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.

In the lac operon, the repressor inhibits transcription when

a. The repressor is bound to the inducer.

b. The repressor is not bound to the inducer.

c. The repressor is bound to glucose.

d. The repressor is not bound to the operator.

Activators bind to regulatory sequences in ________ and to ________ polymerase.

A regulon is a set of genes controlled by

a. One type of regulator of transcription

b. Two or more different alternative sigma proteins

c. Many different types of promoters

d. Glucose

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.

Explain why it makes sense for the lexA regulatory gene of the SOS regulon to be expressed constitutively.

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 a mutant that lacks adenylyl cyclase, the enzyme that synthesizes cAMP, predict which of the following conditions of extracellular lactose and glucose would cause regulation of the lac operon to differ from that of wild-type cells.

a. No lactose, no glucose

b. No lactose, abundant glucose

c. Abundant lactose, no glucose

d. Abundant lactose, abundant glucose

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 Hawaiian bobtail squid (Euprymna scolopes) is able to glow from luminescent Vibrio fischeri bacteria held in its light organs. As it swims at night near the ocean surface, it adjusts the amount of light visible to predators below to match the light from the stars and moon. Predators have difficulty seeing the illuminated squid against the night sky.

The bacteria glow in response to a molecule that regulates expression of genes involved in light-producing chemical reactions. The regulator controls production of the genes' mRNA. Therefore, the light-producing genes are under

a. Transcriptional control.

b. Translational control.

c. Post-translational control.

d. Negative control.

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.

The diagram shown here is a model of the gene regulatory circuit for light production by V. fischeri cells. The lux operon contains genes for luminescence (luxCDABE) and a gene, luxI, that encodes an enzyme that catalyzes the production of an inducer. This inducer easily moves back and forth across the plasma membrane and acts as a signaling molecule. The lux operon is never completely turned off. The luxR gene codes for the activator LuxR. The inducer can bind to LuxR, and when it does, the LuxR–inducer complex can bind to a regulatory site to activate transcription of the lux operon and inhibit transcription of luxR.

Explain how this gene regulatory circuit accounts for bacteria emitting light only when they reach a high cell density.

LuxR is allosterically regulated by the inducer molecule secreted by V. fischeri.

What does it mean that LuxR is allosterically regulated?

What characteristic of the light-producing regulatory circuit is consistent with the idea that it may be a regulon?

What characteristic of this circuit stretches the definition for a regulon?

Quorum sensing (introduced in Ch. 11, Section 11.4) allows bacteria to detect the number of neighboring cells and to trigger a response only when this number reaches a critical level. Quorum sensing is used by V. fischeri in light production and by many pathogenic bacteria, including Vibrio cholerae, to turn on genes for toxin production only when a critical cell density is reached.

Why might quorum sensing be beneficial to pathogenic bacteria?