Describe the lytic and lysogenic life cycles of λ bacteriophage. What roles do λ repressor and Cro protein play in controlling transcription from PM and PRM, and how are these roles linked to lysis and lysogeny?

Key Concepts

Lytic and Lysogenic Life Cycles

The lytic cycle is a viral reproductive process where the virus infects a host cell, replicates its genetic material, and ultimately causes the cell to lyse, releasing new virions. In contrast, the lysogenic cycle involves the integration of the viral genome into the host's DNA, allowing it to replicate along with the host cell without causing immediate harm. This cycle can switch to the lytic phase under certain conditions, leading to cell lysis.

λ Repressor and Cro Protein

λ repressor and Cro protein are key regulatory proteins in the λ bacteriophage life cycle. The λ repressor promotes lysogeny by binding to operator sites and inhibiting transcription of genes necessary for the lytic cycle. Conversely, Cro protein favors the lytic cycle by repressing the expression of the λ repressor, thus allowing the virus to switch from a dormant state to active replication and cell lysis.

Transcription Control from PM and PRM

PM and PRM are promoters that regulate the transcription of genes in the λ bacteriophage. PM drives the expression of genes necessary for the lytic cycle, while PRM is responsible for maintaining the expression of the λ repressor, promoting lysogeny. The balance between these promoters, influenced by the levels of λ repressor and Cro protein, determines whether the bacteriophage enters the lytic or lysogenic cycle, linking transcription control to the fate of the host cell.