Inhibition of growth of rII mutants of bacteriophage T4 by immunity substance of bacteriophage lambda

Abstract

Certain λ lysogens are able to inhibit growth of superinfecting φ80 and λ-P22 hybrid phages with the OPQ region from P22. Superinfection exclusion requires expression of the prophage rex gene and is most evident in λ lysogens in which the prophage carries a mutation (either is or am) in the cI gene. These lysogens also inhibit growth of leaky rII− mutants of T4 which are able to grow in λ+ lysogens. Rex-dependent exclusion of lambdoid phages is not strictly a superinfection exclusion system, as thermal induction of a λrex+clts85(OP)-P22 prophage results in “self-exclusion,” that is, in low phage production. These observations are explained by proposing that Rex activity of a λcits lysogen (and other lysogens which produce a modified repressor) is greater than in lysogens carrying a prophage with a wild-type cI gene. The effects of thermal induction and mitomycin C induction on Rex activity are described, and explanations for increased Rex activity in lysogens carrying a prophage with an altered repressor are discussed.