Antirepressors have been identified as proteins interacting with transcriptional repressors leading to expression of the repressed genes. The defective satellite phage/plasmid P4 has the capacity to derepress the unrelated prophage P2 after infection, thereby getting access to the late functions of the helper that are required for P4 lytic growth. The derepression of prophage P2 is mediated by the P4 E protein that function as an antirepressor by binding to the P2 immunity repressor C. A P2 mutant, sos, has been isolated that is insensitive to the action of the P4 E protein. In the present study, we show that sos is a point mutation in the P2 immunity repressor gene C and that it makes P4 E unable to turn the transcriptional switch of P2 from the lysogenic state to the lytic mode in a two plasmid reporter system. Furthermore, the interaction between C and E, when analysed in the yeast two-hybrid system, is blocked by the sos mutation. An analysis of C mutants indicates that the dimerization function of C is located in the C-terminal part of the protein and the dimerization defective mutants are unable to bind to their operator DNA. The sos mutation does not affect the capacity of the protein to dimerize. Using the yeast two-hybrid system, compensatory E mutants have been isolated that can interact with Sos, but they are unable to turn the transcriptional switch controlled by the Sos repressor. However, one point mutation in the E protein is shown to be unable to turn the transcriptional switch controlled by the wild-type C repressor.