Analysis of bacteriophage P1 immunity by using lambda-P1 recombinants constructed in vitro.

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Abstract
The dissection and reconstruction of a complex control circuit, the P1 immunity system, by a method that involves inserting EcoRI-generated fragments of P1 DNA into [phage] .lambda. vectors that can then be sequentially inserted into a bacterial [Escherichia coli] cell are described. Using these techniques, .lambda.-P1 hybrid phages that express the products of P1 genes c1, c4, ant and ban were isolated and, in appropriately constructed lysogens, the roles played by the first 3 of these products in phage immunity were confirmed. The sites requisite for expression and repression of these gene products were also localized to particular P1 fragments. Apparently, gpant acts in trans to antagonize repression mediated by gpc1, in support of 1 of 2 proposed models for gpant action. Two features of the immunity system are also revealed: a hitherto unknown component that effects gpc1 repression, and an unexpected ability of gpc4 to channel a superinfecting c1+ phage into the lysogenic state, which suggests that gpc4 activity regulates the establishment phase of lysogeny.