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Control of Prokaryotic (Bacterial) Gene Expression

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Figure 18.1 How can this fish’s eyes see equally well in both air and

water?Aka. Quatro ojas

Orchestrating proper gene expression by cells

is crucial to the function of life in (prokaryotes and eukaryotes)

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Regulation of Gene Expression:n Prokaryotes and eukaryotes precisely regulate

gene expression in response to environmental conditions

n In multicellular eukaryotes, gene expression regulates development and is responsible for differences in cell types, e.g., embryonic development

n RNA molecules play many roles in regulating gene expression in eukaryotes

Oh, so that explains it

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Bacterial metabolism:n Bacteria need to respond quickly to changes in

their environment (relatively unstable)u if they have enough of a product,

need to stop productionn why? waste of energy to produce moren how? stop production of enzymes for synthesis

u if they find new food/energy source, need to utilize it quicklyn why? metabolism, growth, reproductionn how? start production of enzymes for digestion

STOP

GO

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Remember Regulating Metabolism?n Feedback inhibition

u product acts as an allosteric inhibitor of 1st enzyme in tryptophan pathway

u but this is wasteful production of enzymes

= inhibition-

-Oh, I

remember thisfrom our

Metabolism Unit!

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A schematic representation of a Generalized Feedback Loop

(two classifications)

Negative Feedback Loop = the effect causes a reversal of the

initial condition. “stimulatory-inhibitory

response”

Positive Feedback Loop = the effect intensifies initial

condition.“stimulatory-stimulatory

response”

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n Gene regulation u instead of blocking

enzyme function, block transcription of genes for all enzymes in tryptophan pathwayn saves energy by

not wasting it on unnecessary protein synthesis

= inhibition-

Now, that’s a good idea from a lowly bacterium!

--

A different way to Regulate Metabolism:

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Gene regulation in bacteria:n Cells vary amount of specific enzymes by

regulating gene transcriptionu turn genes on or turn genes off

n turn genes OFF exampleif bacterium has enough tryptophan then it doesn’t need to make enzymes used to buildtryptophan

n turn genes ON exampleif bacterium encounters new sugar (energy source), like lactose, then it needs to start making enzymes used to digest lactose

STOP

GO

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n Natural selection has favored bacteria that produce only the products needed by that cell

n A cell can regulate the production of enzymes by feedback inhibition or by gene regulation

n One mechanism for control of gene expression in bacteria is the operon model

Bacteria often respond to environmental change by regulating transcription:

Discuss how the processes shown in Figure 18.2 are examples of feedback mechanisms regulating biological systems in bacterial cells.

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Regulation of a metabolic pathway in E. coli for tryptophan synthesis (anabolic). An abundance of Trp can both:

Precursor

(typical response in synthesis pathways)

Tryptophan(b) Regulation of enzyme production.

(at the level of transcription)repress expression of the gene encoding all subunits of enzymes in the pathway;

long-term response

(a) Regulation of enzyme activity.(allosteric regulation), inhibit the

activity of the 1st enzymein the pathway; rapid response

Regulationof gene

expression

trpE

trpD

trpC

trpB

trpA

Enzyme 1

Enzyme 2

Enzyme 3

Figure 18.2 Feedbackinhibition

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Bacteria group genes together:n Operon

u genes grouped together with related functions n example: all enzymes in a metabolic pathway

u promoter = RNA polymerase binding siten single promoter controls transcription of all genes in

operonn transcribed as one unit & a single mRNA is made

u operator = DNA binding site of repressor protein

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n Repressor proteinu binds to DNA at operator site u blocking RNA polymeraseu blocks transcription

So how can these genes be turned off?

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operatorpromoter

Operon model:

DNA

TATA

RNApolymerase

repressor

repressor = repressor protein

Operon: operator, promoter & genes they controlserve as a model for gene regulation

gene1 gene2 gene3 gene4RNApolymerase

Repressor protein turns off gene by blocking RNA polymerase binding site.

1 2 3 4mRNA

enzyme1 enzyme2 enzyme3 enzyme4

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mRNA

enzyme1 enzyme2 enzyme3 enzyme4operatorpromoter

Repressible operon: tryptophan:

DNATATA

RNApolymerase

tryptophan

repressor repressor protein

repressor tryptophan – repressor proteincomplex

Synthesis pathway modelWhen excess tryptophan is present, it binds to tryp repressor protein & triggers repressor to bind to DNAu blocks (represses) transcription

gene1 gene2 gene3 gene4

conformational change in repressor protein!

1 2 3 4

repressortrpRNA

polymerase

trp

trp

trp trp

trp trp

trptrp

trptrp

trp

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Tryptophan operon:What happens when tryptophan is present?

Tryptophan is allosteric regulator of repressor protein

Don’t need to make tryptophan-building enzymes

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mRNA

enzyme1 enzyme2 enzyme3 enzyme4operatorpromoter

Inducible operon: lactose

DNA

TATARNA

polymerase

repressor repressor protein

repressor lactose – repressor proteincomplex

lactose

lac repressor gene1 gene2 gene3 gene4

Digestive pathway modelWhen lactose is present, binds to lac repressor protein & triggers repressor to release DNA

u induces transcription

RNApolymerase

1 2 3 4

lac lac

laclac

laclac

lac

conformational change in repressor protein!

lac

lac

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Lactose operon:What happens when lactose is present?Need to make lactose-digesting enzymes

Lactose is allosteric regulator of repressor protein

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Jacob & Monod: lac Operonn Francois Jacob & Jacques Monod

u first to describe operon systemu coined the phrase “operon”

1961 | 1965

Francois JacobJacques Monod

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Operon summary:n Repressible operon

u usually functions in anabolic pathwaysn synthesizing end products

u when end product is present in excess,cell allocates resources to other uses

n Inducible operonu usually functions in catabolic pathways,

n digesting nutrients to simpler molecules

u produce enzymes only when nutrient is availablen cell avoids making proteins that have nothing to do, cell

allocates resources to other uses

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Don’t be repressed!How can I induce you

to ask Questions?