Lecture 3 Flashcards
Give an example of a catabolic operon
Lac operon
Give an example of a biosynthetic operon
Trp operon
What are biosynthetic operons involved in?
The expression of enzymes that synthesise small molecules (amino acids, nucleotides). If these are not present in the growth media the operon is switched on.
Charles Yanofsky
Discovered the trp operon
What are the 5 structural genes in the trp operon?
&
What do they encode?
trpA, tepB, tepC, trpD, trpE
The encode 5 enzymes in the biosynthetic pathway that leads to the production of a tryptophan amino acid
Which gene codes the repressor for the trp operon?
trpR
Works in transT
Trp operon: apo-repressor
Trp repressor
Trp operon: holo-repressor
Trp repressor & tryptophan
How does tryptophan control its own expression?
When there are high concentrations of tryptophan it binds the trp repressor to form the holo-repressor.
When tryptophan concentration is low tryptophan falls off the trp repressor.
When are biosynthetic operons off?
When the molecule they help synthesise is available
What is positive control?
The activator molecule binds to an operator region & transcription occurs.
Repression occurs when you inactivate the activator molecule
e.g. CAP proteins
What is negative control?
The repressor molecule binds to an operator region to stop transcription.
What is the arabinose operon an example of?
Dual control- protein with repressor & activation activity
What is the order of genes in the arabinose operon?
C O I B A D
C gene in the arabinose operon
ACTIVATOR PROTEIN
control gene
O & I genes in the arabinose operon
AraO and AraI are the control sites
INITIATOR REGION
B & A & D genes in the arabinose operon
STRUCTURAL GENES B = kinase A = isomerase D = epimerase together these convert the sugar arabinose to D-xylase-5-phosphate
What happens in the presence of arabinose?
The operon needs to be switched on to provide enzymes involved in arabinose utilisation.
CAP + cAMP bind
AraC + arabinose bind
These act as an activator to promote transcription
What happens if arabinose is absent?
AraC undergoes a conformational change so it can bind the O & I region. Operon loops to allow this binding and transcription is obstructed
What are the two main methods for controlling transcription?
Choose which genes to start transcribing:
+ alternative sigma factors
+operons: co-ordinated control of gene groups
Deciding where to stop transcribing:
+anti-terminators
+attenuation
Anti-terminators
Anti-termination sites have an anti-terminator protein so RNA polymerase will pick up the anti-terminator protein as it moves along the chromosome. This allows RNA polymerase to pass termination signals and transcribe subsequent genes.
What is attenuation?
A regulatory mechanism for the Tryptophan operon and is focused on the leader sequence.
Yanofsky & attenuation
Saw that bacterium with a deletion mutation in the Trp operon would produce more Trp operon transcripts. Transcripts were longer in these mutants.
The mutation was a deletion at the end of the leader region which was a terminator sequence.
Describe the trpL leader region
146 bp and has 3 regions of inverted repeats
Describe the inverted repeats in trpL
Only 1/3 of the inverted repeats is a functional terminator as it has a run of AAA/UUU. The other inverted repeats only result in stem loop formation.
The inverted repeat regions overlap.
If stem loop 1 and 3 form termination occurs as stem loop 3 is the only loop with the AAA/UUU
If stem loop 2 forms termination does not occur.
Transcription/translation when high tryptophan
RNA polymerase is transcribing the leader region and transcribes the first part of the RNA that contributes to the formation of stem loop 1. The ribosome moves along behind the DNA polymerase.
RNA polymerase transcribes the region that corresponds to stem loop 2.
RNA polymerase completes transcribing the section that corresponds to stem loop 2. The ribosome finishes and dissociates. Stem loop 1 forms folding in on itself
Transcription stops as termination
Transcription/translation when low tryptopha
The ribosome is following RNA polymerase
In stem loop 1 the ribosome needs 2 tryptophans so the ribosome stalls and does not dissociate.
The polymerase has completed transcribing stem loop 1 but it cannot fold as the ribosome is there. RNA polymerase completed transcribing stem loop 2.
Transcription continues as no termination
