Biochemical characterization of a mutant recBCD enzyme, the recB2109CD enzyme, which lacks chi-specific, but not non-specific, nuclease activity.
RecBCD enzyme of Escherichia coli is a DNA helicase which also possesses ATP-dependent nuclease activities. We have purified a mutant recBCD enzyme, designated recB2109CD enzyme, and have examined the nuclease activities of this protein in vitro to determine whether any alteration in these activities is responsible for the recombination-deficient phenotype of the recB2109 strain. The recB2109CD enzyme possesses all of the non-specific nuclease activities (dsDNA exonuclease and ssDNA exo- and endonuclease) associated with wild-type recBCD enzyme although they are reduced approximately 2 to 3-fold relative to the wild-type enzyme. The ATP-dependent dsDNA exonuclease activity of recB2109CD enzyme requires significantly higher ATP concentrations for optimal activity when compared to the wild-type enzyme. The ATP-independent ssDNA endonuclease activity of the two enzymes is similar, but the ATP-stimulated ssDNA endonuclease and ATP-dependent ssDNA exonuclease activities of the mutant enzyme are reduced relative to those of wild-type recBCD enzyme. Despite its ability to degrade linear dsDNA non-specifically, recB2109CD enzyme lacks sequence-specific nicking activity at chi sites, which are hotspots for genetic recombination. Since this interaction with chi significantly attenuates the non-specific dsDNA exonuclease activity of wild-type recBCD enzyme, these results suggest that the non-specific dsDNA exonuclease activity of the mutant enzyme cannot be attenuated, with the consequence that a DNA substrate which is suitable for recombination is not produced.
Department of Cell, Molecular & Structural Biology, Northwestern University Medical School, Chicago, IL 60611.
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Eggleston
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Kowalczykowski
D000255:Adenosine Triphosphate / Q000378:metabolism; D004265:DNA Helicases / Q000235:genetics* / Q000378:metabolism; D004277:DNA, Single-Stranded / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000201:enzymology; D043211:Exodeoxyribonuclease V; D005090:Exodeoxyribonucleases / Q000235:genetics* / Q000378:metabolism; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism; D009154:Mutation; D013379:Substrate Specificity
D004277:DNA, Single-Stranded; D000255:Adenosine Triphosphate; D005090:Exodeoxyribonucleases; D043211:Exodeoxyribonuclease V; D004265:DNA Helicases; D008274:Magnesium