Reduced immunity in lysogens of bacteriophage lambda due to a mutation in the prophage

Analysis of the phase variation in lambda reduced immunity lysogens

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.

Regulation of lambda rex expression after infection of Escherichia coli K by lambda bacteriophage

The ability of Escherichia coli K to support bacteriophage T4rII replication starts to decline at 3 to 6 min after infection by lambda. This inhibition appears to depend on expression of the lambdarex(+) gene, since little inhibition was observed following infection by a lambdarex mutant or by a hybrid bacteriophage lambdai(434) which lacks a functional rex gene. For promotion of the synthesis of rex product, cII(+) and N(+) genes are required and can act trans, whereas cY(+), also required, must be cis to a rex(+) gene. These genes presumably play a role in the transcription of the cI-rex operon because they are also known to be required for repressor (cI product) synthesis. Functional cIII, O, P genes are not necessary for ample rex product synthesis. We also observed full rex expression after infection by lambdasuscI mutants, suggesting that rex and repressor are separate gene products and that repressor is not required for inhibition of T4rII replication. We also report experiments with a rex mutant that is not leaky when in a lysogen but is sufficiently leaky shortly after infection to cause inhibition of T4rII replication.