Rec-mediated recombinational activity of two adjacent Chi elements in bacteriophage lambda

Chi is a sequence of eight nucleotide pairs which stimulate recBC-mediated recombination (Smith, 1983a, b). The effect of two linked Chis on recBC-mediated recombination was tested in bacteriophage lambda. It was noticed that the Chi element located on the right side of the phage chromosome is epistatic on the other Chi. These findings support a model proposed by Stahl et al. (1983) which suggests that the recombination machinery moves unidirectionaly in the phage chromosome from right to left. The results also suggest that in the presence of more than one Chi only the rightmost one stimulates recombination.