16. Regulation of Expression

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

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.

Evaluate these statements about regulation of the lac operon. Select True or False for each statement.

T/FThe lac operon is transcribed at the highest rate when extracellular glucose and lactose are abundant.

T/FThe repressor protein is bound to DNA of the operator when lactose is present.

T/FA mutation in the operator is likely to prevent transcription of the lac operon under any condition.

T/FA 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.

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.

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 the 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.

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?

The light-producing genes of V. fischeri are organized in an operon that is under positive control by an activator protein called LuxR.

Would you expect the genes of this operon to be transcribed when LuxR is bound or not bound to a DNA regulatory sequence? Explain.