systems Biology
5 TOPIC 1 Regulation by RNAs in Bacteria TOPIC 1 Regulation by RNAs in Bacteria. TOPIC 2 RNA Interference Is a Major Regulatory Mechanism in Eukaryotes. TOPIC 3 Synthesis and function of miRNA molecules. TOPIC 4 The Evolution and Exploitation of RNAi. TOPIC 5 Regulatory RNAs and X-inactivation.
6 Chapter 18 Regulatory RNAs PART 1 Regulation by RNAs in Bacteria. PART 2 RNA Interference and miRNA Regulation in Eukaryotes
7 PART 1 Regulation by RNAs in Bacteria Chapter 18 Regulatory RNAsPART 1 Regulation by RNAs in Bacteria
8 Small RNAs (sRNA): regulation by base pairing. Riboswitches: regulation by metabolite-mediated structure changes.Attenuation: Regulation by ribosome stop-mediated formation of terminators
9 Small RNAs (sRNA)---Regulation of translation initiation and transcription termination by altering the accessibility of RBS and the formation of terminator, respectively. [Targets]---Regulation by base pairing with the targeted sequences on mRNAs. [Mechanism]---Acts in trans similar to miRNA, but does not require specific machinery for action. [Mechanism]Figure 18-1
10 2. Riboswitches are regulatory RNA elements that act as direct sensors of small molecule metabolites to control gene transcription or translation.---Regulate translation initiation and transcription termination by altering the accessibility of RBS and the formation of terminator, respectively. [Targets]---Reside upstream of the targeted mRNA, and form specific structure to bind its small molecule ligand. [Mechanism]---Act in cis by alteration of its own structure upon the binding of the small metabolites. [Mechanism]
11 Figure 18-2. The structure of a riboswitch in its regulated mRNA
12 Figure 18-3a. Control of transcription termination by a riboswitch 代谢物代谢物Figure 18-3a. Control of translation initiation by a riboswitch.
13 Figure Alteration of of the structure of the SAM riboswitch upon the binding of SAM (S-adenosyl methionine)
14 The 2nd structures of 7 riboswitches and metabolites that they sense
15 3. Attenuation (衰减作用) RNA Regulation in Bacteria ---A premature transcription termination that switches off gene expression from amino acid biosynthetic operons after the corresponding amino acid is synthesized at an adequate level. [target]---Requires the participation of ribosomes that translate a leader peptide. The premature transcription termination is triggered by formation of an intrinsic terminator when ribosome read through codons of the amino acid that the operon synthesizes. [mechanism].
16 The TRP operonThe trp operon encodes five structural genes required for tryptophan (色胺酸) synthesis.These genes are regulated to efficiently express only when tryptophan is limiting.Two layers of regulation are involved: (1) transcription repression by the Trp repressor (initiation); (2) attenuation
17 Fig 16-19Transcription of the trp operon is prematurally stopped if the tryptophan level is not low enough, which results in the production of a leader RNA of 161 nt. (WHY?)
18 Transcription and translation in bacteria are coupled (细菌体内的转录和翻译是偶联的) Transcription and translation in bacteria are coupled (细菌体内的转录和翻译是偶联的). Therefore, synthesis of the leader peptide immediately follows the transcription of leader RNA.The leader peptide contains two tryptophan codons. If the tryptophan level is very low, the ribosome will pause at these sites.Ribosome pause at these sites alter the secondary structure of the leader RNA, which eliminates the intrinsic terminator structure and allow the successful transcription of the trp operon.
19 The leader RNA and leader peptide of the trp operon
20 High Trp Low Trp Transcription of the leader RNA. RNA PolTranscription of the leader RNA.Low TrpRNA PolTranscription of the trp operon mRNA.
21 Importance of attenuation A typical negative feed-back regulationCellular tryptophan level is controlled by both repression and attenuation.Attenuation alone can provide robust regulation: other amino acids operons like his and leu have no repressors and rely entirely on attenuation for their regulation.The first example of gene regulation mediated by altering RNA secondary structure.
