The recombination hot spot χ is a regulatory element that switches the polarity of DNA degradation by the RecBCD enzyme

Daniel G. Anderson, Stephen C. Kowalczykowski

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143 Scopus citations


Homologous recombination in Escherichia coli is stimulated at DNA sequences known as χ sites. Stimulation requires the multifunctional RecBCD enzyme, which is both a helicase and a 3' → 5' exonuclease. Upon recognition of a properly oriented χ site, the 3' → 5' exonuclease activity is attenuated. Here we show that in addition to attenuation of the 3' → 5' exonuclease activity, recognition of χ by the RecBCD enzyme also up- regulates a nuclease activity of the opposite polarity, resulting in an enzyme that now preferentially degrades 5' → 3'. These results demonstrate that χ is a unique regulatory element that converts the antirecombinogenic form of the RecBCD enzyme into a recombinogenic form by causing two distinct enzymatic changes: attenuation of the 3' → 5' nuclease activity, and up- regulation of the 5' → 3' nuclease activity. The consequence of χ recognition is the production of a recombination intermediate possessing a 3'-ssDNA overhang terminating at the χ sequence. This processing of a dsDNA end to a 3'-ssDNA overhang parallels that which occurs during the initation of homologous recombination in other pathways in E. coli, and in other organisms such as the yeast Saccharomyces cerevisiae.

Original languageEnglish (US)
Pages (from-to)571-581
Number of pages11
JournalGenes and Development
Issue number5
StatePublished - Mar 1 1997



  • χ
  • helicase
  • nuclease
  • RecBCD
  • Recombination
  • regulation

ASJC Scopus subject areas

  • Genetics
  • Developmental Biology

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