A hotspot of spontaneous and UV-induced illegitimate recombination during formation of lambda bio transducing phage.

To study the mechanism of spontaneous and UV-induced illegitimate recombination, we examined the formation of the lambda bio specialized transducing phage in Escherichia coli. Because most lambda bio transducing phages have double defects in the red and gam genes and have the capacity to form a plaque on an E. coli P2 lysogen (Spi- phenotype), we selected lambda bio transducing phage by their Spi- phenotype, rather than using the bio marker. We determined sequences of recombination junctions of lambda bio transducing phages isolated with or without UV irradiation and deduced sequences of parental recombination sites. The recombination sites were widely distributed on E. coli bio and lambda DNAs, except for a hotspot which accounts for 57% of UV-induced lambda bio transducing phages and 77% of spontaneously induced lambda bio transducing phages. The hotspot sites on E. coli and lambda DNAs shared a short homology of 9 bp. In addition, we detected direct repeat sequences of 8 bp within and near both the bio and lambda hotspots. A recA mutation did not affect the frequency of the recombination at the hotspot, indicating that this recombination is not a variant of recA-dependent homologous recombination. We discuss a model in which the short homology as well as the direct repeats play essential roles in illegitimate recombination at the hotspot.