Suppression of gamma ray-induced illegitimate recombination in Escherichia coli by the DNA binding protein H-NS

To study the mechanism of gamma -ray-induced illegitimate recombination, we examined the formation of lambda bio transducing phage in Escherichia coli after gamma -ray irradiation. We show that gamma -ray irradiation enhances the formation of lambda bio transducing phage during prophage induction. Moreover, an hns mutation synergistically enhanced the incidence of gamma -ray-induced illegitimate recombination. Next we determined the sequences at the recombination junctions of the lambda bio transduclllg phages induced by gamma -ray irradiation. Most of the recombination sites coincided with known hotspots. Among them, hotspot I accounted for 67% and 77% of gamma -ray-induced lambda bio transducing phages: in the wild type and the hns mutant, respectively. Therefore, the recombination sites appear to occur mostly at hotspot I or at other hotspots, but rarely at non-hotspot sites. These results suggest that types of DNA damage other than the double-strand breaks induced at random sites are mainly responsible for the introduction of the site-specific or region-specific DNA double strand breaks that lead to recombination at the hotspots. The results also showed that the recombination events took place between DNA sequences possessing short stretches of homology. H-NS protein, which binds to curved DNA, suppresses illegitimate recombination:ln: the presence and absence of gamma -ray irradiation. Models for gamma -ray-induced illegitimate recombination are discussed.