22 PART 2 RNAi and miRNA regulation Chapter 18 Regulatory RNAsPART 2 RNAi and miRNA regulation
23 OutlinesRNAi discovery and mechanismThe discovery of miRNAsmiRNA biogenesis and regulationmiRNA roles in development, cell differentiation and virusmiRNA in cancersiRNA application
24 CHAPTER 18 RNAi and miRNA regulation Topic 1: RNA interference is a major regulatory mechanism in eukaryotes
25 1 Double-stranded RNA inhibits expression of genes homologous to that RNA. [phenomena-现象]
26 2006年的诺贝尔生理学奖获得者：Andrew Z. FireCraig C. Mello发现于1998
29 Fig 2. Analysis of RNA-interference effects in individual cells Fig 2. Analysis of RNA-interference effects in individual cells. Fluorescence micrographs show progeny of injected animals from GFP-reporter strain PD4251 (a C. elegans strain expressing GFP fluorescence protein) (使用外源导入的报告基因).Control dsRNAds-gfp RNAYoung larva (幼虫)Adult (成虫)adult body wall at high magnification (高放大倍数的成虫体壁)
30 Fig 3. Effects of mex-3 RNA interference on levels of the endogenous mRNA (in situ hybridization in embryos) (胚胎的原位杂交).+ hybridization (endogenous mex-3 RNA)No hybridization and staining+antisense+hybridization+ds mex-3 RNA+hybridization
31 Importance of RNAi discovery: explains the virus-induced gene silencing in plants (植物病毒引起的基因沉默) found years ago.Most plant viruses have single-stranded RNA genomes, which are released from the protein coat of their virus particles as they enter a cell. Their genomic RNA is then replicated by the virus encoded RNA-dependent RNA polymerase to produce sense and antisense RNA, which can hybridize to form dsRNA and trigger an RNAi response against their own sequences.
32 2. Short RNAs that silence genes are produced from a variety of sources and direct the silencing of genes in three different ways [机制]
33 Short RNAs (functional input) Different small silencing RNAs are named according to their origin---siRNAs (small interfering RNAs): made artificially or produced in vivo from dsRNA precursors---miRNA (microRNA): derived from precursor RNAs encoded by nuclear genes.
35 Three ways of the RNAi-directed gene silencing (functional output) Trigger destruction of the target mRNA (引起靶标mRNA的降解),Inhibit translation of the target mRNA (抑制靶标mRNA的翻译),Induce chromatin modification (引起靶标启动子的转录沉默).
37 RNAi machinery The heart of the RNAi mechanism Dicer: an RNaseIII-like multidomain ribonuclease that first processes input dsRNA into small fragments, siRNAs or miRNA. Dicer then helps load its small RNA products into RISC.RISC (RNA induced silencing complexes) (RNA诱导的沉默复合体): a large multiprotein complex that direct the bound siRNA or miRNA to its target and inhibit the target gene expression.
38 Dicer: Structural organization: ---A PAZ domain, binds the end of the dsRNA---Two RNase III domains---Other non-conserved domains.贾第鞭毛虫
39 The crystal structure of the Giardia intact Dicer enzyme shows that the PAZ domain, a module that binds the end of dsRNA, is separated from the two catalytic RNase III domains by a flat, positively charged surface.The 65 angstrom distance between the PAZ and RNase III domains matches the length spanned by 25 base pairs of RNA. Thus, Dicer itself is a molecular ruler that recognizes dsRNA and cleaves a specified distance from the helical end.
40 RISC: the key component is Argonaute (AGO) Argonaute (AGO): A large protein family that constitutes key components of RISCs.---AGO proteins are characterized by two unique domains, PAZ and PIWI. PAZ domain binds the 3’ end 2 nt overhangs of the guide strand of siRNA/miRNA, whereas the PIWI domain (RNase III) confers slicer activity. The cleavage occurs in the middle of guide RNA-target RNA duplex. PAZ and PIWI domains are both essential for Ago activity.---Distinct AGO members have distinct functions. For example, human AGO2 programs RISCs to cleave themRNA target, whereas AGO1 and AGO3 do not.
