Superimmunity: Characterization of a new gene in the immunity region of P1

Abstract

Strains carrying chimeras composed of the immunity-specific region of the bacteriophage P1, EcoRI fragment 9, and pBR322 are immune not only to Pl wild type and to P1 clear mutants, but also to Pl vir s and the heteroimmune phage P7. We have termed this phenotype “superimmunity.” Although the Pl EcoRI-9 fragment expresses c4, which is responsible for the specific immunity of a Pl prophage, the c4 gene is neither necessary nor sufficient for expression of superimmunity. Mutations in EcoRI-9 which reduce superimmunity do not inactivate gpc4; furthermore, chimeras carrying an EcoRI-9 fragment of a Pl c4 mutant still confer superimmunity. We propose that a new gene, sim, is responsible for superimmunity at high gene dosage. Although closely linked, the genes c4 and sim do not appear to be cotranscribed. Several alternatives for the mechanism for action of gp sim are presented.