1、會計學1Principal Types of RNAs Produced in Cells: a particular segment of DNA is copied into RNA by the enzyme RNA polymerase: RNAP, RNA pol第1頁/共29頁General themes of transcription第2頁/共29頁Transcription selectively occurs at only certain parts of the genome, and makes a few to thousands copies, transient

2、ly, variably in different cells, in different enviroment.第3頁/共29頁 RNA Polymerases第4頁/共29頁Mg 2+第5頁/共29頁第6頁/共29頁TerminationInitiationElongation第7頁/共29頁RNA polymerase binds at specific location: Promoter strength: 第8頁/共29頁RNA Pol Leaves Its Footprint on a PromoterHow did we identify the promoter in DNA

3、 molecule?第9頁/共29頁In bacteria, its thefactor that makes RNA polymerase initiate transcription only at promotersC- terminal domainfactorHoloenzymeCore enzymeThefactor has four regions第10頁/共29頁Two helices within region 4 form a for DNA binding 第11頁/共29頁An recognizes and interact with bases on non-temp

4、late strand at 10 through its aromatic AAsConformational change, spontaneously but irreversible, in the DNA-enzyme complex to a 第12頁/共29頁RNA polymerase holoenzyme = + 第13頁/共29頁RNA polymerase holoenzyme = + + + + + + + + + + + +The region 3-4 linker of sigma factor (RNA mimic) lies in middle of the R

5、NA exit channel in the open complex第14頁/共29頁RNA polymerase holoenzyme = + 第15頁/共29頁It opens DNA double helix between -11 and +3第16頁/共29頁Initial transcription第17頁/共29頁Initial transcriptionPromoter escape involves breaking polymerase-promoter and the polymerase core-sigma interactionshe region 3-4 lin

6、ker of factor (RNA mimic): the RNAP repeatedly and short RNA products until they reach 10nt, otherwise polymerase is .第18頁/共29頁The polymerase remains stationary and DNA into itPolymerases active center relative to DNA template and synthesizes short transcripts before aborting-repeating until escapin

7、g the promoterEnergy stored in scrunching is released to promoter escape and dislodging of sigma factor第19頁/共29頁Transcription shifts into the elongation phase once a short stretch of RNA ( 10 nt) is synthesizedThe transition requires further conformational change in polymerase that leads it to grip

8、the template more firmly第20頁/共29頁1. in the catalytic cleft2. (Only 8-9 nts remain base-paired with the DNA template at any given time) 3. from the DNA template just behind the RNA pol, then the 第21頁/共29頁Although proofreading for RNA synthesis do exist, transcription() is less accurate than replicati

9、on(); Pyrophosphorolytic editing: remove a correctly or incorrectly inserted ribonucleotide by reincorporation of PPi, but hover longer over mismatchHydrolytic editing: the enzyme backtracks by one or more nucleotides and removes the error-containing sequence. (stimulated by , which both enhance hyd

10、rolytic editing function and serve as elongation stimulating factors: ensure that polymerase elongates efficiently and help overcome “arrest” at the difficult regions)第22頁/共29頁The micrograph(under the electron microscope) shows many molecules of RNA polymerasesimultaneously transcribing each of two

11、adjacent genes. Molecules of RNA polymerase are visible as a series of dots along the DNA with the newly synthesized transcripts (fine threads) attached to them.Multiple RNA pol molecules can transcribe the same gene at the same time, each following closely behind another; 第23頁/共29頁Terminators: the sequences that trigger RNA polymerase to dissociate from the DNARho-independent terminator : a short inverted repeat (20 bp) and a stretch of 8 A:T base pairs. 第24頁/共29頁Have less well-characterized RNA elements rut ( utilization)Rho binding can wrest