BIOL 142 Lecture Notes - Lecture 25: Lac Repressor, Lac Operon, Repressor
30 Apr 2018
School
Department
Course
Professor
1."Which"of"the"following"statements"about"the"lac"operon"is"FALSE?"
When"lactose"binds"to"the"lac"repressor"protein,"the"lac"repressor"protein"can"
no"longer"bind"to"the"operator.
True
○
A.
When+lactose+binds+to+the+operator,+transcription+is+stimulated.+
False:"not"lactose"that"binds"the"operator,"the"repressor"binds"to"the"
operator"DNA"sequence
○
Lactose"sugar"is"an"inducer"that"binds"to"the"repressor,"causing"the"
repressor"to"be"removed"from"the"operator
○
B.
When"the"lac"repressor"protein"binds"to"the"operator,"transcription"is"inhibited."
True
○
C.
When"lactose"binds"to"the"lac"repressor"protein,"the"shape"of"the"lac"repressor"
protein"is"changed."
True:"lactose"binds"a"repressor
○
Repressor"undergoes"conformational"change"that"reduces"affinity"for"
operator"sequence"so"it"releases"from"operator
○
D.
The"lac"repressor"protein"has"binding"sites"for"both"DNA"and"lactose.
True:"system"whereby"genes"needed"to"break"down"lactose"are"under"
control"of"a"operator"and"a"promoter
○
The"operator"in"the"absence"of"lactose"is"bound"by"a"repressor"protein"
that"can"bind"DNA"(operator"region)
○
Repressor"is"removed"from"operator"when"lactose"binds"to"repressor
○
Has"both"lactose"and"DNA"binding"sites
○
E.
Recap:"Because"positive"and"negative"control"elements"are"superimposed,"the"lac"
operon"is"only"fully"activated"when"lactose"is"available"and"when"glucose"is"scarce.
How"transcription"behaves"in"response"to"different"environmental"conditions"
relating"to"sugars
•
When"glucose"is"high"and"lactose"is"low"–allows"lac"repressor"to"bind"to"
operator"to"act"as"a"physical"roadblock"so"RNA"polymerase"doesn't"undergo"
transcription
•
Glucose"high"lactose"high"–lactose"causes"repressor"to"be"removed"from"the"
operator"but"cAMP"levels"are"low,"so"we"lack"positive"control
When"cAMP"are"high"and"glucose"low,"CAP"binds"to"binding"site"and"
promotes"transcription
○
•
Because"positive"and"negative"control"elements"are"superimposed,"the"lac"operon"is"
only"fully"activated"when"lactose"is"available"and"when"glucose"is"scarce
High"levels"of"lac"operon"when"we"remove"the"repressor•
CAP"and"repressor"promotes"removal"transcription•
Lactose"is"the"inducer"of"transcription"by"turning"off"transcription"protein•
Learning"objectives
Compare(and(contrast(the(lac(operon(and(the(trp(operon.1.
Models"1"and"2"are"negative"control"via"repressor"protein
Difference"is"how"other"substances"act"on"repressor•
In"inducible+systems,"an"inducer"from"the"cell’s"environment"prevents"a"
repressor"from"blocking"transcription"(lac"operon)
Shuts"off"repressor"to"stimulate"transcription
○
•
In"repressible+systems,"a"corepressor"produced"by"the"cell"activates"a"repressor,"
enabling"it"to"block"transcription"(trp"operon)
End"product"of"pathway"is"efficient"in"quantity"level,"so"transcription"is"
not"needed"and"shuts"down
○
Co-repressor"bound"to"repressor"to"activate"it"and"shut"down"
transcription
○
•
In"the"absence"of"tryptophan,"RNA"polymerase"transcribes"the"trp"operon,"leading"to"
production"of"enzymes"that"synthesize"tryptophan."
If"synthesis"of"an"enzyme"can"be"turned"off,"it"is"said"to"be"repressible."•
The"trp"operon"in"E."coli"is"repressible."•
When"we"transcribe"operons,"we"make"a"long"mRNA"that"codes"for"the"various"
proteins"coded"from"the"operon"(only"in"prokaryotes)
•
Repressible"system"(no"tryptophan)"leads"to"making"the"enzymes"needed"to"
make"tryptophan
•
When"tryptophan"(an"e.g."of"a"corepressor)"is"present,"it"binds"to"a"repressor,"which"
becomes"active."
