KplE1 is one of the 10 prophage regions of Escherichia coli K12, located at 2464 kb on the chromosome. KplE1 is defective for lysis, but it is fully competent for excisive recombination. In this study, we have mapped the binding sites of the recombination proteins, namely IntS, TorI, and IHF on attL and attR, and the organization of these sites suggests that the intasome is architecturally different from the canonical form. We also measured the relative contribution of these proteins to both excisive and integrative recombination by using a quantitative in vitro assay. These experiments show a requirement of the TorI excisionase for excisive recombination and of the IntS integrase for both integration and excision. Moreover, we observed a strong influence of the supercoiled state of the substrates. The KplE1 recombination module, composed of the integrase and excisionase genes together with the attL and attR DNA regions, is highly similar to that of several phages infecting various E. coli strains as well as Shigella flexneri and Shigella sonnei. The in vitro recombination data reveal that HK620 and KplE1 att sequences are exchangeable. This study thus defines a new sitespecific recombination module, and implications for the mechanism and regulation of recombination are discussed.Phage has long served as a model system for studies of regulated site-specific recombination (1). Indeed, bacteriophages such as may choose between the lytic and the lysogenic cycle for their propagation in the bacterial host (2). In conditions favorable for bacterial growth the phage genome is inserted into the host genome by an integrative recombination reaction, which takes place between DNA attachment sites called attP and attB, in the phage and the bacterial genome, respectively. As a result, the integrated DNA (or prophage DNA) is bounded by hybrid attachment sites, termed attL and attR. In response to a change in the physiological state of the bacteria, mainly in response to stress conditions such as DNA damage, phage DNA is excised from the host chromosome. This excisive reaction recombines attL and attR to restore the attP and attB sites on the circular and Escherichia coli