In this study we used endonuclease IV from Escherichia coli (Endo IV). The enzyme functions as a Class II AP endonuclease and as a 3 0 terminal phosphodiesterase (13, 14) . Endo IV is enzymatically similar to exonuclease III, the major AP endonuclease of E.coli, but it differs from the latter enzyme mainly in lacking the associated exonuclease activity (14) . It was also found that E.coli Endo IV is fairly heat stable to $70 C (15, 16) .