Chi sites are examples of special sites enhancing homologous recombination in their region of the chromosome. Chi, SG-C-T-G-G-T-G-GY, is a recognition site for the RecBC enzyme, which nicks DNA near Chi as it unwinds DNA. A molecular model ofgenetic recombination incorporating these features is reviewed.
Special Sites in Homologous Recombination
During homologous recombination, chromosomes exchange parts to generate recombinant chromosomes. Although exchange can occur at any point of nucleotide sequence homology shared by the parental chromosomes, its frequency is higher in some chromosomal regions than in others. This observation implies the existence of special sites that elevate or depress the frequency of exchange in their vicinity. Such sites were first inferred in the ascomycetes Ascobolus, Neurospora and Saccharomyces.l Sites, called Chi, with similar genetic properties were later identified in bacteriophage A and its host Escherichia coli. Presumably these recombination sites interact with proteins regulating a rate-limiting step of recombination. This article reviews the genetic and molecular properties of Chi and its interaction with RecBC enzyme, and a model of recombination featuring these elements is summarized.