Operons and Gene Expression�in Prokaryotes – V4

Prokaryotic Gene Expression

Prokaryotes like bacteria keep their DNA in a nucleoid region. Bacteria have one circular chromosome and often have extra small circular DNA called plasmids. Prokaryotes also lack a nucleus so there are fewer points for controlling gene expression. Bacterial control of gene expression can happen during transcription, during translation or after the protein is synthesized.

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Operons

An operon is a cluster of genes with a related function. This way the expression of different but related genes can be easily regulated. A promoter region is a sequence of nucleotides upstream from the gene to be transcribed. RNA polymerase recognizes and binds to promoter regions to start transcription. The operator is found between the promoter and the genes to be transcribed. The operator is the on / off switch.

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Types of operon control

There are two types of operon gene expression in bacteria that have been extensively studied:

1. Negative Control (usually off)

- the Lac Operon where the genes are only expressed when there is lactose sugar and little glucose in the environment, to catabolize (breakdown) lactose

2. Positive Control (usually on)

- the tryp operon where the genes that synthesize the amino acid tryptophan are always expressed unless tryptophan amino acids are already available in the environment

Lac Operon Genes

Lactose is a disaccharide made of glucose and galactose. The lac operon in bacteria includes 3 genes which code for 3 enzymes:

LacZ gene for β-galactosidase - lactase enzyme that breaks the covalent bond between glucose and galactose.

LacY gene for Permease - an enzyme to increase the bacterial cell’s permeability to lactose

LacAc gene for transacetylase – transfers an acetyl group to beta-galactose sugars

When to Express lac Genes?

The bacteria prefer glucose to other sugars and only want to catabolize lactose when there is no glucose. There are four possible combinations of glucose and lactose in the cell.

1. High glucose + low lactose = lac operon OFF because glucose present
2. High glucose + high lactose = lac operon ON, but slow transcription because glucose preferred
3. Low glucose + low lactose = lac operon off,because no lactose present to use
4. Low glucose + high lactose = lac operon ON because no glucose present and lactose available

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1. High glucose + low lactose

• Repressor bound to operator (like brakes)
• While RNA polymerase can bind to promotor region the repressor stops transcription
• Energy efficient as best for bacteria to use glucose not lactose

2. High glucose + high lactose

• Lactose binds to and removes the repressor from the operator region
• A low level of transcription occurs

3. Low glucose + low lactose

• The lack of lactose means the repressor prevents transcription
• Low glucose means low [ATP] 🡪 low [ADP] 🡪 high [cAMP]
• The lack of glucose results in high levels of c-AMP which binds to an activator molecule (CAP) that attaches to an activator region
• The c-AMP- CAP binds to a regulatory region to turn ON lac transcription (like gas)

4. Low glucose + high lactose

• Lactose has removed the repressor
• The c-AMP-CAP binds to the regulatory region
• RNA polymerase transcribes A LOT!
• Finally, lots of transcription! The brakes are off and the gas is on.

Regulating the expression of the lac genes�

The goal is to only express a protein when it is needed. The cell always wants to be efficient and get the most work done for the least amount of energy used.

Most genes are always turned off or repressed. A few genes are almost always turned on as these proteins are almost always needed.

In real life these methods interact.

Bonus: name this molecule!

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