Cis- and trans-activity of P22 antirepressor protein against c-repression specified by the closely related Salmonella phages L and Px1.

The ant products of Salmonella phage P22, synthesized by its immI region, releases when acting in cis replication inhibition for phages P22 and L. When ant product acts trans on a coinfecting immunity sensitive phage (Thomas-Bertani-experiment as test for release of replication inhibition) full replication ensues only if both superinfecting phages are homologous in the specificities of their immC and immI regions. If these regions are heterologous, differing in immC, immI or in both, the replication of the phage expected to be complemented by ant is inhibited. This inhibition is observed in both L- and Px-lysogenic bacteria and can be released in case of ant- amber phage by action of ant in cis in su+ lysogenic bacteria.