Hotspots for generalized recombination in the Escherichia coli chromosome

A naturally occurring hotspot for Rec recombination of Escherichia coli was located in the biotin operon. The phenotypes of the bio hotspot as observed in λ bio transducing phage were identical to those of Chi mutations in phage λ. In addition to recA + function, the site-specific stimulation of recombination required recB + function. The stimulation took place when the hotspot was present in only one parent of the cross and even when present opposite a region of heterology. The demonstration of a Chi element in E. coli provoked us to measure the density of Chi elements on the chromosome. E. coli DNA sampled in λ transducing phage (either obtained by induction of secondary site lysogens or made in vitro from EcoRI cleavage fragments) showed one hotspot per 5 to 15 × 10 3 bases. The high density and the fact that Chi stimulation of recombination can span the inter-Chi distance suggest that Chi might be important in Rec recombination in the absence of λ.