The bacteria Escherichia coli contains several exonucleases acting on both double- and single-stranded DNA, and in both a 5′ → 3′ and a 3′ → 5′ direction. These enzymes are involved in replicative, repair and recombination functions. A new exonuclease recently identified in E. coli, termed exonuclease IX, acts preferentially on single-stranded DNA as a 3′ → 5′ exonuclease and also functions as a 3′ phosphodiesterase on DNA containing 3′ incised apurinic/apyrimidinic (AP) sites to remove the product trans-4-hydroxy-2-pentenal-5-phosphate. We now demonstrate that the enzyme is also able to remove 3′ phosphoglycolate end groups from DNA. This activity may have an important role in DNA base excision repair in E. coli.

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