Organization of the immunity region immI of bacteriophage P1 and synthesis of the P1 antirepressor.

The immI region of bacteriophage P1 includes the ant/reb gene, which encodes the antirepressor protein, and the c4 gene, which encodes a repressor molecule that negatively regulates antirepressor synthesis. The antirepressor interferes with the activity of the P1 repressor of lytic function, the product of the c1 gene. We have determined the DNA sequences of the immI region of P1 wild-type and the mutants virs, ant16, ant17, and reb22. Using suitable P1 immI DNA subfragments cloned into a vector of the T7 bacteriophage RNA polymerase expression system the antirepressor protein(s) was overproduced. On the basis of positions of immI mutations and the sizes of ant gene products, the following organizational feature of the P1 immI region is suggested: (1) the genes c4 and ant are cotranscribed in that order from the same promoter in the clockwise direction of the P1 genetic map; (2) an open reading frame for an unknown gene is located in between c4 and ant; (3) the site at which the c4 repressor acts is located within the c4 structural gene; (4) two antirepressor proteins of molecular weights 42,000 and 32,000 are encoded by a single open reading frame, with the smaller protein initiating at an in-frame start codon; (5) transcription of immI is regulated via a c1-controlled operator, Op51, indicating a communication between the immunity systems immC and immI.