LECTURE: REGULATION OF GENE EXPRESSION - PROKARYOTES
LECTURE: REGULATION OF GENE EXPRESSION - PROKARYOTES KIRSTIN BROWN
Lecture: REGULATION OF GENE EXPRESSION - part 1 Outline:
1. Overview 2. Lac operon 3. Mal operon 4. Trp operon
Overview Why cells regulate gene expression o Cells need to be able to “turn on” or “turn off”
expression of genes
➢ Example: genes that encode enzymes
needed to degrade nutrient “X”. It would be a waste of energy to produce the enzymes when there is no nutrient “X” available
➢ Example: genes that encode enzymes
needed to make product “Y”. It would be a waste of energy to produce the enzymes when there is already an abundance of “Y”
➢ Example: in multicellular organisms, some
genes are expressed in certain cell types and not in others (skin cell vs. liver cell)
Overview Prokaryotic regulation of gene expression o In prokaryotes, genes that are regulated and
expressed together are organized into operons
o An operon consists of more than one gene
controlled by one promoter
o A single mRNA containing the transcript for all
of the genes in the operon is produced
o This single mRNA will be translated into
multiple proteins
Promoter
5’ UTR
Gene 1
Gene 1
Protein 1
Gene 2
Gene 2
Protein 2
Gene 3
Gene 3
Protein 3
Terminator
3’ UTR
mRNA
DNA
Overview Repressors and activators o Repressors bind and prevent transcription
Promoter RNA polymerase
Operator
Gene 1
Gene 2
Gene 3
Terminator
Overview Repressors and activators o Repressors bind and prevent transcription ➢ Co-repressors are molecules that act with the
repressor to help turn off gene expression
Promoter RNA polymerase
Operator
Gene 1
Gene 2
Gene 3
Terminator
Overview Repressors and activators o Repressors bind and prevent transcription ➢ Co-repressors are molecules that act with the
repressor to help turn off gene expression
o Activators bind and promote transcription
Operator
Promoter RNA polymerase
Gene 1
Gene 2
Gene 3
Terminator
Overview Repressors and activators o Repressors bind and prevent transcription ➢ Co-repressors are molecules that act with the
repressor to help turn off gene expression
o Activators bind and promote transcription ➢ Inducers are molecules that act to help turn off
gene expression (either by helping the activator or removing the repressor)
Operator
Promoter
Gene 1
Gene 2
Gene 3
Terminator
Gene 1
Gene 2
Gene 3
Terminator
RNA polymerase
Promoter RNA polymerase
Operator
Lac Operon Genes for breakdown of lactose are negatively regulated o In the absence of lactose, transcription is
blocked by a repressor called LacI
Promoter
Operator
lacZ
lacY
lacA
Terminator
RNA polymerase
o In the presence of lactose, repression is
removed by binding of lactose (or allolactose, the inducer) and genes are expressed
Promoter
Operator
lacZ
lacY
lacA
Terminator
Lac Operon Genes for breakdown of lactose are negatively regulated o In the absence of lactose, transcription is
blocked by a repressor called LacI
Promoter
Operator
lacZ
lacY
lacA
Terminator
RNA polymerase
o In the presence of lactose, repression is
removed by binding of lactose (or allolactose, the inducer) and genes are expressed
Promoter RNA polymerase
Operator
lacZ
lacY
lacA
Terminator
Trp Operon Genes for production of tryptophan are negatively regulated o In the absence of tryptophan, there is no repression and genes are expressed Promoter
Operator
trpE
trpD
trpC
trpB
trpA
Terminator
RNA polymerase
o In the presence of tryptophan, the repressor is
bound by tryptophan (the co-repressor), and this promotes binding of the repressor, blocking transcription
Promoter
Operator
trpE
trpD
trpC
trpB
trpA
Terminator
Trp Operon Genes for production of tryptophan are negatively regulated o In the absence of tryptophan, there is no repression and genes are expressed Promoter
Operator
trpE
trpD
trpC
trpB
trpA
Terminator
RNA polymerase
o In the presence of tryptophan, the repressor is
bound by tryptophan (the co-repressor), and this promotes binding of the repressor, blocking transcription
Promoter RNA polymerase
Operator
trpE
trpD
trpC
trpB
trpA
Terminator
Mal Operon Genes for breakdown of maltose are positively regulated o In the absence of maltose, there is no binding of the activator MalT, so no expression Operator
Promoter
malP
malQ
Terminator
RNA polymerase
o In the presence of maltose, the activator is
bound by maltose (the inducer), and this promotes binding of the activator, promoting transcription Operator
Promoter
malP
malQ
Terminator
Mal Operon Genes for breakdown of maltose are positively regulated o In the absence of maltose, there is no binding of the activator MalT, so no expression Operator
Promoter
malP
malQ
Terminator
RNA polymerase
o In the presence of maltose, the activator is
bound by maltose (the inducer), and this promotes binding of the activator, promoting transcription Operator
Promoter RNA polymerase
malP
malQ
Terminator
Practice Question Predict the phenotype of a malT mutant in which MalT can no longer bind the operator
Operator
Promoter
RNA polymerase
malP
malQ
Terminator
Practice Question Predict the phenotype of a lacI mutation in which LacI can no longer bind lactose
