Control of gene expression in prokaryotes: Operons

Which structural genes are included in the lac operon?

lacZ, lacY and lacA

How are the structural genes of the lac operon transcribed?

Transcribed in unison

Which gene regulates the lac operon?

lacI

What does the lacI gene produce?

The lac repressor protein

What do both the regulatory gene and the lac operon contain?

Promoters (Pl and Plac) and terminators

What happens at the promoters Pl and Plac?

RNA polymerase binds

What happens at terminators?

Transcription halts

What does Plac overlap with?

The operator site (O)

What binds to the operator site (O)?

The active form of the repressor protein

What does lacZ encode?

β-Galactosidase

What is the function of β-Galactosidase?

Breaks the glycosidic bond between glucose and galactose

What does lacY encode?

Lactose permease enzyme

What is the function of lactose permease?

Spans the membrane and brings lactose into the cell

What does lacA encode?

Thiogalactoside transacetylase

What is the function of lacA?

Function unknown (possible detoxification of some lactose metabolites)

How are lac genes transcribed?

Transcribed on a single polycistronic mRNA

What is a polycistronic mRNA?

1 RNA translated into multiple protein

How is transcription of the lac operon down-regulated in the absence of lactose?

Through the binding of the lac repressor to the operator

Is the lacI gene always expressed?

Yes, the lacI gene is always expressed

What happens to the repressor in the absence of lactose?

The repressor remains bound to the operator

What does the bound repressor prevent?

Access of the RNA polymerase to the promoter

What happens to transcription when the repressor is bound?

Transcription is blocked

What is the state of the operon in the absence of lactose?

The operon is repressed

What happens to the repressor in the presence of lactose?

The repressor is inactivated and does not bind to the operator

What happens when the repressor does not bind to the operator?

RNA polymerase binds to the promoter and transcribes the downstream structural genes into a single cistronic mRNA

What type of protein is the lac repressor?

An allosteric protein

What is the lac repressor capable of?

Reversible conversion between two alternative forms

What molecules are involved in positive control of the lac operon?

CAP and cAMP

What happens when cAMP binds to CAP?

Complex binds to CAP site near lac promoter

What does the CAP-cAMP complex cause in DNA?

Resulting bend in DNA

What is the effect of the DNA bend?

Enhances RNA polymerase binding which increases transcription

Who studied a constitutive mutant to determine the function of the lac repressor?

Jacob, Monod, and Pardee

What did they find?

Rare mutants with abnormal lactose use

How were the genes of the lac operon expressed in these mutants?

Constitutively

Where were some mutations found?

In the lacI region

What were these strains called?

lacI– (normal strain are lacI+)

What was proposed about lacI–?

Contains a mutation that prevents DNA binding

Which genes are under the control of the lac promoter?

lacZ, lacY and lacA

The binding of ______ to lac repressor causes lac repressor to ______ to the operator site, thereby ______ transcription.

Allolactose, not bind, increasing

What would be the expected effect of a mutation in the Lac operator site that prevented binding of the Lac repressor?

The Lac operon would always be turned on