Exonuclease III

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.

ExoIII activity depends partially on the DNA helical structure and displays sequence dependence (CA=TG).

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

Current Studies

There are many different exonucleases and many are still to be discovered in bacteria, current studies are being conducted in E. coli. Many exonucleases fall into superfamilies with different domains of life proving that exonuclease III has shown to be ancient. Exonucleases evolved early in the history of life and have vital biological roles.

Regulation

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

References

References

1. (March 1995). "Structure and function of the multifunctional DNA-repair enzyme exonuclease III". Nature.
2. Image rendered in MacPyMOL©2006 DeLano Scientific
3. (1980). "Nucleic Acids Part I".
4. (1989). "Molecular cloning: a laboratory manual". Cold Spring Harbor Laboratory.
5. (June 1984). "Unidirectional digestion with exonuclease III creates targeted breakpoints for DNA sequencing". Gene.
6. (December 2011). "The DNA Exonucleases of Escherichia coli". EcoSal Plus.