Exonuclease VII of Escherichia coli. Mechanism of action.

Abstract Exonuclease VII from Escherichia coli is specific for single-stranded DNA, degrading denatured DNA, single-stranded regions extending from the ends of duplex DNA, or displaced single-stranded regions. Hydrolysis is initiated at both 3' and 5' termini. Exonuclease VII can also excise thymine dimers from duplex DNA following treatment of the DNA with the Micrococcus luteus endonuclease specific for thymine dimers. The enzyme has no detectable activity on RNA or DNA-RNA hybrid molecules. The limit products of exonuclease VII action are oligonucleotides bearing 5'-phosphoryl and 3'-hydroxyl termini predominantly in the range of tetramers to dodecamers; no mononucleotide products have been observed. The enzyme acts in a processive fashion, initially releasing large, acid-insoluble oligonucleotides which can be degraded further to produce a limit product of acid-soluble oligonucleotides. The possible role of exonuclease VII in recombination and in the repair of ultra-violet damage to DNA is discussed.