Studies on lysogeny in Escherichia coli with bacteriophage λ physical observation of the insertion process

The kinetics of insertion of λ DNA into a sex factor (F′450) carrying att λ ‡ ‡ Abbreviations used: att λ, the locus at which insertion of λ prophage occurs; gal, operon controlling galactose metabolism; bio, operon controlling biotin synthesis; uvrB, a locus involved in the repair of ultraviolet damage; int, the λ gene controlling prophage integration; red, the λ genes for vegetative recombination; imm λ, the immunity region of λ; imm 434, the immunity region of 434; sus, a suppressor-sensitive mutation. was measured. It has been found that both interruption at att λ and insertion start at about 10 minutes after infection. For the first 30 minutes after infection there is a quantitative conversion of the original sex-factor covalent circles to circles containing one or more λ DNA molecules. At the end of 30 minutes about one-quarter of the F′450 circles are converted to these forms. The remaining F′450 circles continue to be lost, though at a lower rate, and practically none remains after 2 hours. However, the F′450 circles that are lost after this 30-minute period are not recovered as covalent circles. Only when cells are labeled at much later times (about 10 hours after infection) is full recovery of covalently sealed sex factors observed; nearly all of these show the insertion of at least one λ DNA molecule. It is found that int and b2 mutants are neither able to insert nor interrupt the bacterial att, while N mutants are able to insert but to a lesser extent. Evidence is presented which suggests that most multiple prophages arise by a one-step insertion of multiple-size λ circles.