The activity of ant product of the Salmonella phage P22 against the closely related but heteroimmune phage L.

P22 mutants defective in the early gene 24 are complemented by phage L in mixed infection. P22 12- and P22 23- mutants are not complemented by phage L. Gene function 24 of an L prophage is turned on by a superinfecting P22 24- mutant and complements the missing function of the defective P22 phage. Since this transactivation of prophage gene 24 depends on a functional gene ant in the superinfecting P22 mutant, it indicates derepression for leftward directed gene expression in prophage L. On the contrary neither the rightward directed expression of gene 12 nor of gene 23 in prophage L. can be turned on by superinfecting P22 24- 12- or P22 24- 23- mutants (and also not by P22 12- and P22 23-) to a degree sufficient for complementation of simultaneously superinfecting L virB 12- or L virB 23- mutants. The failure to detect release of repression for rightward directed gene expression of prophage L corresponds to the earlier observation (Prell, 1975) that P22 superinfecting L lysogens cannot release replication inhibition for simultaneously infecting phage L. The results are discussed with respect to the mechanism underlying the different action of P22 antirepressor in L and in P22 lysogens.