EXONUCLEASE-IX OF ESCHERICHIA-COLI

The bacteria Escherichia coli contains several exonucleases acting on both double-and single-stranded DNA and in both a 5' --> 3' and 3' --> 5' direction. These enzymes are involved in replicative, repair and r ecombination functions. We have identified a new exonuclease found in E.coli, termed exonuclease IX, that acts preferentially on single-stra nded DNA as a 3' --> 5' exonuclease and also functions as a 3' phospho diesterase on DNA containing 3'-incised apurinic/ apyrimidinic (AP) si tes to remove the product trans-4-hydroxy-2-pentenal 5-phosphate. The enzyme showed essentially no activity as a deoxyribophosphodiesterase acting on 5'-incised AP sites, The activity was isolated as a glutathi one S-transferase fusion protein from a sequence of the E.coli genome that was 60% identical to a 260 bp region of the small fragment of the DNA polymerase I gene. The protein has a molecular weight of 28 kDa a nd is free of AP endonuclease and phosphatase activities. Exonuclease IX is expressed in E.coli, as demonstrated by reverse transcription-PC R, and it may function in the DNA base excision repair and other pathw ays.