16. Regulation of Expression

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

What would occur if the repressor of an inducible operon were mutated so it could not bind the operator?

a. Irreversible binding of the repressor to the promoter

b. Reduced transcription of the operon's genes

c. Buildup of a substrate for the pathway controlled by the operon

d. Continuous transcription of the operon's genes

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

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?