The Role of   Integrase in Integration and Excision

Bacteriophage λ lysogenizes Escherichia coli by inserting its DNA into a specific site in the bacterial chromosome and growing thereafter in synchrony with its host (Campbell 1962). Upon lysogenic induction, the union between prophage and bacterial chromosomes is broken and the prophage is excised. At first sight, it might appear that excision is a simple reversal of insertion. Such a view does indeed account for the observations that both reactions require the phage int-gene protein, that the substrates—the attachment sites of the phage and host—are apparently unaltered after a cycle of insertion and excision, and that a point mutation in an attachment site reduces both insertion and excision (see Gottesman and Weisberg 1971; Shulman and Gottesman 1973). Nevertheless, it is contradicted by several other observations. First, the phage xis-gene protein is required for excision but not for insertion (Guarneros and Echols 1970). Second, integration is thermosensitive and excision is thermoresistant...