Homoimmune vs heteroimmune phage


A large group of phage (called "lambdoid" phage) share a very similar genome organization and life-style with phage lambda. Different lambdoid phage often have different repressor genes and operator sites. Although the repressor from each lambdoid phage binds its own operator sites, it is unable to repress phage with a different operator sites. Recombination between different lambdoid phage can produce a hybrid phage with the regulatory region derived from one phage (called the "immunity" or "imm" region) and the rest of the phage genome derived from another phage. An example of phage lambda, the closely related phage 434, and a hybrid between lambda and 434 (called lambda imm434) is shown in the figure below.

If a superinfecting phage infects a lysogen producing the cognate cI protein that can bind to the OL and OR sites of the incoming phage, as soon as the phage enters the cell the repressor protein made in the lysogen will turn off gene expression from the incoming phage so no plaque will result -- that is, the lysogen is "immune" to the superinfecting phage. In contrast, if the superinfecting phage infects a lysogen producing a cI protein that is unable to bind to the OL and OR sites of the incoming phage, the lysogen will not affect growth of the incoming phage so when the phage enters the cell it will begin normal lytic or lysogenic development -- that is, the lysogen is "heteroimmune" to the superinfecting phage. An example of typical results is shown in the table below.



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Last modified November 26, 2003