Treatment of DNA containing AP sites with either T4 UV endonuclease or with E. coli endonuclease III followed by a human class II AP endonuclease releases a putative β-elimination product. This result suggests that both the T4 endonuclease and E. coli endonuclease III class I AP endonucleases catalyze pbosphodiester bond cleavage via a lyase- rathar than a hydrolase mechanism. Indeed, we have not detected a class I AP endonuclease which hydrolytically catalyzes phosphodiester bond cleavage. Whereas these enzymes use a lyase-like rather than a hydrolytic mechanism, they nonetheless catalyze phosphodiester bond cleavage. We suggest that the term endonuclease can be properly applied to them.
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