42 RNAi output 1: A model for siRNA-guided mRNA cleavage by Ago
43 RNAi output 2: Mechanisms of miRNA-Mediated Translational Inhibition Eulalio et al. Cell 132, 2008
44 (A) Inhibition of translation elongation: miRNAs repress translation of target mRNAs by blocking translation elongation or by promoting premature dissociation of ribosomes (ribosome drop-off)(B) Co-translational protein degradation: The protein is normally translated after which it is immediately degraded proteolytically.(C) Competition for the cap structure: Argonaute proteins compete with eIF4E for binding to the cap structure.(D) Inhibition of ribosomal subunit joining: Argonaute proteins recruit eIF6, which prevents the large ribosomal subunit from joining the small subunit.
45 RNAi output 3: Transcriptional Gene Silencing by Directing Chromatin Modification Figure Silencing of the centromere in S. pombe (裂殖酵母)
50 The discovery of miRNAs Victor AmbrosGary RuvkunmiRNA was first discovered in 1993 by Victor Ambros at Harvard (lin-4)The second miRNA Let-7 was discovered in 2000 by Frank Slack as a postdoc at Harvard (Ruvkun lab)
51 The first discovered miRNA lin-4 in 1993 Ruvkun G, Wightman B, Ha I. The 20 years it took to recognize the importance of tiny RNAs. Cell Jan 23;116 (2 Suppl):S93-6.Lee R, Feinbaum R, Ambros V. A short history of a short RNA. Cell Jan 23;116 (2 Suppl):S89-92Thought to be an oddity not a general phenomenon
52 Breakthrough with BlastN of the second miRNA (stRNA) let-7 Pasquinelli AE, Reinhart BJ, Slack F, Martindale MQ, Kuroda MI, Maller B, Hayward DC, Ball EE, Degnan B, Muller P, Spring J, Srinivasan A, Fishman M, Finnerty J, Corbo J, Levine M, Leahy P,Davidson E, Ruvkun G. Conservation of the sequence and temporal expression of let-7 heterochronic regulatory RNA. Nature Nov 2;408(6808):86-9.
54 后基因组时代的基因调控：RNA 调控Most of the RNA transcribed from your genome doesn’t make protein. Carina Dennis talks to the revolutionaries who believe that it functions in gene-regulatory networks that underlie the complexity of higher organisms.
56 microRNAs had been neglected for so many years because of their small size. The underlying reason is: people never dream that small RNAs will have important biological roles.
57 The number of the identified miRNAs is growing rapidly in recent years (now approaching late log phase) miRNAs have been found until March, 2009 (The miRBase Sequence Database).These miRNAs are from primates, rodents, birds, fish, worms, flies, plants and viruses. Among which 706 miRNAs from human, which are named as has-miRx.The data are freely available to all through the web interface at and in flatfile form from ftp://ftp.sanger.ac.uk/pub/mirbase/sequences/.
58 Topic 3: miRNA biogenesis and regulation CHAPTER 18 RNAi and miRNA regulation三、miRNA生成和调控Topic 3: miRNA biogenesis and regulation
60 MicroRNA (miRNA): A type of non-coding small RNA (~21–23 nucleotides) produced by Dicer from a stem-loop structured RNA precursor (~70-90 nts ong) (结构和来源). miRNAs are widely expressed in animal and plant cells and functions in the form of RNA–protein complexes, termed miRISCs. miRNAs have been implicated in the control of development because they lead to the destruction or translational suppression of target mRNAs with homology to the miRNA (生物学功能和机制).
61 The miRNA genes and Structure of pri-miRNAs Pri-miRNAs bear the 5’ cap and 3’ poly(A) tails
62 miRNA processing Pri-miRNA (miRNA初级转录产物) Drosha (1) pre-miRNA Dicer(2)miRNAExportin 5 (Exp5) transports pre-miRNA to the cytoplasm
63 A typical metazoan pri-miRNA consists of a stem of approximately 33 bp, with a terminal loop and flanking segments.The terminal loop is unessential, whereas the flanking ssRNA segments are critical for processing.The cleavage site is determined mainly by the distance (approximately 11 bp) from the stem-ssRNA junction.
69 CHAPTER 18 RNAi and miRNA regulation 四、MicroRNA广泛而重要的生物学功能Topic 4: The biological roles of miRNAs (development, cell differentiation, virus-host interaction etc)
70 1. miRNA in C. elegans development Victor R. Ambros秀丽线虫 C. elegans
71 lin-4 and let-7 miRNAs control the developmental timing of C. elegans.
