The Lambda Repressor and Its Operators Define the Immunity Region

KEY CONCEPTS
- Several lambdoid phages have different immunity regions.
- A lysogenic phage confers immunity to further infection by any other phage with the same immunity region.
The presence of lambda repressor explains the phenomenon of immunity. If a second lambda phage DNA enters a lysogenic cell, repressor protein synthesized from the resident prophage genome will immediately bind to O_L and O_R in the new genome. This prevents the second phage from entering the lytic cycle.

The operators were originally identified as the targets for repressor action by virulent mutations (λvir). These mutations prevent the repressor from binding at O_Lor O_R , with the result that the phage inevitably proceeds into the lytic pathway when it infects a new host bacterium. Note that λvir mutants can grow on lysogens because the virulent mutations in O_L and O_R allow the incoming phage to ignore the resident repressor and thus enter the lyticcy cle. Virulent mutations in phages are the equivalent of operatorconstitutive mutations in bacterial operons.

A prophage is induced to enter the lytic cycle when the lysogenic circuit is broken. This happens when the repressor is inactivated. The absence of repressor allows RNA polymerase to bind at P_L and P_R , starting the lytic cycle, as shown in FIGURE 1.

FIGURE 1. In the absence of repressor, RNA polymerase initiates at the left and right promoters. It cannot initiate at P_RM in the absence of repressor.
The autoregulatory nature of the repressor maintenance circuit creates a sensitive response. The presence of the lambda repressor is necessary for its own synthesis; therefore, expression of the cI gene stops as soon as the existing repressor is destroyed. Thus, no repressor is synthesized to replace the molecules that have been damaged. This enables the lytic cycle to start without interference from the circuit that maintains lysogeny.
The region including the left and right operators, the cI gene, and the cro gene determines the immunity of the phage. Any phage that possesses this region has the same type of immunity, because it specifies both the repressor protein and the sites on which the repressor acts. Accordingly, this is called the immunity region . Each of the four lambdoid phages ϕ80, 21, 434, and λ has a unique immunity region. When we say that a lysogenic phage confers immunity to any other phage of the same type, we mean more precisely that the immunity is to any other phage that has the same immunity region (irrespective of differences in other regions).

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