lac operon

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Sample Pathway for Control of Tryptophan (trp) Expression Gene 1 Gene 2 Gene 3 Gene 4 Gene 5 transcription translation Enzyme 1 Enzyme 2 Enzyme 3 Enzyme 4 Enzyme 5 Each enzyme catalyzes the next series of reactions necessary for tryptophan production Promotor (RNA polymerase binding site) DNA mRNA (transcription unit) 2 Negative Feedback shut-off systems trp

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Page 1: Lac Operon

Sample Pathway for Control of Tryptophan (trp) Expression

Gene 1

Gene 2

Gene 3

Gene 4

Gene 5

tran

scrip

tion

tran

slat

ion

Enzyme 1

Enzyme 2

Enzyme 3

Enzyme 4

Enzyme 5

Each enzyme catalyzes the next series of reactions necessary for tryptophan production

Promotor (RNA polymerase binding site)

DNA mRNA (transcription unit)

2 Negative Feedbackshut-off systems

trp

Page 2: Lac Operon

Basic Vocabulary

mRNA Transcription Unit:

- grouping of mRNA genes with related function

- 2 advantages ~

1. organization

2. easy regulation – one “on/off” switch can be used to control an entire cluster of related genes

Page 3: Lac Operon

Basic Vocabulary

Operon:

- an entire gene-regulating system in PROKARYOTES, which includes the operator, promotor, and all corresponding genes

Page 4: Lac Operon

Basic Vocabulary

Operator:

- “on-off” switch to genes

- controls the access of RNA polymerase to genes

- found within the promotor region or between the promoter and gene-encoding regions

Let’s take a closer look at the

tryptophan (trp) operon…an example of a

“repressor” system

Page 5: Lac Operon

Overview of Repressor Trp Operon Components

r. Trp Repressor Gene (always “on”)

promotor

rp. RNA Pol. binding site

o. Operator

r. rp. o. Gene 1

5 Genes needed for tryptophan production

mRNA transcription unit

Enz 1 Enz 2 Enz 3 Enz 4 Enz 5

Gene 1Gene 3 Gene 4 Gene 5

trp

mRNA for trp repressor

Trp repressor protein translated in its “inactive” form

RNA polymerase

Trp operon controls the production of the amino acid tryptophan

Page 6: Lac Operon

How is the trp operon turned off once enough trp is made?

Trp repressor gene

RNA pol binding site

operator

Increased concentration of trp (co-repressor) increases chances of trp binding to allosteric site of inactive trp repressor

Inactive trp repressor protein

trp

trp

trp

trp trp

Page 7: Lac Operon

How is the trp operon turned off once enough trp is made?

Active trp repressor can now bind to operator

RNA polymerse, therefore is physically blocked from transcribing genes for trp

Page 8: Lac Operon

Why is this a repressor system?

The trp operon is a repressor operon, meaning gene expression of the operon is repressed by the presence of the co-repressor, tryptophan.

Page 9: Lac Operon

Watch this animation to fully understand and review this operon, then take the on-line quiz together

as a class

http://bcs.whfreeman.com/thelifewire/content/chp13/1302002.html

Page 10: Lac Operon

Overview of Inducible Lac Operon

The Lac operon controls the production of the ß-galactosidase, an enzyme that catalyzes the hydrolysis (break-down) of lactose into glucose and galactose.

This is an inducible operon, meaning gene expression ß-galactosidase is stimulated by the presence of an co-inducer, lactose.

Page 11: Lac Operon

Inducible Lac operon

I. crp. rp. o.

I. Lac Inducer

crp: helps RNA pol. to bind

rp: RNA pol.binding site

o. operator

promotorGenes that promote ß-galactosidase production

mRNA for inducer protein

inducer protein in “active” form

RNA pol. blocked from ß-galactosidase transcription without lactose co-inducer

Page 12: Lac Operon

TWO regulatory mechanisms used to turn on lac operon

1) Presence of lactose as a co-inducer

** reason?

ß-galactosidase is not needed unless lactose needs to be broken down

2) Low amounts of glucose

** reason?

Recall that lactose breaks down into glucose and galactose. Low glucose levels signals the cell for more lactose to be broken down.

Both conditions must be met for the lac operon to turn on.

Page 13: Lac Operon

TWO regulatory mechanisms used to turn on lac operon

1) Allolactose co-inducer attaches to allosteric site of inducer protein, inactivating it

AlloLactose

2. cAMP attaches to CRP transcription factor when glucose is scarce

CRP Transcription Factor

cAMP

RNA polymerase can bind only with the help of CRP transcription factor

RNA pol.

Page 14: Lac Operon

Lac Operon Animation

Watch this animation to fully understand and review the lac operon!

http://highered.mcgraw-hill.com/olc/dl/120080/bio27.swf

Page 15: Lac Operon

How are genes controlled in eukaryotes?

1) Regulation of Chromatin Structure

2) Pre and post Transcriptional Regulation

3) Pre and Post Translational Regulation

Page 16: Lac Operon

Regulation of Chromatin Structure

N-terminus (amino group) of histone proteins face outwards from nucleosome

Tails are thus able to be modified chemically

Page 17: Lac Operon

Regulation of Chromatin Structure

• Histone Acetylation – neutralizes (+) charges on tails, which prevents binding to adjacent nucleosome loose chromatin structure results, allowing for increased transcription

• Methylation Promotes condensation• Phosphorylation can prevent condensation, if

phosphorylation is adjacent to methyl group

Page 18: Lac Operon

Pre-Transcriptional Regulation• Similar to methods used in bacterial

operons, using proteins that inhibit or promote binding of RNA pol.– Distal and Proximal Control Elements– Proteins involved include:

• Transcription factors• Activators• Mediator Proteins

Page 19: Lac Operon
Page 20: Lac Operon

Post-Transcriptional Regulation

RNA Processing

– differential/alternative splicing can produce different 20 mRNA transcript

Differential splicing redefines which RNA segments are considered introns and which are exons

Page 21: Lac Operon

Post-Transcriptional Regulation

• Time of mRNA degradation can vary

Page 22: Lac Operon

Pre-Translational Regulation

Initiation of Translation

- can be blocked by regulatory proteins that prevent ribosome binding

- shortened polyA tails in mRNA prevents translation (polyA tails can be added during appropriate time)

- global regulatory control of all mRNAs in cell

Page 23: Lac Operon

Post-Translational Regulation

• During protein processing, folding

• Timing of protein degredation can vary

• Proteasomes degrade proteins that are tagged by ubiquitine molecules