Aberrant immunity behaviour of hybrid λimm 21 phages containing the DNA of ColE1-type plasmids

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Abstract
Hybrid λ and λimm 21 bacteriophages carrying various ColE1-type plasmids have been constructed in vitro. The λimm 21/plasmid recombinants display aberrant immunity behaviour, giving clear plaques under conditions where the parental phages give turbid ones and being able to grow on homoimmune lysogens. λimm λ/plasmid recombinants show no such unusual behaviour. Studies with hybrids of a λimm 21 cITS phage carrying pMB9 DNA showed the operation of the plasmid's replication system to be the basic cause of the aberrant immunity behaviour. The plasmid replication system could act as a complete alternative to the phage system during vegetative phage growth. The probable reason that λimm 21 phages show such altered phenotypes when carrying a functional plasmid replication origin, whereas λimm λ and λimm 434 (Mukai et al., 1978) phages do not, is the relative ease of titration of the phage 21 repressor to allow transcription from p R 21. Various uses are considered for the altered phenotypic behaviour of λimm 21/ColE1-type plasmid hybrids.