Lambda mutants which persist as plasmids.

Lambda phages mutated in gene N do not kill sensitive host bacteria, but persist as plasmids. Plasmids are formed by genomes containing cI(+), and also by sus, ts, or c mutants of cI. Bacteria infected with two or more phage particles give rise to clones in which most of the bacteria are carriers. The introduced lambda genomes replicate more than once per bacterial division until there are 10 to 20 lambda plasmids per host genome. In bacteria containing both F and lambda plasmids, both replicate independently, and elimination by growth in acridine orange is also independent. Carriers of lambda Nsus plasmids are not immune, and there is complementation between the plasmids and superinfecting lambda mutants.