Control Mechanisms (Prokaryote)

SBI4U

Controlling Expression

When a gene is being used by a cell, it gets transcribed, and then the mRNA is translated to produce the polypeptide. We say that the gene has been expressed.

How does a cell control, or regulate, which genes are expressed, and when they are expressed?

Gene regulation involves turning certain genes on or off depends on whether their products are

required

Regulators are the proteins that switch genes on or off activators, or repressors

Recall…Recall…

Protein synthesis occurs in two distinct stages:

1. Transcription, in the _________◦Product: ____

2. Translation, in the ______________◦Product: __________

Four Levels of Control

Transcriptional – which genes are transcribed

Post-transcriptional – mRNA modifications

Translational – ribosome activity vs. mRNA degradation

Post-translational – regulation of protein activity

Transcriptional regulation

Two classic examples:

lac operon – negative regulation trp operon – positive regulation

Operons

Cluster of genes, which generally code for the enzymes of a biochemical pathway,

all genes are under the control of one set of regulatory sequences promoter and operator sequences regulation is dependent on the presence or absence

of effector molecules

The genes are transcribed as a single piece of RNA, and are translated simultaneously. In this way, the entire biochemical pathway becomes active at the same time.

only in prokaryotic

cells

Operator sequence

Sequence that the regulator protein binds to – the operon’s “ON-OFF” switch.

Depending on the regulator, binding to the operator causes either: activation of transcription repression of transcription

Effector

Acts on the regulator protein

Can be an inducer stimulates transcription

Can be a co-repressor inhibits transcription

Lac operon

lactose = glucose + galactose

E. coli: ß-galactosidase catalyses the cleavage of the bond in lactose

Not economical to produce ß-galactosidase when lactose is not present

Structure of lac operon

Cluster of genes: lacZ, lacy, lacAEach codes a different part of the enzymeALL are under the control of one promoter

lac Operon

lac Operon

WATCH THIS!!

http://www.sumanasinc.com/webcontent/animations/content/lacoperon.html

Summary: Regulation of lac operon

lactose is absent lactose is present•LacI binds to lac promoter. •No transcription.

Lactose binds to LacI protein.

LacI changes conformation → unable to bind lac operator.

RNA polymerase can access lac promoter; transcription occurs.

What is the effector molecule?

_______________

It acts as an inducer – it induces transcription by deactivating the repressor protein.

The trp operon

The trp operon

WATCH THIS!!

WATCH THIS TOO!!

What is the effector molecule?

_______________

It acts as a co-repressor. It binds to a repressor to activate it in order to repress transcription.

Summary – Regulation of trp operon

trp absent trp present•trp operon is transcribed (default)

•trp itself will bind to the repressor protein.

•conformational shape occurs•allows repressor to bind to the trp operator → shuts off transcription

SUMMARY

Recap: Vocabulary

Basic terms:

Gene regulation: Describes any process that alters the rate of gene expression.

Operon: A cluster of closely-related genes. They are all controlled by one set of regulatory sequences.

Types of molecules involved:

Regulators: Molecules that carry out translational gene regulation. Classified as either activators, or

repressors. Example: LacI repressor; trp repressor

Effectors: Substances to which regulator proteins respond. Inducers or Co-repressors Example: Lactose, Tryptophan

Regulatory DNA sequences:

Promoter: DNA sequence to which RNA polymerase binds to begin transcription.

Operator: DNA sequence to which a transcription factor binds in order to alter transcription. Close to the promoter.

Credits

http://commons.wikimedia.org/wiki/File:Lac-operon.jpeg

http://commons.wikimedia.org/wiki/File:Trpoperon.svg