16. Regulation of Expression

The Lac Operon

Gene Regulation lac operon

Gene Regulation and the Lac Operon

Why are the genes involved in lactose metabolism considered to be an operon?
a. They occupy adjacent locations on the E. coli chromosome.
b. They have a similar function.
c. They are all required for normal cell function.
d. They are all controlled by the same promoter.

IPTG is a molecule with a structure much like lactose. IPTG can be transported into cells by galactoside permease and can bind to the lac repressor protein. However, unlike lactose, IPTG is not broken down by ββ-galactosidase. Predict what would occur to lac operon regulation if IPTG were added to E. coli growth medium containing no glucose or lactose.

In the Absence of Lactose

Lac Operon

The lac operon is a(n) _______________ operon that is typically ______________.

X-gal is a colorless, lactose-like molecule that can be split into two fragments by ββ-galactosidase. One of these product molecules creates a blue color. The photograph here shows E. coli colonies growing in a medium that contains X-gal. Find three colonies whose cells have functioning copies of ββ-galactosidase. Find three colonies whose cells might have mutations in the lacZ or the lacY genes. Suppose you analyze the protein-coding sequence of the lacZ and lacY genes of cells from the three mutant colonies and find that these sequences are wild type (normal). What other region of the lac operon might be altered to account for the mutant phenotype of these colonies?

Animation: The lac Operon in E. coli 

Mutations can alter the function of the lac operon (see Module 11.1). Predict how the following mutations would affect the function of the operon in the presence and absence of lactose:
a. Mutation of regulatory gene; repressor cannot bind to lactose.
b. Mutation of operator; repressor will not bind to operator.
c. Mutation of regulatory gene; repressor will not bind to operator.
d. Mutation of promoter; RNA polymerase will not attach to promoter.

The protein that binds to the operator of the lac operon to prevent transcription is encoded by which gene?

It serves no function in regulating the lac operon.

In the lac operon, which of the following functions does the lactose molecule serve:

Evaluate these statements about regulation of the lac operon. Select True or False for each statement.
T/F The lac operon is transcribed at the highest rate when extracellular glucose and lactose are abundant.
T/F The repressor protein is bound to DNA of the operator when lactose is present.
T/F A mutation in the operator is likely to prevent transcription of the lac operon under any condition.
T/F A mutation that alters the catabolite activator protein is predicted to alter the regulation of many different operons.

Predict what would happen to regulation of the lac operon if the lacI gene were moved 50,000 nucleotides upstream of its normal location.

In the Presence of Lactose

The repressor will bind the operator allowing transcription of the lac operon genes.

The repressor will not bind the operator preventing transcription of the lac operon genes.

The repressor will not bind the operator allowing transcription of the lac operon genes.

The repressor will bind the operator preventing transcription of the lac operon genes.

If E. coli bacteria are grown in the presence of lactose:

No. The repressor protein is already present and bound to the operator.

No. The operator is a nucleotide sequence.

No. A bacterial cell will not know what to do with human growth factor.

No. Prokaryotes do not use transcription factors.

The operon is still regulated as the lac operon.

You have inserted the gene for human growth factor into the E. coli lactose operon, replacing the structural genes with the gene for human growth factor. What substance must you add to your culture of bacteria to cause them to produce human growth factor for you?

Prokaryotic Gene Regulation: Lac Operon

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