the lac operon - mcccblinderl/documents/ch17_lacoperonpost.pdf1 the lac operon 1961, jacob and monod...

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The Lac Operon 1961, Jacob and Monod

E. coli and other bacteria

Bacterial Genes

Many genes constitutively expressed

“housekeeping” genes

Other genes are more regulated

Can be turned on, or off depending on cell needs

Operon

group of coordinately regulated genes

1 promoter for a number of genes

Polycistronic mRNA

Inducer molecule – turns operon on

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E. Coli Lac Operon

E. coli cells normally grown in glucose

BUT, if lactose is used instead:

convert lactose to glucose and galactose

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The Lac Operon allows for

coordinate gene expression

Note: 1 mRNA, promoter, 3 genes

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THE OPERON HAS

A. 3 STUCTURAL GENES = Z, Y, A

Lac Z gene encodes betagalactosidase

b-gal

lactose ------------- glucose + galactose

substrate products

No lactose present ~ 3 molecules of bgal

Add lactose 3,000 molecules of bgal

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So…………………..

b- gal is an inducible enzyme

What is the role of lactose?

inducer

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Lac Operon

B. promoter = allows transcription of ZYA

C. operator = must be unbound for P to be

“open”

What molecule are ALL of the components above?

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Lac Y gene encodes permease that transports lactose into cell

Lac A encodes a transacetylase

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D. Lac I gene

Encodes a repressor protein

Repressor binds to operator

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Is this operon ON or OFF?

Is lactose PRESENT or ABSENT?

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Which components act in cis?

In trans?

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Regulation of the Lac Operon

Its normally repressed! = OFF

Because lactose is absent

Therefore, it is an inducible operon

When lactose is present

INDUCER (LACTOSE SUGAR)

1. Lactose enters cell

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2. Binds repressor protein causing

conformational change

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3. repressor cannot bind operator

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4. RNA polymerase transcribes genes

5. Cell metabolizes lactose

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Lactose (the inducer) enters the cell

Binds repressor protein causing a conformational change

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repressor binds operator

polymerase cannot bind promoter

no transcription of ZYA genes

No lactose:

Lac operon animation

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Operon mutants

Mutant Mutant Phenotype

lac I -

constitutive expression because…

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This operon is not inducible

Mutant Mutant Phenotype

Oc constitutive expression because …

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Mutant Mutant Phenotype

P- operon always off because……

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Mutant phenotype

Z- operon is?

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Operon on, or off in the absence of

lactose?

Lac Oc I+P+OcZ+Y+A+

In presence of lactose?

Is it Inducible?

Operon on, or off in the absence

of lactose?

Lac I- I-P+O+Z+Y+A+

In presence of lactose?

Is it Inducible?

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Partial diploid cells contain a plasmid

F’ I+ Inducible?

I-P+O+Z+Y+A+

F’ I- P- Z+Y-A-

I+P+O+Z- Y+A+

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Inducible?

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Repressor and polymerase = proteins

diffusible

Proteins can bind DNA

act in TRANS

promoter, operator, and ZYA and I = genes

cannot move

act in CIS

Tips for plasmid analysis