Promoter RNA polymerase
Operator
lacZ
lacY
lacA
Terminator
Lecture: REGULATION OF GENE EXPRESSION - part 1 Outline:
1. Overview 2. Lac operon 3. Mal operon 4. Trp operon
Overview Why cells regulate gene expression o Cells need to be able to “turn on” or “turn off”
expression of genes
➢ Example: genes that encode enzymes
needed to degrade nutrient “X”. It would be a waste of energy to produce the enzymes when there is no nutrient “X” available
➢ Example: genes that encode enzymes
needed to make product “Y”. It would be a waste of energy to produce the enzymes when there is already an abundance of “Y”
➢ Example: in multicellular organisms, some
genes are expressed in certain cell types and not in others (skin cell vs. liver cell)
Overview Prokaryotic regulation of gene expression o In prokaryotes, genes that are regulated and
expressed together are organized into operons
o An operon consists of more than one gene
controlled by one promoter
o A single mRNA containing the transcript for all
of the genes in the operon is produced
o This single mRNA will be translated into
multiple proteins
Promoter
5’ UTR
Gene 1
Gene 1
Protein 1
Gene 2
Gene 2
Protein 2
Gene 3
Gene 3
Protein 3
Terminator
3’ UTR
mRNA
DNA
Overview Repressors and activators o Repressors bind and prevent transcription
Promoter RNA polymerase
Operator
Gene 1
Gene 2
Gene 3
Terminator
Overview Repressors and activators o Repressors bind and prevent transcription ➢ Co-repressors are molecules that act with the
repressor to help turn off gene expression
Promoter RNA polymerase
Operator
Gene 1
Gene 2
Gene 3
Terminator
Overview Repressors and activators o Repressors bind and prevent transcription ➢ Co-repressors are molecules that act with the
repressor to help turn off gene expression
o Activators bind and promote transcription
Operator
Promoter RNA polymerase
Gene 1
Gene 2
Gene 3
Terminator
Overview Repressors and activators o Repressors bind and prevent transcription ➢ Co-repressors are molecules that act with the
repressor to help turn off gene expression
o Activators bind and promote transcription ➢ Inducers are molecules that act to help turn off
gene expression (either by helping the activator or removing the repressor)
Operator
Promoter
Gene 1
Gene 2
Gene 3
Terminator
Gene 1
Gene 2
Gene 3
Terminator
RNA polymerase
Promoter RNA polymerase
Operator
Lac Operon Genes for breakdown of lactose are negatively regulated o In the absence of lactose, transcription is
blocked by a repressor called LacI
Promoter
Operator
lacZ
lacY
lacA
Terminator
RNA polymerase
o In the presence of lactose, repression is
removed by binding of lactose (or allolactose, the inducer) and genes are expressed
Promoter
Operator
lacZ
lacY
lacA
Terminator
Lac Operon Genes for breakdown of lactose are negatively regulated o In the absence of lactose, transcription is
blocked by a repressor called LacI
Promoter
Operator
lacZ
lacY
lacA
Terminator
RNA polymerase
o In the presence of lactose, repression is
removed by binding of lactose (or allolactose, the inducer) and genes are expressed
Promoter RNA polymerase
Operator
lacZ
lacY
lacA
Terminator
Trp Operon Genes for production of tryptophan are negatively regulated o In the absence of tryptophan, there is no repression and genes are expressed Promoter
Operator
trpE
trpD
trpC
trpB
trpA
Terminator
RNA polymerase
o In the presence of tryptophan, the repressor is
bound by tryptophan (the co-repressor), and this promotes binding of the repressor, blocking transcription
Promoter
Operator
trpE
trpD
trpC
trpB
trpA
Terminator
Trp Operon Genes for production of tryptophan are negatively regulated o In the absence of tryptophan, there is no repression and genes are expressed Promoter
Operator
trpE
trpD
trpC
trpB
trpA
Terminator
RNA polymerase
o In the presence of tryptophan, the repressor is
bound by tryptophan (the co-repressor), and this promotes binding of the repressor, blocking transcription
Promoter RNA polymerase
Operator
trpE
trpD
trpC
trpB
trpA
Terminator
Mal Operon Genes for breakdown of maltose are positively regulated o In the absence of maltose, there is no binding of the activator MalT, so no expression Operator
Promoter
malP
malQ
Terminator
RNA polymerase
o In the presence of maltose, the activator is
bound by maltose (the inducer), and this promotes binding of the activator, promoting transcription Operator
Promoter
malP
malQ
Terminator
Mal Operon Genes for breakdown of maltose are positively regulated o In the absence of maltose, there is no binding of the activator MalT, so no expression Operator
Promoter
malP
malQ
Terminator
RNA polymerase
o In the presence of maltose, the activator is
bound by maltose (the inducer), and this promotes binding of the activator, promoting transcription Operator
Promoter RNA polymerase
malP
malQ
Terminator
Practice Question Predict the phenotype of a malT mutant in which MalT can no longer bind the operator
Operator
Promoter
RNA polymerase
malP
malQ
Terminator
Practice Question Predict the phenotype of a lacI mutation in which LacI can no longer bind lactose
Promoter RNA polymerase
Operator
lacZ
lacY
lacA
Terminator
