BIOC0005 EXAM QUESTIONS AND ANSWERS WITH
COMPLETE SOLUTIONS VERIFIED LATEST UPDATE
What is the order of the trp operon?
trpL, trpE, trpD, trpC, trpB, trpA
What is the trp operon controlled by?
tryptophan concentration
Where is trpa?
a small sequence embedded in the leader sequence trpL
What does the trp operon do?
convert chorismic acid into tryptophan
What happens to the trp operon when [trp] is high?
operon is not transcribed
trpR gene - to left of operon promoter, has own promoter, encodes aporepressor
(inactive)
aporepressor binds to a trp molecule (co-repressor) and then this binds to another apo-
trp complex to form dimer
active repressor binds operon promoter operator region
RNA pol can't bind to promoter
operon is not transcribed
What happens to the trp operon when [trp] is low?
,operon must be transcribed for cell to produce trp
RNA pol binds promoter
operon transcribed
trp produced
repressor still produced but remains as aporepressor monomer/dimer without trp
How does attenuation play a role in regulation of the trp operon?
attenuation = incomplete transcription
only 161 (trpL) of the 7000 bases (operon) transcribed when attenuation occurs
prevents transcription when [trp] is high
trpa encodes attenuator sequence
How does attenuation occur in trp operon?
in leader-attenuator mRNA there are 4 complementary GC rich regions - 1 can pair to 2,
2 can pair to 3, 3 can pair to 4
when 3 pairs with 4, terminator stem loop forms and attenuation occurs
tryptophan starvation - transcription proceeds, ribosome stalls in region 1 at the two trp
codons, region 2 pairs with 3, prevents 3 pairing with 4, no stem loop formed, no
attenuation
tryptophan abundance - transcription progression prevented, ribosome translates leader
peptide, falls off at stop signal, region 1 pairs with 2, 3 pairs with 4, terminator stem loop
formed, attenuation
Why is attenuation needed for trp operon?
repression of trp operon is weak, attenuation gives more control
repressed level of transcription only 70-fold lower than when repressor not bound -
,attenuation increases this another 10-fold
(10 x 70 = 700-fold)
what is a regulon?
collection of genes regulated as a unit and transcribed in response to the same signal,
each gene has their own promoter
What is the pho regulon?
phosphate regulon
found in E. coli
bacterial phosphate in form of orthophosphate anion or inorganic phosphate
What does the pho regulon encode?
proteins for sensing [phosphate]
activator of gene transcription
~30 genes whose activation leads to increased [phosphate]
What regulatory system does the pho regulon use?
2 component
sensor protein - PhoR, histidine kinase, senses [phosphate], transmits signal of
decreased [phosphate] to regulator protein
regulator protein - PhoB, DNA binding protein, activator of transcription
What is PhoR?
histidine kinase
sensor protein
What is PhoB?
DNA binding protein
, How does the Pho regulon function when [phosphate] is sufficient?
PstS in periplasm has binding sites for phosphate
PstS binds phosphate and moves from periplasm to periplasmic membrane where it
makes contact with PstA and PstC
PstA and PstC make contacts with PstB (dimer) in cytoplasm - forms an open channel
phosphate flows into cytoplasm
PhoU interacts with PstB and PhoR protein
PhoR is inactive when channel is open
what are the PstSCAB components?
PstS, PstC, PstA, PstB
Pst = phosphate-specific transporter
What does PstSCAB do?
assists in phosphate transport into cytoplasm
What is PhoU?
phosphate-specific transport system accessory protein
What does PhoU do?
interacts with transporter and PhoR
How does the Pho regulon function in phosphate starvation?
PstS doesn't bind phosphate in periplasm
PstS doesn't make contact with PstA and PstC which don't make contact with PstB
forms a closed channel
no phosphate travels into cytoplasm
conformational change causes conformational change in PhoU
COMPLETE SOLUTIONS VERIFIED LATEST UPDATE
What is the order of the trp operon?
trpL, trpE, trpD, trpC, trpB, trpA
What is the trp operon controlled by?
tryptophan concentration
Where is trpa?
a small sequence embedded in the leader sequence trpL
What does the trp operon do?
convert chorismic acid into tryptophan
What happens to the trp operon when [trp] is high?
operon is not transcribed
trpR gene - to left of operon promoter, has own promoter, encodes aporepressor
(inactive)
aporepressor binds to a trp molecule (co-repressor) and then this binds to another apo-
trp complex to form dimer
active repressor binds operon promoter operator region
RNA pol can't bind to promoter
operon is not transcribed
What happens to the trp operon when [trp] is low?
,operon must be transcribed for cell to produce trp
RNA pol binds promoter
operon transcribed
trp produced
repressor still produced but remains as aporepressor monomer/dimer without trp
How does attenuation play a role in regulation of the trp operon?
attenuation = incomplete transcription
only 161 (trpL) of the 7000 bases (operon) transcribed when attenuation occurs
prevents transcription when [trp] is high
trpa encodes attenuator sequence
How does attenuation occur in trp operon?
in leader-attenuator mRNA there are 4 complementary GC rich regions - 1 can pair to 2,
2 can pair to 3, 3 can pair to 4
when 3 pairs with 4, terminator stem loop forms and attenuation occurs
tryptophan starvation - transcription proceeds, ribosome stalls in region 1 at the two trp
codons, region 2 pairs with 3, prevents 3 pairing with 4, no stem loop formed, no
attenuation
tryptophan abundance - transcription progression prevented, ribosome translates leader
peptide, falls off at stop signal, region 1 pairs with 2, 3 pairs with 4, terminator stem loop
formed, attenuation
Why is attenuation needed for trp operon?
repression of trp operon is weak, attenuation gives more control
repressed level of transcription only 70-fold lower than when repressor not bound -
,attenuation increases this another 10-fold
(10 x 70 = 700-fold)
what is a regulon?
collection of genes regulated as a unit and transcribed in response to the same signal,
each gene has their own promoter
What is the pho regulon?
phosphate regulon
found in E. coli
bacterial phosphate in form of orthophosphate anion or inorganic phosphate
What does the pho regulon encode?
proteins for sensing [phosphate]
activator of gene transcription
~30 genes whose activation leads to increased [phosphate]
What regulatory system does the pho regulon use?
2 component
sensor protein - PhoR, histidine kinase, senses [phosphate], transmits signal of
decreased [phosphate] to regulator protein
regulator protein - PhoB, DNA binding protein, activator of transcription
What is PhoR?
histidine kinase
sensor protein
What is PhoB?
DNA binding protein
, How does the Pho regulon function when [phosphate] is sufficient?
PstS in periplasm has binding sites for phosphate
PstS binds phosphate and moves from periplasm to periplasmic membrane where it
makes contact with PstA and PstC
PstA and PstC make contacts with PstB (dimer) in cytoplasm - forms an open channel
phosphate flows into cytoplasm
PhoU interacts with PstB and PhoR protein
PhoR is inactive when channel is open
what are the PstSCAB components?
PstS, PstC, PstA, PstB
Pst = phosphate-specific transporter
What does PstSCAB do?
assists in phosphate transport into cytoplasm
What is PhoU?
phosphate-specific transport system accessory protein
What does PhoU do?
interacts with transporter and PhoR
How does the Pho regulon function in phosphate starvation?
PstS doesn't bind phosphate in periplasm
PstS doesn't make contact with PstA and PstC which don't make contact with PstB
forms a closed channel
no phosphate travels into cytoplasm
conformational change causes conformational change in PhoU
