STRUCTURE AND FUNCTION OF THE MULTIFUNCTIONAL DNA-REPAIR ENZYME EXONUCLEASE-III

THE repair of DNA requires the removal of abasic sites, which are cons tantly generated in vivo both spontaneously(1) and by enzymatic remova l of uracil(2), and of bases damaged by active oxygen species, alkylat ing agents and ionizing radiation The major apurinic/apyrimidinic (AP) DNA-repair endonuclease in Escherichia coli is the multifunctional en zyme exonuclease In, which also exhibits 3'-repair diesterase, 3'-->5' exonuclease, 3'-phosphomonoesterase and ribonuclease activities(5). W e report here the 1.7 Angstrom resolution crystal structure of exonucl ease III which reveals a 2-fold symmetric, four-layered ap fold,vith s imilarities to both deoxyribonuclease I-6 and RNase H-7. In the ternar y complex determined at 2.6 Angstrom resolution, Mn2+ and dCMP bind to exonuclease III at one end of the alpha beta-sandwich, in a region do minated 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 throu gh a nucleophilic attack facilitated by a single bound metal ion.