Chi sites are recombination hot spots in the genomes of enteric bacteria. Chi is the DNA sequence 5' GCTGGTGG 3'. When RecBCD encounters a Chi sequence, its function changes from an exonuclease to a helicase, producing a single strand of DNA that can invade homologous double-stranded DNA mediated by RecA (i.e., after encountering Chi, RecBCD behaves like a RecD mutant.)
Chi occurs approximately once every 5 Kb on the chromosome, about 1000x more frequent than expected for a sequence of this length. Most of the Chi sites are oriented in the same direction as DNA replication.
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Last modified July 15, 2002