16. Regulation of Expression

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?

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?