A NEW-TYPE OF ILLEGITIMATE RECOMBINATION IS DEPENDENT ON RESTRICTION AND HOMOLOGOUS INTERACTION

Illegitimate (nonhomologous) recombination requires little or no seque nce homology between recombining DNAs and has been regarded as being a process distinct from homologous recombination, which requires a long stretch of homology between recombining DNAs, Under special condition s in Escherichia coli, we have found a new type of illegitimate recomb ination that requires an interaction between homologous DNA sequences. It was detected when a plasmid that carried 2-kb-long inverted repeat s was subjected to type II restriction in vitro and type I (EcoKI) res triction in vivo within a Delta rac recBC recG ruvC strain, Removal of one of the repeats or its replacement with heterologous DNA resulted in a reduction in the level of recombination. The recombining sites th emselves shared, at most, a few base pairs of homology. Many of the re combination events joined a site in one of the repeats with a site in another repeat. In two of the products, one of the recombining sites w as at the end of one of the repeats, Removal of one of the EcoKI sites resulted in decreased recombination. We discuss the possibility that some structure made by homologous interaction between the long repeats is used by the EcoKI restriction enzyme to promote illegitimate recom bination. The possible roles and consequences of this type of homologo us interaction are discussed.