What is the primary action of Cro protein in the lytic state?

The primary action of Cro protein in the lytic state is to block lambda repressor synthesis. In the context of the bacteriophage lambda life cycle, when the virus is in the lytic phase, the goal is to replicate itself rapidly and produce new virions. Cro protein functions as a transcriptional repressor that inhibits the expression of the lambda repressor (cI protein). By doing so, Cro facilitates the transition from the lysogenic to the lytic cycle, allowing for the transcription of genes necessary for the replication of the phage’s genome and the production of phage proteins, ultimately leading to the lysis of the bacterial cell.

The action of Cro protein thus directly promotes the lytic phase by reducing the levels of the lambda repressor, which is essential for maintaining the lysogenic state. This mechanism of action is crucial for ensuring that the phage can effectively enter the lytic phase when environmental conditions are conducive to viral replication and spread.