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...