Question

Keeping in mind the life cycle of bacteriophages discussed earlier in the text (see Chapter 6 ), consider the following problem: During the reproductive cycle of a temperate bacteriophage, the viral DNA inserts into the bacterial chromosome where the resultant prophage behaves much like a Trojan horse. It can remain quiescent, or it can become lytic and initiate a burst of progeny viruses. Several operons maintain the prophage state by interacting with a repressor that keeps the lytic cycle in check. Insults (ultraviolet light, for example) to the bacterial cell lead to a partial breakdown of the repressor, which in turn causes the production of enzymes involved in the lytic cycle. As stated in this simple form, would you consider this system of regulation to be operating under positive or negative control?

Short answer

Answer: The regulation system operates under negative control, as the repressor inhibits the lytic cycle when it is present.

Step-by-step solution

Understand positive and negative control

Positive and negative control are two regulation mechanisms for controlling gene expression. In positive control, the regulatory protein (activator) binds to the DNA sequence and increases the expression, while in negative control, the regulatory protein (repressor) binds to the DNA sequence and decreases the expression. Since the question states that the repressor keeps the lytic cycle in check, you will need to determine if this represents positive or negative control.

Analyze the regulation mechanism described in the exercise

The exercise states that during the reproductive cycle of a temperate bacteriophage, the viral DNA inserts into the bacterial chromosome, and several operons maintain the prophage state by interacting with a repressor. This repressor keeps the lytic cycle in check. Insults to the bacterial cell, such as ultraviolet light, lead to a partial breakdown of the repressor and cause the production of enzymes that are involved in the lytic cycle.

Determine if the regulation mechanism represents positive or negative control

In this situation, the repressor acts to keep the lytic cycle in check by preventing the production of enzymes involved in the lytic cycle. When the repressor is broken down, the operons are no longer repressed, and the lytic cycle is activated. This regulatory mechanism functions by inhibiting the lytic cycle when the repressor is present, which means that it operates under negative control. Therefore, the regulation system described in the exercise can be considered as operating under negative control.