Phosphodiester phosphoester

Exonuclease III (Exo III) of E. coli is a monomeric multifunctional enzyme (31 kDa) that catalyzes the hydrolysis of at least four different types of phosphoester bonds in dsDNA (Fig. 3.4). The main enzymatic activity of Exo III is the 3 — 5 -exonuclease activity that carries out the successive release of 5 -P-mononucleotides from the 3 ends of dsDNA. The second activity is the DNA 3 -phosphatase activity that hydrolyzes 3 -terminal phosphomonoesters. In fact, Exo III was initially discovered as a DNA 3 -phosphatase in E. coli (1,2). Exo III has a third activity which degrades the RNA strand in a DNA RNA heteroduplex, thus the RNase H activity. The fourth activity of Exo III is an AP endonuclease which cleaves phosphodiester bonds at apurinic or apyrimidinic sites. 

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