Lysogeny Is Maintained by the Lambda Repressor Protein

KEY CONCEPTS
- The lambda repressor, encoded by the cI gene, is required to maintain lysogeny.
- The lambda repressor acts at the O_L and O_R operators to block transcription of the immediate early genes.
- The immediate early genes trigger a regulatory cascade; as a result, their repression prevents the lytic cycle from proceeding. 

Looking at the lambda lytic cascade, we see that the entire program is set in motion by the initiation of transcription at the two promoters P_L and P_R for the immediate early genes N and cro. Lambda uses antitermination to proceed to the next stage of (delayed early) expression; therefore, the same two promoters continue to be used throughout the early period.

The expanded map of the regulatory region drawn in FIGURE 1 shows that the promoters P_L and P_R lie on either side of the cI gene. Associated with each promoter is an operator (O_L , O_R ) at which repressor protein binds to prevent RNA polymerase from initiating transcription. The sequence of each operator overlaps with the promoter that it controls, and because this occurs so often these sequences are described as the P_L /O_L and P_R /O_R control regions.

FIGURE 1. The lambda regulatory region contains a cluster of trans-acting functions and cis-acting elements.
As a result of the sequential nature of the lytic cascade, the control regions provide a pressure point at which entry to the entire cycle can be controlled. By denying RNA polymerase access to these promoters, the lambda repressor protein prevents the phage genome from entering the lytic cycle. The lambda repressor functions in the same way as repressors of bacterial operons: It binds to specific operators.
The lambda repressor protein is encoded by the cI gene. Note in Figure 1 that the cI gene has two promoters, P_RM (promoter right maintenance) and P_RM (promoter right establishment). Mutants in this gene cannot maintain lysogeny but always enter the lytic cycle. In the time since the original isolation of the lambda repressor protein, the characterization of the repressor protein has shown how it both maintains the lysogenic state and provides immunity for a lysogen against superinfection by new phage lambda genomes.

The lambda repressor binds independently to the two operators,O_L and O_R . Its ability to repress transcription at the associated promoters is illustrated in FIGURE 2 .

FIGURE 25.15 Repressor acts at the left operator and right operator to prevent transcription of the immediate early genes (N and cro). It also acts at the promoter P_RM to activate transcription by RNA polymerase of its own gene.
At O_L , the lambda repressor has the same sort of effect as has already been discussed for several other systems: It prevents RNA polymerase from initiating transcription at P_L . This stops the expression of gene N. P_L is used for all leftward early gene transcription; thus, this action prevents expression of the entire leftward early transcription unit, blocking the lytic cycle before it can proceed beyond early stages.
At O_R , repressor binding prevents the use of P_R , and so cro and the other rightward early genes cannot be expressed. The lambda repressor protein binding at O_R also stimulates transcription of cI, its own gene from P_RM .
The nature of this control circuit explains the biological features of lysogenic existence. Lysogeny is stable because the control circuit ensures that, so long as the level of lambda repressor is adequate, expression of the cI gene continues. The result is that O_L and O_R remain occupied indefinitely. By repressing the entire lytic cascade, this action maintains the prophage in its inert form.