Illegitimate recombination mediated by double-strand break and end-joining in Escherichia coli.

The frequency of illegitimate recombination has been measured by a lambdabio transducing phage assay during the induction of the E. coli lambda c1857 lysogen. Illegitimate recombination falls into two classes, short homology-independent and short homology-dependent illegitimate recombination. The former involves sequences with virtually no homology, and is mediated by DNA topoisomerases and controlled by the DNA binding protein HU. The latter is induced by UV irradiation or other DNA damaging agents and requires short regions of homology, usually contain 4 to 13 base pairs, at sites involved in recombination. It has been shown that the RecJ exonuclease promotes short homology-dependent illegitimate recombination, but that the RecQ helicase suppresses it. In addition, we have shown that the overexpression of RecE and RecT enhances the frequencies of spontaneous and UV-induced illegitimate recombination and that the RecJ, RecF, RecO, and RecR functions are required for this RecE-mediated illegitimate recombination. Moreover, we have also indicated that RecQ plays a role in the suppression of RecEmediated illegitimate recombination, with the participation of DnaB, Fis, Exol, and H-NS. Models have been proposed for these modes of recombination: the DNA gyrase subunit exchange model for short homology-independent illegitimate recombination and the "double-strand break and join" model for short homologydependent illegitimate recombination. Many features of these models remain to be tested in future studies.