χ recombination activity in phage λ decays as a function of genetic distance

In Escherichia coli, χ is a recombination hotspot that stimulates RecBCD- dependent exchange at and to one side of itself. χ active is highest at χ and decreases with distance from χ. The decrease in χ activity may be a simple property of the physical distance over which χ can stimulate recombination. Alternatively, the decay in χ activity with distance may reflect the high likelihood that χ-stimulated recombination occurs in a single χ-proximal act, to the exclusion of additional χ-stimulated exchanges more distal to χ. To test the models, we determined if χ activity decreases as a function of physical distance (i.e., DNA base pairs) or genetic distance (homologous DNA base pairs). Our results indicate that χ activity decays as a function of genetic distance. In addition, we found that the sbcB gene product (exonuclease 1, a 3' → 5' ssDNA exonuclease) modulates the distance over which χ can act. In contrast, the recJ gene product (a 5' → 3' ssDNA exonuclease) does not alter the decay of χ activity.