Exonuclease III

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].

Function
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].

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