Effects of mutations in the immunity system of bacteriophage P1

A mutant of bacteriophage P1 that made an altered c1 repressor is described. The mutant c1 product had two configurations: in lysogens, at high temperatures, it permitted constitutive expression of the normally repressed DNA replication function ban and was insensitive to the action of ant, a product expressed by the virulent mutant P1virs and by the heteroimmune phage P7 (formerly phiamp+) and normally able to overcome c1 repression; in mutant lysogens at low temperatures, the mutant repressor was apparently normal (able to repress ban and sensitive to ant action). Genetic studies of this mutant led to the isolation of a derivative that formed unstable lysogens. These studies suggested that the ban product was normally under c1 control; they further showed that ant overcame c1 repression by inactivating c1 rather than by creating a bypass of repressor activity.