The Role of Specialized Nucleoprotein Structures in Site-specific Recombination and Initiation of DNA Replication

For a number of years the evidence has been compelling that bacteriophage λ inserts into and detaches from the bacterial chromosome by the recombinational mechanism suggested by Campbell (1962) (reviewed by Nash 1981; Weisberg and Landy 1983). Given its recombinational nature, the integration-excision reaction has two remarkable properties: site specificity and directional control (Echols et al. 1968; Weil and Signer 1968; Echols 1970) (reviewed by Echols and Guarneros 1983; Weisberg and Landy 1983). The extraordinary localization of site-specific recombination by λ is a particularly interesting property in terms of the DNA-protein interactions that govern the reaction. Integration at the aggregate of “secondary sites” is more than 100-fold less frequent than insertion at the normal site; this high specificity is crucial for the life-style of the virus, because a misintegrated λ is highly defective in excision (Shimada et al. 1972).