72 Expression of lin-4 allows C Expression of lin-4 allows C. elegans to proceed to the late developmental stage
73 lin-4 binds its target mRNAs by imperfect base pairing.
74 2. miRNAs play important roles in vertebrate development.
75 Learning the miRNA function from its expression pattern 斑马鱼青鳉小鼠果蝇Figure 2. Expression of miR-124a and miR-1 in Zebrafish, Medaka, Mouse, and Fly.miR-124a is restrictedly expressed in the brain and the spinal cord in fish and mouse or to the ventral nerve cord in the fly. The expression of miR-1 is restricted to the muscles and the heart in the mouse.
76 3. miRNA controls plant phenotypes and more (控制植物表型特征) Jaw-miRNA 控制拟南芥叶形变化(Nature, 2003)
77 4. miRNA controls the differentiation of the hematopoietic stem cell (调控造血干细胞的分化) ( Science 2004)
83 Figure 3, Comparison between normal and tumor samples reveals global changes in miRNA expression.
84 One mechanism of miRNA controlling oncogene expression
85 c-Myc is a helix–loop–helix leucine zipper transcription factor that regulates an estimated 10–15% of genes in the human and Drosophila genomes.c-Myc activates expression of a cluster of six miRNAs (mir cluster) on human chromosome 13. (Figure 1)E2F1 is the transcription factor, which is a target of c-Myc that promotes cell cycle progression.Expression of E2F1 is negatively regulated by two miRNAs in this cluster, miR-17-5p and miR-20a. (Figure 4)
90 Figure 1. The mir-17–92 cluster shows increased expression in B-cell lymphoma samples and cell lines. The level of mir-17–92 pri-miRNA was determined by real-time quantitative RT-PCR in 46 lymphomas and 47 colorectal carcinomas, and compared to levels found in corresponding normal tissues from five individuals.
91 Figure 2. Overexpression of the mir-17–19b cluster accelerates c-myc-induced lymphomagenesis in mice.
93 1. siRNA application in mammalian Transfection of exogenous siRNAChemical synthesis: fast, but expensive and short-termGenerate esiRNA (enzymatic produced siRNA) from in vitro transcribed long dsRNA by E. coli RNase III or RNase III-like DICER: less expensive, large-scale, but short-term.Expression of siRNAsiRNA produced with pol III promoter from the transfected DNA plasmids. Stable and long-term, but slower
94 Expression of hairpin RNA (shRNA) using a Pol III promoter. Expression of siRNA in cultured cells or in animal modelsExpression of hairpin RNA (shRNA) using a Pol III promoter.通过将编码shRNA的质粒转染进细胞内。转染分两类：瞬时转染（transient transfection）和稳定转染（Stable cell lines）Transcription from RNAP III promoters of U6 and H1 are well characterized. RNAP III transcription uses a well-defined termination signal (TTTTT) and the products have no extra sequence.2. Transcription from these promoters is very efficient in various tissues.
95 A mammalian expression vector designed to direct the intracellular synthesis of shRNAs, which will then be processed into siRNAs.
96 Advantages of expression of siRNA in living cells Create induced phenotypes that can be observed over long time spans.Create a stably engineered cells can be assayed either in vitro (体外-细胞水平) or in vivo（体内）, allowing the test of the angiogenic (血管生成) or metastatic (转移) potentials of tumor cells in xenograft models (异种移植模型)。Create hypomorphic alleles (亚等位基因) rapidly in transgenic mice. [Hypomorphic alleles are those in which function is reduced, but not completely lost]
97 4. Combine shRNAs with existing high-efficiency gene delivery vehicles to create bona fide RNAi-based therapeutics. For example, ultimately, to silence a disease-causing mutant allele specifically.
98 Research Applications of RNAi: A new strategy of reverse genetics & a novel way of gene knock-out It can be used in reverse genetics (反向遗传学) to identify the cellular or biological function of a gene.It can be combined with genomics to perform large-scale genetic screens aimed at gene discovery.
99 Therapeutic Applications of RNAi: A new strategy to invitation of new drugs and gene therapy siRNAs can be used to counter viral infection by specifically destroying the mRNAs of the pathogenic viruses, such as HIV and HBV.siRNAs can be applied to counter cancers by specifically down-regulate the expression of genes related to oncogenesis.