The"molecule"that"binds"and"activates"a"repressor"is"called"a"corepressor and"
may"be"the"end"product"of"the"operon"(as"in"the"case"of"tryptophan).
Once"levels"are"high,"tryptophan"acts"as"a"corepressor"
○
Tryptophan"levels"will"drop"and"stimulate"the"pathway"again
○
•
The"activated"repressor"binds"to"the"operator of"the"trp"operon,"blocking"
production"of"enzymes"for"tryptophan"synthesis.
•
2."Which"of"the"following"would"you"expect"to"find"in"an"inducible"system?"
A+repressor+protein,+which+is+bound+to+DNA+in+absence+of+any+other+factor.+
Lac"operon,"repressor"binds"to"the"operator"in"the"absence"of"lactose
○
A.
A"repressor"protein,"which"is"bound"to"DNA"in"the"presence"of"a"corepressor."B.
An"activator"protein,"which"is"bound"to"DNA"in"absence"of"any"other"factor."C.
An"activator"protein,"which"is"bound"to"DNA"only"in"the"absence"of"an"inhibitorD.
3."When"tryptophan"is"present"in"the"culture"medium,"transcription"of"tryptophan-
synthesizing"genes"in"E."coli"is"stopped"by"a"repressor"binding"to"the:"
trp"repressor"A.
trp"operon"B.
trp"promoter"C.
trp+operator+
Trp"operon"–repressor"when"bound"to"co-repressor,"the"active"repressor"
will"bind"to"the"trp"operator
○
D.
trp"polymeraseE.
Prokaryotic*Gene*Regulation*III
Monday,"April"16,"2018
11:59"AM
1."Which"of"the"following"statements"about"the"lac"operon"is"FALSE?"
When"lactose"binds"to"the"lac"repressor"protein,"the"lac"repressor"protein"can"
no"longer"bind"to"the"operator.
True
○
A.
When+lactose+binds+to+the+operator,+transcription+is+stimulated.+
False:"not"lactose"that"binds"the"operator,"the"repressor"binds"to"the"
operator"DNA"sequence
○
Lactose"sugar"is"an"inducer"that"binds"to"the"repressor,"causing"the"
repressor"to"be"removed"from"the"operator
○
B.
When"the"lac"repressor"protein"binds"to"the"operator,"transcription"is"inhibited."
True
○
C.
When"lactose"binds"to"the"lac"repressor"protein,"the"shape"of"the"lac"repressor"
protein"is"changed."
True:"lactose"binds"a"repressor
○
Repressor"undergoes"conformational"change"that"reduces"affinity"for"
operator"sequence"so"it"releases"from"operator
○
D.
The"lac"repressor"protein"has"binding"sites"for"both"DNA"and"lactose.
True:"system"whereby"genes"needed"to"break"down"lactose"are"under"
control"of"a"operator"and"a"promoter
○
The"operator"in"the"absence"of"lactose"is"bound"by"a"repressor"protein"
that"can"bind"DNA"(operator"region)
○
Repressor"is"removed"from"operator"when"lactose"binds"to"repressor
○
Has"both"lactose"and"DNA"binding"sites
○
E.
Recap:"Because"positive"and"negative"control"elements"are"superimposed,"the"lac"
operon"is"only"fully"activated"when"lactose"is"available"and"when"glucose"is"scarce.
How"transcription"behaves"in"response"to"different"environmental"conditions"
relating"to"sugars
•
When"glucose"is"high"and"lactose"is"low"–allows"lac"repressor"to"bind"to"
operator"to"act"as"a"physical"roadblock"so"RNA"polymerase"doesn't"undergo"
transcription
•
Glucose"high"lactose"high"–lactose"causes"repressor"to"be"removed"from"the"
operator"but"cAMP"levels"are"low,"so"we"lack"positive"control
When"cAMP"are"high"and"glucose"low,"CAP"binds"to"binding"site"and"
promotes"transcription
○
•
Because"positive"and"negative"control"elements"are"superimposed,"the"lac"operon"is"
only"fully"activated"when"lactose"is"available"and"when"glucose"is"scarce
High"levels"of"lac"operon"when"we"remove"the"repressor
•
CAP"and"repressor"promotes"removal"transcription
•
Lactose"is"the"inducer"of"transcription"by"turning"off"transcription"protein
•
Learning"objectives
Compare(and(contrast(the(lac(operon(and(the(trp(operon.
