Homology-dependent cutting in trans of DNA in extracts of Escherichia coli: an approach to the enzymology of genetic recombination.

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Abstract
An in vitro system is described in which the cutting of crosslinked .vphi.X replicative form (RF)I DNA molecules by the uvr system of E. coli induces the cutting of homologous undamaged DNA during incubation with crude extracts of thermally induced E. coli (.lambda. precA+) lysogens. This reaction, which was also observed in intact E. coli lysogens infected with .lambda. phages, is dependent on the presence of functional recA+ and uvrB+ gene products. Extracts from thermally induced .lambda. precA+ lysogens of E. coli were substantially more active than extracts from nonlysogenic cells of the same strain. The results provide preliminary evidence for an endonuclease activity that cuts intact superhelical DNA in response to interaction with homologous damaged DNA. An in vitro system is described in which both the endonucleolytic cutting of DNA containing crosslinks and the induced cutting of undamaged DNA can be studied without purification of the participating enzymes. Although the information obtained is fragmentary and often puzzling, this system can contribute to an understanding of the complex mechanisms involved in repair and recombination.