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

Verified step by step guidance

1. Understand the two life cycles of λ bacteriophage: The lytic cycle involves the virus replicating and lysing the host cell to release new phages, while the lysogenic cycle integrates the phage DNA into the host genome, allowing it to replicate passively with the host cell without causing immediate harm.
2. Explain the role of the λ repressor (also called CI protein): The λ repressor binds to the operator regions OL and OR, blocking transcription from the lytic promoters PL and PR. This repression ensures the virus enters and maintains the lysogenic cycle by preventing the expression of genes required for lysis.
3. Explain the role of the Cro protein: Cro protein binds to the same operator regions as the λ repressor but with different affinities. It represses transcription from PRM (the promoter for the λ repressor) and promotes transcription from PL and PR, favoring the lytic cycle by allowing the expression of lytic genes.
4. Describe the competition between λ repressor and Cro protein: The balance between these two proteins determines the life cycle. High levels of λ repressor maintain lysogeny by repressing lytic genes, while high levels of Cro protein shift the balance toward the lytic cycle by repressing the λ repressor and activating lytic genes.
5. Link the roles of PM and PRM to the life cycles: PM (maintenance promoter) is responsible for the continued expression of the λ repressor during lysogeny, while PRM (repressor maintenance promoter) is activated to produce more λ repressor. Cro protein inhibits PRM, tipping the balance toward lysis. This regulatory network ensures the phage can switch between cycles based on environmental conditions.

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

The λ repressor is a protein that maintains the lysogenic state by binding to specific DNA sites, inhibiting the expression of genes necessary for the lytic cycle. Conversely, Cro protein promotes the lytic cycle by repressing the expression of the λ repressor. The balance between these two proteins determines whether the bacteriophage will enter the lytic or lysogenic cycle, influencing the fate of the host cell.

Transcription Control from PM and PRM

PM and PRM are promoters that regulate the transcription of genes involved in the lytic and lysogenic cycles. PM drives the expression of genes necessary for the lytic cycle, while PRM is responsible for the maintenance of the lysogenic state by promoting λ repressor production. The interplay between these promoters, influenced by the levels of λ repressor and Cro protein, is crucial for determining whether the bacteriophage will undergo lysis or establish lysogeny.