Coupling with Packaging Explains Apparent Recombination of Bacteriophage Lambda by Reca and Recbc Functions Nonreciprocality

Chi (x, 5'-GCTGGTGG) is a recombinator in RecAand RecBC-mediated recombination in Escherichia coli. In vegetative recombination between two bacteriophage lambda strains, one with and the other without Chi (a+x+bX u-Xob+), the X-containing recombinant (a-X+b-) is less abundant than the nonx-containing recombinant (a+xob+). Previously this was taken was evidence for nonreciprocality of X-stimulated exchange. This inequality, however, is now seen to result from an event at cos (X's packaging origin) that both activates Chi and initiates DNA packaging. An event at rightward cos leads to activation of leftward x on the same chromosome for an exchange to its left. From the resulting circulating dimer (-cos-a+-X"-b+-cos-a--x+-b--), the cos that activated x is more likely to be used for rightward packaging initiation than is the cos from the other parent. Consistent with this coupling model is "biased packaging" in X carrying two cos sites per monomer genome. When their maturation is dependent on dimerization by X-stimulated exchange, the phage particles result more often from packaging from the cos that activates x than from packaging from the other cos. Since Chi activation and packaging can be uncoupled, we infer that some early and reversible step in packaging activates x. A strong candidate for this step is a double-strand break at cos that provides an oriented entry site for a recombinase. OLARITY in gene conversion implies special sites on DNA (recombinators) P that locally enhance homologous genetic recombination (LISSOUBA et al. 1962; MURRAY 1963; see STAHL 1979a,b for review). Chi is a special site for the RecBC pathway of recombination in Escherichia coli (for reviews see STAHL 1979a,b; SMITH 1983). When Chi stimulates recombination on bacteriophage lambda, the stimulated crossovers are located primarily to the left of the Chi on the lambda genetic map (leftuard bias) (Figure la) (STAHL and STAHL 1975; STAHL et al. 1980). By inverting DNA segments carrying Chi, FAULDS et al. (1979) and YAGIL et al. (1980) showed that only one orientation of Chi had ' Present address: University of Tokyo Faculty of Medicine, Department of Bacteriology, Hongo 7-3-1, Tokyo-1 13, Japan. Genetics 108: 773-794 December, 1984.