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?

Key Concepts

Bacteriophage Life Cycle

The life cycle of bacteriophages includes two main phases: the lytic cycle, where the virus replicates and causes cell lysis, and the lysogenic cycle, where the viral DNA integrates into the host's genome as a prophage. Understanding this cycle is crucial for analyzing how bacteriophages can switch between dormant and active states, influencing their interaction with bacterial hosts.

Prophage and Repressor Proteins

A prophage is the dormant form of a bacteriophage that integrates into the bacterial chromosome. Repressor proteins play a key role in maintaining this dormant state by inhibiting the expression of genes necessary for the lytic cycle. The balance between the repressor and the environmental factors determines whether the prophage remains quiescent or activates the lytic cycle.

Positive vs. Negative Control in Gene Regulation

Gene regulation can occur through positive or negative control mechanisms. Negative control involves repressors that inhibit gene expression, while positive control involves activators that enhance gene expression. In the context of the prophage system, the repressor's role in preventing the lytic cycle suggests that this regulation operates under negative control, as it suppresses the genes required for lytic activity.