Escherichia coli endonuclease IV hydrolyses the C(3′)-O-P bond 5′ to a 3′-terminal base-free deoxyribose. It also hydrolyses the C(3′)-O-P bond 5′ to a 3′-terminal base-free 2′,3′-unsaturated sugar produced by nicking 3′ to an AP (apurinic or apyrimidinic) site by beta-elimination; this explains why the unproductive end produced by beta-elimination is converted by the enzyme into a 3′-OH end able to prime DNA synthesis. The action of E. coli endonuclease IV on an internal AP site is more complex: in a first step the C(3′)-O-P bond 5′ to the AP site is hydrolysed, but in a second step the 5′-terminal base-free deoxyribose 5′-phosphate is lost. This loss is due to a spontaneous beta-elimination reaction in which the enzyme plays no role. The extreme lability of the C(3′)-O-P bond 3′ to a 5′-terminal AP site contrasts with the relative stability of the same bond 3′ to an internal AP site; in the absence of beta-elimination catalysts, at 37 degrees C the half-life of the former is about 2 h and that of the latter 200 h. The extreme lability of a 5′-terminal AP site means that, after nicking 5′ to an AP site with an AP endonuclease, in principle no 5′----3′ exonuclease is needed to excise the AP site: it falls off spontaneously. We have repaired DNA containing AP sites with an AP endonuclease (E. coli endonuclease IV or the chromatin AP endonuclease from rat liver), a DNA polymerase devoid of 5′----3′ exonuclease activity (Klenow polymerase or rat liver DNA polymerase beta) and a DNA ligase. Catalysts of beta-elimination, such as spermine, can drastically shorten the already brief half-life of a 5′-terminal AP site; it is what very probably happens in the chromatin of eukaryotic cells. E. coli endonuclease IV also probably participates in the repair of strand breaks produced by ionizing radiations: as E. coli endonuclease VI/exonuclease III, it is a 3′-phosphoglycollatase and also a 3′-phosphatase. The 3′-phosphatase activity of E. coli endonuclease VI/exonuclease III and E. coli endonuclease IV can also be useful when the AP site has been excised by a beta delta-elimination reaction.

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