Trp Gene Regulation Flashcards
trp operon expression is controlled at which two points?
-transcription initiation (very similar to lac operon)
-transcription termination
mutations in two different sites can lead to constitutive expression of trp operon. what are these two sites?
-trpO (operator site, overlaps promoter)
-trpR (encodes repressor protein that binds to operator & prevents RNAP from transcribing)
when does TrpR bind operator? (opposite to LacI)
when tryptophan is present
tryptophan is the ____ and trpR is _____ (type of repressor)
-co-repressor
-aporepressor
how does tryptophan binding to TrpR affect Trp operon expression?
binding of tryptophan to TrpR changes its conformation so it can bind DNA and repress operon expression
In the absence of tryptophan, how does trp operon work?
-trpR gene produces aporepressor protein but no tryptophan means it can’t bind to DNA (trpO) so transcription of trp operon continues (operon on)
In the presence of tryptophan, how does trp operon work?
-trpR gene produces aporepressor protein. tryptophan present bind to protein and allows it to bind to DNA (trpO) so transcription of trp operon stops (operon off)
catabolic pathways aka _____. anabolic pathways aka _____ (type of operon)
-breaking down/degradative operon
-building things up/biosynthetic operon
degradative operon: function? expression? regulation by? effector molecules? presence of effector causes?
-degrade compound
-induced by compound
-repressor
-inducer
-repressor loses DNA binding ability
biosynthetic operon: function? expression? regulation by? effector molecules? presence of effector causes?
-synthesize compound
-repressed by compound
-repressor
-corepressor
-repressor gains DNA binding ability
repressors of anabolic pathways exist in 2 states. what are they?
aporepressor: repressor protein in absence of effector/corepressor
co-repressor: effector molecule that binds to aporepressor & facilitates DNA binding
what does the trp operon encode?
-enzymes for synthesizing l-tryptophan (a.a that is part of most proteins, need to made if not present)
trp operon contains what?
5 genes transcribed from single promoter P-trp
where the TrpR repressor encoded?
separate place in genome, unlinked (not in operon)
trp operon activity when tryptophan is high? low?
-operon not expressed
-operon is turned on
contro; of trp operon expression at transcription termination allows for what?
finer control of transcription in proportion to [tryptophan] aka attenuation (controls gene expression @ intermediate levels)
how does attenuation work?
transcription start normally but is terminated before RNAP reaches 1st structural gene of operon (happens in operons that have leader region that encodes leader peptide)
why does attenuation occur (hint :ribosome)
occurs because ribosome movement during translation affects 2 structure of mRNA
3 genetic evidence for 2nd control mechanism/attenuation of Trp operon
- in trpR mutants, higher levels of trp enzymes when tryptophan is not present
-mutations that lower levels of tRNA-Trp increase trp operon expression
-deletion of trp leader region eliminates this regulation
attenuation of trp operon model (brief)
presence of excess tRNA-Trp, transcription of the trp operon stops in leader region
description of trp leader region
-leader peptide with 4 regions of sequence (1,2,3,4) that can form alternative hairpin structures (1:2 & 3:4 or 2:3); short peptide encoded in leader that contains to trp residues
How does the trp leader region cause attenuation?
-leader region of mRNA can form intrinstic transcription termination structure; terminator only forms when [try] is high in cell due to different rates of leader peptide translation
how to leader peptide translation initiated?
RNAP initiates translation of P-trp. After region1 & 2 of leader are transcribed, stem loop forms, then RNAP pauses and ribosome then initiates leader peptide translation
when tryptophan is sufficient, how does attenuation occur after leader peptide translation?
-tRNA-trp is a lot so ribosome translates to end of leader peptide; ribosome at stop codon blocks pairing of segment 2 & 3 of leader mRNA & segment 3 & 4 of leader mRNA form terminator hairpin = attenuation, producing 140 bp transcript (RNA pol terminates)
when tryptophan is insufficient, how does attenuation occur after leader peptide translation?
tRNA-trp wil;l be low; ribosome pauses at adjacent trp codons so segment 2 & 3 of leader mRNA can form hairpin (antiterminator) before segment 4 is transcribed ; blocks pairing of 3 & 4 (terminator hairpin). so no transcription termination, and remainder of trp operon transcribed
3 pieces of genetic evidence for attenuation model
-mutations that prevent formation of 2:3 hairpin terminate transcription in leader region constitutively
-mutation of AUG start codon of leader peptide yields constitutive transcritipn in leader region (no ribosome stalled on Trp regions, so hairpin1:2 stays, preventing 2:3, so 3:4 forms and stop transcription)
-bacteria starved of other a.a in arg leader peptide also show attenuation
