27 - RNA Processing; Termination of Prokaryotic Transcription

27 - RNA Processing; Termination of Prokaryotic Transcription

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1 Prokaryotic Transcription Termination In E. coli transcription termination- 2 mechanisms: – RNA polymerase only – RNA polymerase + protein Rho Rho-dependent vs. Rho-independent termination Also: premature termination- attenuation
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2 • about ½ of E. coli terminators require rho (ρ - stands for “release”) RNA rho is hexameric RNA helicase • has RNA-binding domain (N-term) and ATPase (C-term) activities • binds to rut site (rho utilization) on mRNA - rho has no affinity for RNAP • tracks along RNA until it catches-up to RNAP RNAP pauses at termination site → provides time for catch-up (termination site often requires hairpin to be formed in secondary structure of RNA being transcribed) • rho unwinds DNA-RNA hybrid (requires ATP) → mRNA, rho, RNAP all released (Lewin, 2006. Fig. 11.33) Rho-dependent Termination
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3 Rho -Independent Termination Rho-independent termination site which have two characteristics: – GC rich self- complementary region with several intervening nucleotides When this region is transcribed - forms stem loop structure – Followed by series of U residues – Experiments with mutants both loop forming sequences and U stretch are important
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4 Mutation analysis: –Loop interacts with the “surface” of RNAP which just synthesized short polyU region –This interaction causes RNAP to pause –This U region at 3’ end of nascent RNA forms very weak bonds with complementary (template) A region –Bonds melt during RNAP pause on the loop –RNA chain is released –DNA strands re-anneal –RNAP dissociates from the DNA Rho -Independent Termination cont.
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5 Attenuation and Termination Review: Negative regulation of trp Operon by TrpR
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6 Attenuation and Termination Repressor- not the only control mechanism Attenuation - regulated premature termination of transcription Does not involve Rho Premature termination by attenuation at specific site- attenuator site - DNA sequence where RNA polymerase either terminates or continues with transcription - different from “normal” termination site (t site) Several operons, including phenylalanine, histidine…
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7 Analysis of E. coli Trp mutants which have high level of Trp synthesizing enzymes even when repressed ( as Trp is high, it binds to aporepressor → formed
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This note was uploaded on 12/23/2011 for the course BIOL 308 taught by Professor Miskovic during the Fall '09 term at Waterloo.

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27 - RNA Processing; Termination of Prokaryotic Transcription

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