1.
Models"1"and"2"are"negative"control"via"repressor"protein
Difference"is"how"other"substances"act"on"repressor
•
In"inducible+systems,"an"inducer"from"the"cell’s"environment"prevents"a"
repressor"from"blocking"transcription"(lac"operon)
Shuts"off"repressor"to"stimulate"transcription
○
•
In"repressible+systems,"a"corepressor"produced"by"the"cell"activates"a"repressor,"
enabling"it"to"block"transcription"(trp"operon)
End"product"of"pathway"is"efficient"in"quantity"level,"so"transcription"is"
not"needed"and"shuts"down
○
Co-repressor"bound"to"repressor"to"activate"it"and"shut"down"
transcription
○
•
In"the"absence"of"tryptophan,"RNA"polymerase"transcribes"the"trp"operon,"leading"to"
production"of"enzymes"that"synthesize"tryptophan."
If"synthesis"of"an"enzyme"can"be"turned"off,"it"is"said"to"be"repressible."•
The"trp"operon"in"E."coli"is"repressible."•
When"we"transcribe"operons,"we"make"a"long"mRNA"that"codes"for"the"various"
proteins"coded"from"the"operon"(only"in"prokaryotes)
•
Repressible"system"(no"tryptophan)"leads"to"making"the"enzymes"needed"to"
make"tryptophan
•
When"tryptophan"(an"e.g."of"a"corepressor)"is"present,"it"binds"to"a"repressor,"which"
becomes"active."
The"molecule"that"binds"and"activates"a"repressor"is"called"a"corepressor and"
may"be"the"end"product"of"the"operon"(as"in"the"case"of"tryptophan).
Once"levels"are"high,"tryptophan"acts"as"a"corepressor"
○
Tryptophan"levels"will"drop"and"stimulate"the"pathway"again
○
•
The"activated"repressor"binds"to"the"operator of"the"trp"operon,"blocking"
production"of"enzymes"for"tryptophan"synthesis.
•
2."Which"of"the"following"would"you"expect"to"find"in"an"inducible"system?"
A+repressor+protein,+which+is+bound+to+DNA+in+absence+of+any+other+factor.+
Lac"operon,"repressor"binds"to"the"operator"in"the"absence"of"lactose
○
A.
A"repressor"protein,"which"is"bound"to"DNA"in"the"presence"of"a"corepressor."B.
An"activator"protein,"which"is"bound"to"DNA"in"absence"of"any"other"factor."C.
An"activator"protein,"which"is"bound"to"DNA"only"in"the"absence"of"an"inhibitorD.
3."When"tryptophan"is"present"in"the"culture"medium,"transcription"of"tryptophan-
synthesizing"genes"in"E."coli"is"stopped"by"a"repressor"binding"to"the:"
trp"repressor"A.
trp"operon"B.
trp"promoter"C.
trp+operator+
Trp"operon"–repressor"when"bound"to"co-repressor,"the"active"repressor"
will"bind"to"the"trp"operator
○
D.
trp"polymeraseE.
Prokaryotic*Gene*Regulation*III
Monday,"April"16,"2018 11:59"AM
Document Summary
When lactose binds to the lac repressor protein, the lac repressor protein can no longer bind to the operator. When lactose binds to the operator, transcription is stimulated. False: not lactose that binds the operator, the repressor binds to the operator dna sequence. Lactose sugar is an inducer that binds to the repressor, causing the repressor to be removed from the operator. When the lac repressor protein binds to the operator, transcription is inhibited. When lactose binds to the lac repressor protein, the shape of the lac repressor protein is changed. Repressor undergoes conformational change that reduces affinity for operator sequence so it releases from operator. The lac repressor protein has binding sites for both dna and lactose. True: system whereby genes needed to break down lactose are under control of a operator and a promoter. The operator in the absence of lactose is bound by a repressor protein that can bind dna (operator region)
