In bacteriophages, lysozyme (endolysin) that lyses the bacterial cell to release mature virions is encoded by which phase of viral gene expression?

Introduction:
Bacteriophage replication is temporally regulated. Distinct waves of gene expression (immediate-early, early, delayed-early, and late) accomplish host takeover, genome replication, assembly, and release. The timing of endolysin expression explains when cell lysis occurs to liberate progeny virions.

Given Data / Assumptions:

• We are focusing on phage lysozyme (endolysin), which degrades peptidoglycan.
• Endolysin must act after virion assembly to release progeny.
• Options cover temporal classes of gene expression.

Concept / Approach:
Phages typically express structural proteins and lysis proteins during the late phase. Late genes include capsid proteins, tail components, scaffolding factors, holins, and endolysins. Holins form membrane lesions allowing endolysins to access the cell wall, coordinating lysis after assembly is complete.

Step-by-Step Solution:
Relate function to timing: lysis should follow assembly. Structural and release functions cluster in the late gene class. Therefore, endolysin is a late gene product.

Verification / Alternative check:
Genetic maps of classic phages (e.g., T-even phages, lambda under lytic conditions) place lysis genes in late transcriptional units, ensuring lysis occurs only after successful assembly.

Why Other Options Are Wrong:

• Immediate-early / Delayed-early: Involved in host takeover and DNA metabolism, not final lysis.
• All of these: Incorrect because expression is temporally restricted.
• Non-coding regulatory region only: Lysis requires a protein product.

Common Pitfalls:
Assuming any essential function must be early; in fact, precise timing prevents premature cell death and maximizes burst size.

Final Answer:
Late genes encode endolysin for phage release.