BISC 1112 Chapter Notes - Chapter 18: Hydrolysis, Nonsteroidal, Genomic Imprinting
Quiz 3 Chapter 18.1-18.2, 18.5: Gene Regulation BISC 1115
1
18.1
• Metabolic control occurs on 2 levels
o 1) Cells can adjust the activity of enzymes already present
▪ Feedback Inhibition- Activity of the first enzyme in the pathway in inhibited by the
pathways end product. Common in anabolic pathways
• EX- If there is excess tryptophan, it shuts down the synthesis of more
tryptophan by inhibiting enzyme activity
o 2) Cells can adjust the production level of a certain enzyme and regulate the expression of the
genes encoding the enzymes
▪ EX- If the environment provides all the tryptophan the cell needs, the cell stops
making the enzymes that catalyze the synthesis of tryptophan
▪ Control of enzyme production occurs at the level of transcription- Synthesis of
mRNA
▪ Operon Model- Basic mechanism of controlling gene expression in bacteria
Operon Model- A single promoter serves all 5 genes, which together constitute a transcription unit
• Promoter- a site where RNA polymerase can bind to DNA and begin transcription
• Transcription gives rise to one long mRNA molecule that codes for the 5 polypeptides making up
the enzymes in the tryptophan pathway
o The cell can translate this one mRNA into 5 separate polypeptides
• coordinately controled- a single “on-off-switch” can control the whole cluster of functionally related
genes
• Operator- controls the access of RNA polymerase to the genes. Positioned within the promotor
(sometimes between the promoter and the enzyme-coding genes). Acts as the operon’s switch for
controlling transcription
o Operon- the operator, the promoter, and the genes they control
o When E. coli must make tryptophan for itself because the nutrient medium lacks this amino
acid, all the enzymes for the metabolic pathway are synthesized at one time
▪ By itself the trp operon is turned on, allowing RNA polymerase to bind to the
promoter and transcribe the genes of the operon
• Repressor- switches the operon on or off. Binds to the operator and blocks attachment of RNA
polymerase to the promoter preventing transcription of the genes. Repressor proteins are specific
for the operator of a particular operon.
o The binding of repressors to operator is reversible
▪ Operator alternates between 2 states
• 1) One with the repressor bound
• 2) One without the repressor bound
• Relative duration of the repressor-bound state increases when there are more
active repressor molecules present
o trp repressor- protein product of a regulatory gene called trpR; has its own promoter.
Allosteric protein with 2 alternative shapes (active & inactive)
▪ Allosteric Protein- a protein with multiple ligand-binding sites such that ligand
binding at one site affects ligands binding at another site (cooperative binding)
▪ Enzyme can convert itself between active and inactive conformations
▪ Only when a tryptophan molecule binds to the trp repressor at an allosteric site does
the repressor protein change its shape to the active form which can attach to the
operator, turning the operator off
▪ Tryptophan functions in this system as a corepressor
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Quiz 3 Chapter 18.1-18.2, 18.5: Gene Regulation BISC 1115
2
• Corepressor- a small molecule that cooperates with a repressor protein to switch an operon odd
o As tryptophan accumulates, more tryptophan molecules associate with trp repressor
molecules. These can then bind to the trp operator and shut down production of the
tryptophan pathway enzymes. If tryptophan drops, transcription of the operon’s genes
resumes
Repressible and Inducible Operons (2 types of negative gene regulation)
• Repressible operon- its transcription is usually on but can be inhibited (repressed) when a specific
small molecule (tryptophan) binds allosterically to a regulatory protein
o Enzymes for tryptophan synthesis said to be repressible enzymes because they function in
anabolic pathways -synthesizing essential end products from raw materials
• Inducible Operon- is usually off but can be stimulated when a specific small molecule interacts with
a regulatory protein.
