ORIENTATION-DEPENDENT RECOMBINATION HOTSPOT ACTIVITY IN BACTERIOPHAGE-LAMBDA

Promoters of genetic exchange by the Escherichia coli Rec system, Chi elements, have been analyzed in λ phages carrying bacterial EcoRI restriction fragments. Some fragments confer Chi+ phenotype in one orientation and Chi- in the opposite orientation. The inactivity of Chi in one orientation explains why all active Chi elements in λ manifest a certain recombinational bias of the same sense. When these studies were undertaken, we rather expected to find two classes of Chi, one class which stimulated recombinant formation stronger to its left and one class stimulating recombinant formation more strongly to its right. The failure to find the second class is now understandable by supposing that the orientation of Chi which would have permitted it to act rightward is the orientation in which Chi has no activity at all. Several models are proposed for the orientation dependence of Chi activity. © 1979.