Gene Expression in Prokaryotes

Topic 5

lac operon

An operon whose genes encode for proteins that allow the bacteria to use lactose and an energy source

What makes the lac operon turn on?

The Lac Operon only turns on when lactose is available and glucose is not available. This is because glucose is more energy-efficient to break down.

Structure of lac operon

Promoter, operator, lacZ, lacY, lacA, and [most of the time] a repressor

How does the repressor act when NO lactose is present?

The repressor will be bound to the operator, thus blocking and preventing RNA polymerase from transcribing and encoding for proteins that break down lactose.

How does the repressor act when lactose IS present?

Lactose will bind to the repressor, making it inactive and let go of the operator, allowing RNA polymerase to transcribe for proteins that break down lactose.

Negative Transcription Regulation

The genes in the operon are expressed unless they are switched off by a repressor protein.

Positive Transcription Regulation

The genes are expressed only when an active regulator protein, e.g. an activator, is present.

Most energy efficient level of regulation?

Transcriptional regulation

Fastest level of of regulation?

Post-translational

Genes that are constitutively transcribed…

are considered housekeeping genes (essential for basic cellular functions and are expressed in all cells under normal conditions) and have NO operator

Genes that are regulated…

may be induced or repressed, are specialized genes (people are coming over!), and have an operator

Trp operon

A group of genes that encode for enzymes involved in the synthesis of tryptophan

When is the trp operon turned off?

When tryptophan is present, it attaches to the repressor molecules and changes its shape so it becomes active, successfully blocking RNA polymerase from transcribing

When is the trp operon turned on?

When tryptophan is not present, there is nothing to activate the repressor molecule, so it does not attach to the operator and RNA polymerase is allowed to transcribe.

How is the trp operon an example of negative regulation?

RNA polymerase will only be able to transcribe when the repressor is inactive

How is the lac operon an example of negative regulation?

RNA polymerase will only be able to transcribe when the repressor is inactive.

Properties or negative control/regulation

ALWAYS involve operator and repressor

CAP sequence

The CAP sequence is an activator sequence found upstream of the promoter in a lac operon

cAMP

When low levels of glucose are detected, cAMP is produced, which binds to CAP

CAP

When cAMP binds to CAP, it then binds to the DNA. CAP helps RNA polymerase bind to the lac operon promoter, resulting in high levels of transcription

What happens to cAMP when glucose levels are high?

No CAMP is made, and CAP cannot bind to DNA without cAMP, so transcription only occurs at low levels

How is the lac operon an example of positive control?

The CAP-cAMP complex enhances transcription with the help of an activator

Properties of positive control/regulation

Regulatory protein is upstream of the promoter, and transcription is only in full force when activator binds upstream from promoter

Allosteric effector

A molecule that binds to a protein at a site other than the active site (so it binds to the allosteric site) and alters the proteins activity, either activating or inhibiting it

Allosteric Inhibitor

Binds to the allosteric site of a protein and changes the shape of the active site through multiple pathways, so now the substrate can no longer bind to the active site

Allosteric Activator

Binds to the allosteric site of a protein and changes the shape of the active site through multiple pathways so the substrate can bind to the active site

lacZ

Cleaves lactose into its two components: glucose and galactose

lacY

Brings lactose into the cell

Is tryptophan an activator or repressor in the trp operon?

Is it an activator/corepressor because when the repressor becomes active and block RNA polymerase when it is bound

Is lactose in the lac operon an activator or inhibitor?

It is an inhibitor/inducer because when the repressor becomes inactive when it binds, allowing RNA polymerase to transcribe

Catabolic…

inducible

Anabolic…

repressible