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Spring 09 lecture 20

Spring 09 lecture 20 - Review control of initiation of...

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Review: control of initiation of transcription in E. coli- different ways Repressor ( trans-acting factor ) will block transcription initiation when bound to the operator ( cis-acting element ) – negative control One way: sites for RNAP (promoter) and repressor (operator) could overlap - repressor binding blocks RNAP from interacting with DNA at the start site Presence of auxiliary operators additional repressor binding sites Affinity of repressor for operator depends on presence/absence of effector (repressor / inducer and corepressor / aporepressor) – its binding changes repressor’s conformation ° change in activity There are proteins - activators that could increase binding of RNAP to start site - positive control Binding sites for activators are called enhancers (elements) Affinity of activator for enhancer also sometimes depends on presence/absence of effector
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Some proteins are regulators (could execute positive or negative regulation depending on presence/absence of effector . different conformation = affinity for different cis elements). Regulation through two component regulatory system (two proteins involved: sensor-transmitter AND response regulator) DNA bending (as a consequence of binding of a trans factor) could lead to negative or positive regulation - blocking or helping RNAP to interact with DNA at the start site (promoter) When promoter and regulatory protein’s binding site are close or overlap, regulatory protein can directly contact RNAP causing conformational changes in RNAP which might promote formation of open complex Direct contact could be achieved even when promoter and regulatory protein’s binding site are far apart – DNA looping Control of initiation of transcription in E. coli - different ways (cont):
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Overview of Eukaryote RNAPs Learning objectives: Describe the criteria used to separate and purify RNAPs Describe the subunit structure of each RNAP - discuss similarities between subunit structures of eukaryotic and prokaryotic RNAPs - discuss the complexity of eukaryotic RNAPs Describe the structure of RNAP II including CTD tail Describe how the knowledge of RNAP II crystal structure has improved the understanding of transcription mechanism
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Eukaryotic vs. prokaryotic transcription 1. Transcription and translation occur in separate compartments 2. Eukaryote pre-mRNAs are subject to extensive post- transcriptional modification - processing 3. Chromatin structure in eukaroytes limits accessibility (Transcription is tightly regulated. Only 0.01% of genes in a typical eukaryotic cell are undergoing transcription at any given moment ) 4. Eukaryotic RNAP does not recognize binding site – needs general transcription factors to help 5. Mostly multicellular organisms (different cells/tissues) 6.
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