Operons và quá trình điều hòa hoạt động của gen

Trp repressor gene RNA pol binding site operator Increased concentration of trp repressor increases chances of trp binding to allosteric site of inactive trp repressor... Active trp repr

Chapter 18 - Operons

Control of Gene Expression in

Prokaryotes

Sample Pathway for Control of Tryptophan (trp) Expression

Gene 1 Gene 2 Gene 3 Gene 4 Gene 5

necessary for tryptophan production

Basic Vocabulary

mRNA Transcription Unit:

- grouping of mRNA genes with related

Basic Vocabulary

Operon:

- an entire gene-regulating system in

PROKARYOTES, which includes the

operator, promotor, and all corresponding genes

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

Overview of Repressor Trp Operon

mRNA transcription unit

Enz 1 Enz 2 Enz 3 Enz 4 Enz 5

Gene 1Gene 3 Gene 4 Gene 5

Trp operon controls the production

of the amino acid tryptophan

How is the trp operon turned off

once enough trp is made?

Trp repressor gene

RNA pol binding site

operator

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

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

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

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

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

Inducible Lac operon

I Lac Inducer

crp: helps RNA pol

to bind

rp: RNA pol.binding site

o operator

promotor

Genes that promote ß- galactosidase production

ß-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.

TWO regulatory mechanisms used

to turn on lac operon

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

co-AlloLactose

2 cAMP attaches to CRP transcription factor when glucose is scarce

RNA pol.

Lac Operon Animation

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

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

How are genes controlled in

eukaryotes?

1) Regulation of Chromatin Structure

2) Pre and post Transcriptional Regulation3) Pre and Post Translational Regulation

Regulation of Chromatin Structure

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

Tails are thus able to be modified chemically

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

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

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

Post-Transcriptional Regulation

• Time of mRNA degradation can vary

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

Post-Translational Regulation

• During protein processing, folding

• Timing of protein degredation can vary

• Proteasomes degrade proteins that are tagged by ubiquitine molecules