title: The recombination hot spot chi is a regulatory element that switches the polarity of DNA degradation by the RecBCD enzyme.
abstract: Homologous recombination in Escherichia coli is stimulated at DNA sequences known as chi sites. Stimulation requires the multifunctional RecBCD enzyme, which is both a helicase and a 3' --> 5' exonuclease. Upon recognition of a properly oriented chi site, the 3' --> 5' exonuclease activity is attenuated. Here we show that in addition to attenuation of the 3' --> 5' exonuclease activity, recognition of chi 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 chi 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 chi recognition is the production of a recombination intermediate possessing a 3'-ssDNA overhang terminating at the chi 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.
authors.affiliation: Genetics Graduate Group, University of California at Davis 95616-8665, USA.
authors.forename: D G
authors.initials: DG
authors.lastname: Anderson
authors.forename: S C
authors.initials: SC
authors.lastname: Kowalczykowski
mesh_terms: D001665:Binding Sites; D004247:DNA / Q000235:genetics / Q000378:metabolism*; D004277:DNA, Single-Stranded / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli / Q000235:genetics; D043211:Exodeoxyribonuclease V; D005090:Exodeoxyribonucleases / Q000235:genetics* / Q000378:metabolism*; D011995:Recombination, Genetic; D012045:Regulatory Sequences, Nucleic Acid; D013379:Substrate Specificity; D015854:Up-Regulation
chemical_list: D004277:DNA, Single-Stranded; D004247:DNA; D005090:Exodeoxyribonucleases; C020133:exodeoxyribonuclease III; D043211:Exodeoxyribonuclease V