Major apurinic-apyrimidinic endonuclease of E.coli. It removes the damaged DNA at cytosines and guanines by cleaving on the 3'-side of the AP site by a beta-elimination reaction. It exhibits 3'-5'-exonuclease, 3'-phosphomonoesterase, 3'-repair diesterase and ribonuclease H activities (By similarity). Exonucleolytic cleavage in the 3'- to 5'-direction to yield nucleoside 5'-phosphates. Probably binds two magnesium or manganese ions per subunit. Monomer. Belongs to the DNA repair enzymes AP/ExoA family.