Structures of oversized genomes of phage P1 derivatives carrying lac genes of Escherichia coli K12

Physical analyses of two newly isolated oversized P1lac phage genomes showed that they are partly diploid in P1 genes and that they carry a 60–70-kb segment of host DNA. The transposable element γδ is present at one of the junctions between host and P1 DNA, and IS1 is at the other junction. These elements must thus have been actively involved in the formation of these P1lac prophages. The genome of a third oversized P1lac has a segment with dispensable P1 genes deleted. The absence of any known recombinogenic element at one of its junctions between P1 and host DNA suggests non-homologous recombination to have been involved in its formation. Non-homologous recombination might have also taken place in one of the final steps of the formation of the former P1lac genomes.