o Lactose pathway is considered inducible enzymes because they function in catabolic
pathways, which break down a nutrient to simpler molecule synthesis from being induced by
a chemical signal (allolactose)
• Lactose Pathway metabolism begins with hydrolysis of the disaccharide into its component
monosaccharides glucose and galactose. Reaction is catalyzed by the enzyme -galactosidase
o The gene -galactosidase is part of the lac operon
o Entire transcription unit is under the command of one main operator and promoter
o lacl is a regulatory gene located outside the operon. Codes for an allosteric repressor protein
that can switch off the lac operon by binding to the operator
▪ Different than the regulation of trp repressor protein
▪ trp repressor protein is inactive by itself and requires tryptophan as a corepressor in
order to bind to the operator
▪ lac repressor is active by itself and binds to the operator, switching the lac operon off
▪ A specific molecule called an inducer inactivates the repressor. In lac operon, the
inducer is allolactose
▪ Allolactose is an isomer of lactose formed in small amounts from lactose that enter
the cell
• If lactose is absent, the lac repressor is in its active shape, and the genes of
the lac operon are silenced
• If lactose is present, allolactose binds to the lac repressor and alter its shape,
inhibiting the repressor’s from attaching to the operator
• Without the repressor bound, the lac operon is transcribed into mRNA for
the lactose-utilizing enzyme
o Regulation of lac and trp involves the negative control of genes because the operons are
switched off by the active form of the repressor protein; said to be positive only when a
regulatory protein interacts directly with the genome to switch transcription on
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Quiz 3 Chapter 18.1-18.2, 18.5: Gene Regulation BISC 1115
3
• Positive Gene Regulation- When glucose and lactose are both present in its environment, E. coli
preferentially uses glucose
o E. coli- Cyclic AMP (cAMP)- Accumulates when glucose is scares
o Lactose is present and Glucose is in short supply (**Diagram Below**)- E.coli use lactose
as an energy source
o Activator- a protein that binds to DNA and stimulates transcription of a gene
▪ Catabolite activator protein (CAP)- binds to regulatory protein and assumes its
active shape, attaching to its specific site at the upstream end of the lac promoter
• This increases the affinity of RNA polymerase for the promoter which is
actually low even when no repressor is bound to the operator
▪ Attachment of CAP to the promoter directly stimulates gene expression by
facilitating the binding of RNA polymerase to the promoter, increasing the rate of
transcription. (This mechanism is positive regulation)
o When glucose in the cell increases- cAMP concentration falls; Without cAMP, CAP
detaches from the operon
▪ Because CAP is inactive, RNA polymerase binds less efficiently to the promoter, and
transcription of the lac operon proceeds only at a low level
▪ In this mechanism, the lac operon is under dual control
• Negative control by the lac repressor
• Positive control by CAP
o The state of the lac repressor determines whether or not transcription of the lac operon’s
genes occurs at all (with or without bound allolactose)
o The state of the CAP controls the rate of transcription if the operon is repressor-free (with or
without cAMP)
o Positive control of the lac operon by Catabolite activator protein (CAP)
▪ RNA polymerase has high affinity for the lac promoter only when CAP is bound to a
DNA site at the upstream end of the promoter
▪ CAP, in turn, attaches to its DNA site only when associated with cyclic AMP
(cAMP)
▪ cAMP’s concentration in the cell rises when the glucose concentration falls
▪ Thus, when glucose is present (even if lactose is also present), the cell preferentially
catabolizes glucose
• Differential Gene Expression- The expression of different genes by cells with the same genome
• Gene Expression in a Eukaryotic cell- have more control mechanisms operating before transcription
and after translation
o The nuclear envelope separating transcription from translation in eukaryotic cells offers
things for post transcriptional control in the form of RNA process that is ABSENT in
prokaryotes
o Each stage is a potential control point at which gene expression can be turned on or off,
accelerated, or slowed down
o In all organisms, gene expression is controlled at transcription. Regulation at this stage
often occurs in response to signals coming from outside the cell (Hormones or other signaling
molecules)
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Document Summary
Bisc 1115: metabolic control occurs on 2 levels. 18. 1: 1) cells can adjust the activity of enzymes already present, feedback inhibition- activity of the first enzyme in the pathway in inhibited by the pathways end product. Positioned within the promotor (sometimes between the promoter and the enzyme-coding genes). Binds to the operator and blocks attachment of rna polymerase to the promoter preventing transcription of the genes. Bisc 1115: corepressor- a small molecule that cooperates with a repressor protein to switch an operon odd, as tryptophan accumulates, more tryptophan molecules associate with trp repressor molecules. These can then bind to the trp operator and shut down production of the tryptophan pathway enzymes. If tryptophan drops, transcription of the operon"s genes resumes. Reaction is catalyzed by the enzyme -galactosidase: the gene -galactosidase is part of the lac operon, entire transcription unit is under the command of one main operator and promoter lacl is a regulatory gene located outside the operon.
