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Exonuclease III

Exonuclease III
Symbol  ?
Alt. Symbols ExoIII
Entrez  ?
RefSeq  ?
UniProt P09030
Other data
EC number
Locus Chr. N/A [1]

Exonuclease III (ExoIII) is an enzyme that belongs to the exonuclease family. ExoIII catalyzes the stepwise removal of mononucleotides from 3´-hydroxyl termini of duplex DNA [1]. A limited number of nucleotides are removed during each binding event, resulting in coordinated progressive deletions within the population of DNA molecules [2].

Additional recommended knowledge


The preferred substrates are blunt or recessed 3´-termini, although ExoIII also acts at nicks in duplex DNA to produce single-strand gaps. The enzyme is not active on single-stranded DNA, and thus 3´-protruding termini are resistant to cleavage. The degree of resistance depends on the length of the extension, with extensions 4 bases or longer being essentially resistant to cleavage. This property is used to produce unidirectional deletions from a linear molecule with one resistant (3´-overhang) and one susceptible (blunt or 5´-overhang) terminus [3].

ExoIII activity depends partially on the DNA helical structure [4] and displays sequence dependence (C>A=T>G) [5].

ExoIII has also been reported to have RNase H, 3´-phosphatase and AP-endonuclease activities [1].


Temperature, salt concentration and the ratio of enzyme to DNA greatly affect enzyme activity, requiring reaction conditions to be tailored to specific applications.


  1. Exonuclease III of Escherichia coli K-12, an AP endonuclease. Methods Enzymol. 1980; 65: 201-11.
  2. Molecular Cloning: A Laboratory Manual. 1989, 2nd. Ed., 5.84-5.85.
  3. Unidirectional digestion with exonuclease III creates targeted breakpoints for DNA sequencing. Gene. 1984; 28: 351-9 Abstract
  4. A deoxyribonucleic acid phosphatase-exonuclease from Escherichia coli. II. Characterization of the exonuclease activity. J Biol Chem. 1964; 239: 251-8 Free text (PDF - 937KB)
  5. Sequence specificity of exonuclease III from E. coli. Nucleic Acids Res. 1982; 10: 4845-59. Free text
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Exonuclease_III". A list of authors is available in Wikipedia.
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