The superimmunity gene sim of bacteriophage P1 causes superinfection exclusion.

Previous work has shown that the sim gene of bacteriophage P1, if cloned into a multicopy vector, confers immunity against P1 infection to cells. We show that a 1.85-kb DNA fragment from the sim region of P1 (in the multicopy plasmid pMK4) expresses immunity and encodes three proteins with molecular weights of about 25, 24, and 15 kDa. Deletion of 650 bp from the sim region abolished synthesis of all three proteins and of the sim phenotype. Expression of sim did not prevent adsorption of P1 to cells. Successful transfection with linear P1 DNA suggests that the recombinational circularization of P1 DNA is not inhibited in the presence of sim. Plasmid pMK4 and a P1 prophage can be stably maintained in the cell indicating that replication of the prophage is not disturbed by sim. The prophage can be induced in the presence of sim. This shows that the sim phenotype is not caused by preventing lytic replication or phage maturation. In cells with pMK4 there is no expression of genes from infecting phages and transduction frequency is drastically reduced. We suggest that sim functions as a superinfection exclusion system by preventing transfer of DNA from the adsorbed phages into the cytoplasm.