Analysis of the phase variation in lambda reduced immunity lysogens

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Two distinct phases characterized by different levels of immunity that appear in some E. coli strains lysogenic for reduced immunity mutants of bacteriophage lambda are identified as single and tandem double lysogens respectively on the basis of DNA-DNA hybridization experiments and the requirement of the phage xis function for the transition from a single to a double, and of the host recA function for the transition from a double to a single lysogen (in a xis- condition). Rim lysogens with a further increase in immunity, containing some 5 copies of the lambda genome per host genome, have also been observed. It is argued that the different levels of immunity are a direct reflection of the CI gene dosage effect. An unexplained finding is that rim single lysogens yield double lysogens with a frequency of near 1% per generation, whereas cured cells fail to appear even at a frequency 100 times lower.