title: Nuclease activity is essential for RecBCD recombination in Escherichia coli.
abstract: RecBCD has two conflicting roles in Escherichia coli. (i) As ExoV, it is a potent double-stranded (ds)DNA exonuclease that destroys linear DNA produced by restriction of foreign DNA. (ii) As a recombinase, it promotes repair of dsDNA breaks and genetic recombination in the vicinity of chi recombination hot-spots. These paradoxical roles are accommodated by chi-dependent attenuation of RecBCD exonuclease activity and concomitant conversion of the enzyme to a recombinase. To challenge the proposal that chi converts RecBCD from a destructive exonuclease to a recombinogenic helicase, we mutated the nuclease catalytic centre of RecB and tested the resulting mutants for genetic recombination and DNA repair in vivo. We predicted that, if nuclease activity inhibits recombination and helicase activity is sufficient for recombination, the mutants would be constitutive recombinases, as has been seen in recD null mutants. Conversely, if nuclease activity is required, the mutants would be recombination deficient. Our results indicate that 5' --> 3' exonuclease activity is essential for recombination by RecBCD at chi recombination hot-spots and at dsDNA ends in recD mutants. In the absence of RecB-dependent nuclease function, recombination becomes entirely dependent on the 5' --> 3' single-stranded (ss)DNA exonuclease activity of RecJ and the helicase activity of RecBC(D).
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Miami School of Medicine, PO Box 016129, Miami, FL 33101-6129, USA.
authors.forename: M E
authors.initials: ME
authors.lastname: Jockovich
authors.forename: R S
authors.initials: RS
authors.lastname: Myers
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000502:physiology; D010582:Bacteriophage lambda / Q000235:genetics / Q000502:physiology; D004260:DNA Repair / Q000528:radiation effects; D004261:DNA Replication / Q000528:radiation effects; D004268:DNA-Binding Proteins / Q000235:genetics / Q000502:physiology; D004926:Escherichia coli / Q000201:enzymology* / Q000235:genetics* / Q000528:radiation effects / Q000821:virology; D029968:Escherichia coli Proteins; D043211:Exodeoxyribonuclease V; D005090:Exodeoxyribonucleases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D005838:Genotype; D008957:Models, Genetic; D016296:Mutagenesis / Q000528:radiation effects; D011995:Recombination, Genetic / Q000528:radiation effects; D013379:Substrate Specificity; D014466:Ultraviolet Rays
chemical_list: D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; D029968:Escherichia coli Proteins; C489340:recF protein, E coli; C066218:recF protein, Bacteria; D005090:Exodeoxyribonucleases; D043211:Exodeoxyribonuclease V; C468322:exodeoxyribonuclease V, E coli