Because a lysogen resists lysis by superinfecting phage, the prophage repressor is also called the:

Introduction:
Temperate phages provide their hosts with “immunity” to further infection by the same or closely related phages. The protein responsible for this phenotype has a conventional name that reflects its function.

Given Data / Assumptions:

• Lysogens resist lytic development by superinfecting related phages.
• This resistance is mediated by a prophage-encoded repressor.

Concept / Approach:
The repressor binds operator regions of lytic promoters in incoming phage DNA, preventing expression of lytic genes. Hence it confers “immunity” to superinfection, and is aptly termed an “immunity repressor.” Lambda phage cI is a paradigmatic example.

Step-by-Step Solution:
Identify the phenotype: superinfection immunity. Match phenotype to the responsible protein: the phage repressor. Recall conventional term: immunity repressor.

Verification / Alternative check:
Genetic studies of lambda and related phages consistently use “immunity” terminology to describe repressor-governed resistance to superinfection.

Why Other Options Are Wrong:

• Immunity operon / Operon repressor: Misleading phrasing; the standard term is immunity repressor (a protein), not an operon name.
• None / Superinfection lysozyme: Lysozyme is unrelated; “none” does not fit the well-established term.

Common Pitfalls:
Confusing repressor-mediated immunity with nonspecific resistance; immunity is sequence- and immunity-group–specific.

Final Answer:
The repressor is called the immunity repressor.