title: Structure and function of the multifunctional DNA-repair enzyme exonuclease III.
abstract: The repair of DNA requires the removal of abasic sites, which are constantly generated in vivo both spontaneously and by enzymatic removal of uracil, and of bases damaged by active oxygen species, alkylating agents and ionizing radiation. The major apurinic/apyrimidinic (AP) DNA-repair endonuclease in Escherichia coli is the multifunctional enzyme exonuclease III, which also exhibits 3'-repair diesterase, 3'-->5' exonuclease, 3'-phosphomonoesterase and ribonuclease activities. We report here the 1.7 A resolution crystal structure of exonuclease III which reveals a 2-fold symmetric, four-layered alpha beta fold with similarities to both deoxyribonuclease I and RNase H. In the ternary complex determined at 2.6 A resolution, Mn2+ and dCMP bind to exonuclease III at one end of the alpha beta-sandwich, in a region dominated by positive electrostatic potential. Residues conserved among AP endonucleases from bacteria to man cluster within this active site and appear to participate in phosphate-bond cleavage at AP sites through a nucleophilic attack facilitated by a single bound metal ion.
authors.affiliation: Department of Molecular Biology, Scripps Research Institute, La Jolla, California 92037.
authors.forename: C D
authors.initials: CD
authors.lastname: Mol
authors.forename: C F
authors.initials: CF
authors.lastname: Kuo
authors.forename: M M
authors.initials: MM
authors.lastname: Thayer
authors.forename: R P
authors.initials: RP
authors.lastname: Cunningham
authors.forename: J A
authors.initials: JA
authors.lastname: Tainer
mesh_terms: D000595:Amino Acid Sequence; D003196:Computer Graphics; D018360:Crystallography, X-Ray; D004260:DNA Repair / Q000502:physiology*; D003843:Deoxycytidine Monophosphate / Q000737:chemistry; D003850:Deoxyribonuclease I / Q000737:chemistry; D004563:Electrochemistry; D004926:Escherichia coli / Q000201:enzymology; D005090:Exodeoxyribonucleases / Q000737:chemistry* / Q000502:physiology*; D006801:Humans; D008345:Manganese / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D016914:Ribonuclease H / Q000737:chemistry; D013329:Structure-Activity Relationship
chemical_list: D003843:Deoxycytidine Monophosphate; D008345:Manganese; D005090:Exodeoxyribonucleases; C020133:exodeoxyribonuclease III; D003850:Deoxyribonuclease I; D016914:Ribonuclease H