ROLE OF THE RECJ GENE-PRODUCT IN UV-INDUCED ILLEGITIMATE RECOMBINATION AT THE HOTSPOT

Illegitimate recombination between a prophage and adjacent bacterial D NA is the first step in the formation of specialized transducing phage . Such recombination is rare, but it is greatly enhanced by UV irradia tion. We studied the mechanism of UV-induced illegitimate recombinatio n by examining the effect of rec mutations on the frequency of lambda bio transducing phage and found that an Escherichia coil recJ mutation reduces it by 3- to 10-fold. In addition, the recombination hotspot, which accounts for approximately 60% of lambda bio transducing phages in wild-type bacteria, was not detected in the recJ mutant. Introducti on of a RecJ overexpression plasmid into the recJ mutant recovered the recombination at the hotspot. These results indicate that the Red pro tein preferentially stimulates illegitimate recombination at the hotsp ot. Both the hotspot and the non-hotspot sites have short regions of h omology, but only the hotspot sites contain common direct-repeat seque nces. We propose a model based on the 5'-3' exonuclease activity of Re cJ to explain the involvement of this protein in illegitimate recombin ation at the hotspot.