Clustering of mutations inactivating a Chi recombinational hotspot

Chi sites promote Rec-mediated recombination in bacteriophage λ. Nine independent, nitrous acid-induced mutations were obtained within one of these sites, χ +C. Eight of the mutations completely inactivated the Chi site, while one mutation left partial activity. Nucleotide sequence analysis showed that the mutations were located at four different sites one to four base-pairs from the site of the χ +C mutation that created the active Chi locus. This interval is within a region of homology common to the χ +C locus and another sequenced Chi locus, χ +B. These results support the view that Chi is a unique nucleotide sequence and suggest the extent of the Chi sequence.