Prophage P1, an Extrachromosomal Replication Unit

In the lysogenic state, most temperate phages in Escherichia coli K12 are known to locate on specific sites of the bacterial chromosome. However, mating experiments do not reveal any such specific site for prophage P1 (Jacob and Wollman, 1959; Boice and Luria, 1963). It may therefore be suspected either that the prophage P1 associates with different chromosomal sites and is readily detached from the chromosome, or that it exists as an episome without a chromosome-associated phase. The first experiment to examine the molecular structure of the prophage P1 was carried out by Inselburg (1968) who analyzed the distribution of P1-specific DNA of lysogenic cells in a sucrose density gradient measured by hybridization of the DNA with P1-DNA trapped in agar (McCarthy and Bolton, 1963). The result was interpreted to indicate that prophage P1 is associated with the host chromosome. However, later development of a convenient method of DNA-DNA hybridization using...