第二十章 轉(zhuǎn)錄的起始（Initiation of transcription）

1、Chapter 20Initiation of transcription20.1 Eukaryotic RNA polymerases consist of many subunits20.3 Promoter elements are defined by mutations and footprinting20.4 RNA polymerase I has a bipartite promoter20.5 RNA polymerase III uses both downstream and upstream promoters20.6 The startpoint for RNA po

2、lymerase II20.7 TBP is a universal factor20.8 TBP binds DNA in an unusual way20.9 The basal apparatus assembles at the promoter20.10 Initiation is followed by promoter clearance20.11 A connection between transcription and repair20.12 Promoters for RNA polymerase II have short sequence elements20.13

3、Some promoter-binding proteins are repressors20.14 Enhancers contain bidirectional elements that assist initiation20.15 Independent domains bind DNA and activate transcription20.16 The two hybrid assay detects protein-protein interactions20.17 Interaction of upstream factors with the basal apparatus

4、Enhancer element is a cis-acting sequence that increases the utilization of (some) eukaryotic promoters, and can function in either orientation and in any location (upstream or downstream) relative to the promoter.20.1 IntroductionFigure 20.1 A typical gene transcribed by RNA polymerase II has a pro

5、moter that extends upstream from the site where transcription is initiated. The promoter contains several short (200 bp. An enhancer containing a more closely packed array of elements that also bind transcription factors may be located several kb distant. (DNA may be coiled or otherwise rearranged s

6、o that transcription factors at the promoter and at the enhancer interact to form a large protein complex.)20.1 IntroductionAmanitin (more fully a-amanitin)is a bicyclic octapeptide derived from the poisonous mushroom Amanita phalloides; it inhibits transcription by certain eukaryotic RNA polymerase

7、s, especially RNA polymerase II.20.2 Eukaryotic RNA polymerases consist of many subunits Eukaryotic RNA polymerase II has 10 subunits. 20.2 Eukaryotic RNA polymerases consist of many subunits Cotransfection is the simultaneous transfection of two markers.20.3 Promoter elements are defined by mutatio

8、ns and footprinting Figure 20.3 Promoter boundaries can be determined by making deletions that progressively remove more material from one side. When one deletion fails to prevent RNA synthesis but the next stops transcription, the boundary of the promoter must lie between them. 20.3 Promoter elemen

9、ts are defined by mutations and footprinting Figure 20.4 Transcription units for RNA polymerase I have a core promoter separated by 70 bp from the upstream control element. UBF1 binds to both regions, after which SL1 can bind. RNA polymerase I then binds to the core promoter. The nature of the inter

10、action between the factors bound at the upstream control element and those at the core promoter is not known. 20.4 RNA polymerase I has a bipartite promoter Preinitiation complex in eukaryotic transcription describes the assembly of transcription factors at the promoter before RNA polymerase binds.2

11、0.5 RNA polymerase III uses both downstream and upstream promoters Figure 20.5 Deletion analysis shows that the promoter for 5S RNA genes is internal; initiation occurs a fixed distance (55 bp) upstream of the promoter. 20.5 RNA polymerase III uses both downstream and upstream promoters Figure 20.6

12、Promoters for RNA polymerase III may consist of bipartite sequences downstream of the startpoint, with boxA separated from either boxC or boxB. Or they may consist of separated sequences upstream of the startpoint (Oct, PSE, TATA).20.5 RNA polymerase III uses both downstream and upstream promoters F

13、igure 20.7 Initiation via the internal pol III promoters involves the assembly factors TFIIIA and TFIIIC, the initiation factor TFIIIB, and RNA polymerase III. 20.5 RNA polymerase III uses both downstream and upstream promoters TATA box is a conserved AT-rich septamer found about 25 bp before the st

14、artpoint of each eukaryotic RNA polymerase II transcription unit; may be involved in positioning the enzyme for correct initiation.20.6 The startpoint for RNA polymerase II Figure 20.8 RNA polymerases are positioned at all promoters by a factor that contains TBP. 20.7 TBP is a universal factorFigure

15、 20.9 A view in cross-section shows that TBP surrounds DNA from the side of the narrow groove. TBP consists of two related (40% identical) conserved domains, which are shown in light and dark blue. The N-terminal region varies extensively and is shown in green. The two strands of the DNA double heli

16、x are in light and dark grey. Photograph kindly provided by Stephen Burley. 20.7 TBP is a universal factorFigure 20.10 The cocrystal structure of TBP with DNA from -40 to the startpoint shows a bend at the TATA box that widens the narrow groove where TBP binds. Photograph provided by Stephen Burley.

17、 20.7 TBP is a universal factorFigure 20.11 An initiation complex assembles at promoters for RNA polymerase II by an ordered sequence of association with transcription factors. 20.8 The basal apparatus assembles at the promoterFigure 20.12 Two views of the ternary complex of TFIIB-TBP-DNA show that

18、TFIIB binds along the bent face of DNA. The two strands of DNA are green and yellow, TBP is blue, and TFIIB is red and purple. Photograph kindly provided by Stephen Burley. 20.8 The basal apparatus assembles at the promoter Phosphorylation of the CTD by the kinase activity of TFIIH may be needed to

19、release RNA polymerase to start transcription. 20.8 The basal apparatus assembles at the promoter Mfd recognizes a stalled RNA polymerase and directs DNA repair to the damaged template strand. 20.9 A connection between transcription and repair Figure 14.28 The Uvr system operates in stages in which

20、UvrAB recognizes damage, UvrBC nicks the DNA, and UvrD unwinds the marked region. 20.9 A connection between transcription and repair Figure 20.15 The TFIIH core may associate with a kinase at initiation and associate with a repair complex when damaged DNA is encountered. 20.9 A connection between tr

