Rec-mediated recombinational hot spot activity in bacteriophage lambda. IV. Effect of heterology on Chi-stimulated crossing over.

A Chi mutation in phage lambda stimulates Rec-mediated crossing over more to one side of itself than to the other; stimulation, which is maximal near Chi, can occur at some distance from the Chi site as well. A gross heterology differentiating the two recombining parents does not interfere with the distant Chi-stimulated crossover whether the heterology is at the Chi site or between the Chi site and the distant interval in which recombination is monitored. These conclusions hold whether recombination is measured "genetically" in standard crosses or "physically" in density-labeled crosses conducted in the absence of DNA replication.