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4 Leader RNA structure
4 complentary sequences named sequence 1,2,3,4 from different base-paired structure 1:2; 2:3; 3:4 only 3:4 hairpin form terminator
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6 Attenuation in the trp operon
Effectively adds a precise adjustment to the regulation of the trp operon. Several key points: 1. Transcription & translation are tightly coupled in bacteria (attenuation requires this). 2. translation can occur as an mRNA is being transcribed .
4
P = promoter T = terminator O = operator
The trp operon
trpR （separate promotor and terminator ） trp promotor (-10 and -35) trp operator ,active repressor binding site trpL ,162bp ,containing an attenuator five structural gene E,D,C,B,A ,but only one polycistronic
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Key notes
1 2 3 4 5 6 7 8 the tryptophan operon the trp repressor the attenuator leader RNA structure the leader peptide attenuation importance of attenuation operon summary
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The leader mRNA of E. coli
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5 The leader peptide
14 AA leader peptide 10th and 11th condons encode 2 successive tryptophan residues as Trp is a rare amino acid ,the ribosonme would be expected to pause at this site
5
2
the trp repressor
a gene product of the trpR operon a dimer of 2 subunits inactive itself specifically interacts with the operator site only binding co-repressor trp binding the palindrome
1 3 4
2
CACCAUGCGUAAAGCAAUCAGAUACCCAGCCCGCCUAAUGAGCGGGCUUUU Met-Gln-Thr-Gln-Lys-Pro UUUU-GAACAAAAUUAGAGAAUAACA-AUG-CAA-ACA-CAA-AAA-CCG trpE . . . Terminator
L2 THE TRP OPERON
lecturer :Hongyang Wang major :Developmental Biology tutor :Hongtao Li
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Difference
off usually active repressor inducer turn on when lac as a carbon source on usually inactive repressor co-repressor turn off when trp is high
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The Trp Leader peptide
Met-Lys-Ala-Ile-Phe-ValAAGUUCACGUAAAAAGGGUAUCGACA-AUG-AAA-GCA-AUU-UUC-GUALeu-Lys-Gly-Trp-Trp-Arg-Thr-Ser-STOP CUG-AAA-GGU-UGG-UGG-CGC-ACU-UCC-UGA-AACGGGCAGUGUAUU
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The trp repressor
调节区
trpR RNA聚合酶 RNA聚合酶 O P RNA聚合酶 RNA聚合酶
结构基因
Trp 低时
mRNA
Trp 高时 Trp
At first ,it was thought the repressor was responsible for all the transcriptional regulation of the trp operon . However ,it was observed that the deletion of a sequence between the operator and trpE resulted in an increase in both the basal and the activated levels of transcription . There must be another transcription regulation
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1 The trp operon
TrpR (repressor) P leader PO
trpR
mRNA
T
trpE trpD trpC trpB trpA
Polycistronic mRNA (encodes 5 proteins)
T
TrpR 5 separate proteins that were synthesized from one mRNA
8 Operon summary
We have considered in detail three operons: the lac operon, the ara operon, and the trp operon. The first two are operons concerned with the control of catabolic processes (utilization of energy substrates) while the third is concerned with anabolic processes (synthesis of a molecule the cell needs).
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Transcription And Translation In Prokaryotes
5’ 3’
3’
5’
RNA Pol.
Ribosome
mRNA
5’
Ribosome
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17
Double regulation
repressor--------whether to start transcribing as a rough regulation attenuator-------whether to continue to transcribe after starting transcription as a precise regulation
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7 Importance of attenuation
10-fold repression 700-fold regulation combined with repressor occurs in at least 6 operons ie His a more preciser regulation transcription regulation by translation as feedback regulation
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All three share negative control features, using repressor proteins binding to operators to place the operon in the "off" state. The first two have positive control features that increase transcription in response to low glucose (CAP-cAMP binding to the CAP site). This is not the case for the tryptophan operon. The tryptophan operon, however, has the additional negative control feature of attenuation. These features are summarized in the following table:
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3
The attenuator
towards the end of the leader sequence precedes the trpE initiator codon a short GC-rich palindrome followed by 8 successive U residues . able to form a hairpin structure in RNA transcript