21、anscription and repair Figure 14.37 A helicase unwinds DNA at a damaged site, endonucleases cut on either side of the lesion, and new DNA is synthesized to replace the excised stretch.20.9 A connection between transcription and repair CAAT box is part of a conserved sequence located upstream of the

22、startpoints of eukaryotic transcription units; it is recognized by a large group of transcription factors.20.10 Promoters for RNA polymerase II have short sequence elements Figure 20.16 Saturation mutagenesis of the upstream region of the b-globin promoter identifies three short regions (centered at

23、 -30, -75, and -90) that are needed to initiate transcription. These correspond to the TATA, CAAT, 20.10 Promoters for RNA polymerase II have short sequence elements Figure 20.17 Promoters contain different combinations of TATA boxes, CAAT boxes, GC boxes, and other elements. 20.10 Promoters for RNA

24、 polymerase II have short sequence elements Table 20.17 Upstream transcription factors bind to sequence elements that are common to mammalian RNA polymerase II promoters. 20.10 Promoters for RNA polymerase II have short sequence elements ModuleConsnesusDNA boundFactorTATA boxTATAAAA10bpTBPCAAT boxGG

25、CCAATCT22bpCTF/NF1GC boxGGGCGG20bpSP1OctamerATTTGCAT20bpOct-1OctamerATTTGCAT23bpOct-2kBGGGACTTTCC10bpNF kBATFGTGACGT20bpATFTable 20.17 Upstream transcription factors bind to sequence elements that are common to mammalian RNA polymerase II promoters. 20.10 Promoters for RNA polymerase II have short s

26、equence elements ModuleConsnesusDNA boundFactorTATA boxTATAAAA10bpTBPCAAT boxGGCCAATCT22bpCTF/NF1GC boxGGGCGG20bpSP1OctamerATTTGCAT20bpOct-1OctamerATTTGCAT23bpOct-2kBGGGACTTTCC10bpNF kBATFGTGACGT20bpATFFigure 20.18 A transcription complex involves recognition of several elements in the sea urchin H2

27、B promoter in testis. Binding of the CAAT displacement factor in embryo prevents the CAAT-binding factor from binding, so an active complex cannot form. 20.10 Promoters for RNA polymerase II have short sequence elements Enhancer element is a cis-acting sequence that increases the utilization of (som

28、e) eukaryotic promoters, and can function in either orientation and in any location (upstream or downstream) relative to the promoter.20.11 Enhancers contain bidirectional elements that assist initiation Figure 19.39 Indirect end-labeling identifies the distance of a DNAase hypersensitive site from

29、a restriction cleavage site. The existence of a particular cutting site for DNAase I generates a discrete fragment, whose size indicates the distance of the DNAase I hypersensitive site from the restriction site. 20.11 Enhancers contain bidirectional elements that assist initiation Figure 19.40 The

30、SV40 minichromosome has a nucleosome gap. Photograph kindly provided by Moshe Yaniv. 20.11 Enhancers contain bidirectional elements that assist initiation Figure 20.19 An enhancer contains several structural motifs. The histogram plots the effect of all mutations that reduce enhancer function to 75%

31、 of wild type. Binding sites for proteins are indicated below the histogram. 20.11 Enhancers contain bidirectional elements that assist initiation Figure 20.16 Saturation mutagenesis of the upstream region of the b-globin promoter identifies three short regions (centered at -30, -75, and -90) that a

32、re needed to initiate transcription. These correspond to the TATA, CAAT, 20.11 Enhancers contain bidirectional elements that assist initiation Figure 20.20 An enhancer may function by bringing proteins into the vicinity of the promoter. An enhancer does not act on a promoter at the opposite end of a

33、 long linear DNA, but becomes effective when the DNA is joined into a circle by a protein bridge. An enhancer and promoter on separate circular DNAs do not interact, but can interact when the two molecules are catenated. 20.11 Enhancers contain bidirectional elements that assist initiation Figure 20

34、.21 DNA-binding and activating functions in a transcription factor may comprise independent domains of the protein. 20.12 Independent domains bind DNA and activate transcription Figure 20.22 The GAL4 protein has independent regions that bind DNA, activate transcription (2 regions), dimerize, and bin

35、d the regulator GAL80. 20.12 Independent domains bind DNA and activate transcription Figure 20.23 The ability of GAL4 to activate transcription is independent of its specificity for binding DNA. When the GAL4 DNA-binding domain is replaced by the LexA DNA-binding domain, the hybrid protein can activ

36、ate transcription when a LexA operator is placed near a promoter.20.12 Independent domains bind DNA and activate transcription Figure 20.24 The activating domain of the tat protein of HIV can stimulate initiation if it is tethered in the vicinity by binding to the RNA product of a previous round of

37、transcription. Activation is independent of the means 20.12 Independent domains bind DNA and activate transcription Figure 20.25 The two hybrid technique tests the ability of two proteins to interact by incorporating them into hybrid proteins where one has a DNA-binding domain and the other has a tr

38、anscription-activating domain. 20.12 Independent domains bind DNA and activate transcription Figure 20.21 DNA-binding and activating functions in a transcription factor may comprise independent domains of the protein. 20.13 Interaction of upstream factors with the basal apparatus Figure 20.26 An ups

39、tream transcription factor may bind a coactivator that contacts the basal apparatus. 20.13 Interaction of upstream factors with the basal apparatus Figure 20.24 The activating domain of the tat protein of HIV can stimulate initiation if it is tethered in the vicinity by binding to the RNA product of a previous round of transcription. Activation is independent of the means 20.13 Interaction of upstream factors with the basal apparatus Figure 20.11 An initiation comple