Question

A particular gene (call it xyz) is, like the lac operon, regulated both by Negative and by Positive control. However, unlike the lac operon, xyz's Negative control system is repressible. We'll call the repressor signal "signal 1." The Positive control system of xyz, like that of lac, is inducible. We'll call the inducer "signal 2." I want you to sketch the xyz gene in the presence and absence of each signal, as follows, and then determine whether the xyz gene would be transcribed under each of the four conditions. Is each regulatory protein produced in an active or inactive conformation by default? Use the following symbols: active regulatory protein = square labeled either A or R, inactive regulatory protein = circle labeled either A or R, environmental signal = triangle labeled 1 or 2. Condition . . .

Signal 1. Signal 2

A) Present Absent

B) Absent Present

C) Absent Absent

D) Present Present

Answer 1

Positive control: The regulator of the operon acts as an activator and is required in its active form for transcription.

Negative control: The regulator of the operon acts as a repressor that prevents transcription when it is active.

An inducible operon requires an inducer molecule to stimulate transcription either by inactivating a repressor protein in a negative inducible operon or by stimulating the activator protein in a positive inducible operon.

In a repressible operon, transcription is turned off either by the repressor becoming active in a negative repressible operon or by the activator becoming inactive in a positive repressible operon.

Negative repressible operons is when an inducer is need to bind to the operon to prevent transcription.