Site-Specific Recombination and the Segregation of Circular Chromosomes

Abstract

Site-specific recombination provides a mechanism for precise programmed DNA rearrangements. It was discovered through its involvement in the integration and excision of bacteriophage lambda into and out of the E. coli chromosome (reviewed in Landy 1989). Subsequently, site-specific recombination has been shown to be involved in the integration/excision of many other viruses; in converting the initial products of replicative intermolecular transposition to final products (Arthur and Sherratt 1979); in various inversion gene switches (Johnson 1991); and in the copy number control and stable inheritance of circular replicons (Colloms et al. 1990; Blakely et al. 1991). Related mechanisms and enzymes also appear to be involved in unprogrammed reactions that lead to less precise rearrangements, for example in the process of conjugative transposition and in the movement of integrons, bacterial genetic elements that are responsible for the movement of antibiotic resistance genes into and out of plasmids and transposable elements (Murphy 1989; Schmidt et al. 1989). In this short report, an E. coli chromosomally encoded site-specific recombination system that was initially identified through its role in the stable inheritance of multicopy plasmids is described in some detail. Subsequently, it has been shown that this system additionally functions in the normal segregation of the bacterial chromosome. We propose that the use of site-specific recombination to segregate circular replicons at cell division may be ubiquitous.