The Genetics of the Anti-immune Phenotype of Defective Lambda Lysogens

Abstract

Heat inducible λ lysogens surviving induction carry two prophage mutations: one in gene N, the other in x, zd, O, or P . The lysogens carrying the defective prophage N − zd − , N − O − or N − P − can exist in two distinct physiological states: immune (im + phase) and anti-immune (im − phase). The im + phase is characterized by the presence of immunity against the homoimmune superinfecting phage and repression of early phage functions. The im − phase is characterized by the absence of immunity, derepression of the x – O operon, and channeling of the superinfecting homoimmune phage towards the lytic cycle. When the prophage contains a mutation in x, the x – O operon cannot be transcribed and the im − phase cannot be established (Neubauer and Calef, 1970). The im + and im − phases are hereditary for the whole line of cells. However, the cell populations always contain 0.1–1% of cells which give rise to colonies of the opposite phase. This phenomenon, called the “immunity phase shift,” is of a nonmutational character; when such temperature sensitive defective lysogens are deprived of immunity by heat, the whole im + population shifts to the im − phase. From these results the following conclusions were possible: (a) The two prophage mutations are sufficient to allow the bacteria to survive the absence of immunity. (b) The presence of an immunity specific cytoplasmic factor, called anti-immunity, antagonizes the establishment of immunity in the im − phase. Starting from these points and aiming towards a definition of the genetics of the novel anti-immune phenotype we tried...