Chi site

A Chi site or Chi sequence is a short stretch of DNA in the genome of a bacterium near which homologous recombination is more likely than expected to occur. Chi sites serve as stimulators of DNA double-strand break repair in bacteria, which can arise from radiation or chemical treatments, or result from replication fork breakage during DNA replication. The sequence of the Chi site is unique to each group of closely related organisms; in E. coli and other enteric bacteria, such as Salmonella, the sequence is 5'-GCTGGTGG-3'. The existence of Chi sites was originally discovered in the genome of bacteriophage lambda, a virus that infects E. coli, but is now known to occur about 1000 times in the E. coli genome.