Exonuclease - E. Coli Types

E. Coli Types

In 1971, Lehman IR discovered exonuclease I in E. coli. Since that time, there have been numerous discoveries including: exonuclease, II, III, IV, V, VI, VII, and VIII. Each type of exonuclease has a specific type of function or requirement.

Exonuclease I breaks apart single-stranded DNA in a 3’ → 5’ direction, releasing deoxyribonucleoside 5'-monophosphates one after another. It does not cleave DNA strands without terminal 3'-OH groups because they are blocked by phosphoryl or acetyl groups.

Exonuclease II is associated with DNA polymerase I, which contains a 5’ exonuclease that clips off the RNA primer contained immediately upstream from the site of DNA synthesis in a 5’ → 3’ manner.

Exonuclease III has four catalytic activities:

• 3’ to 5’ exodeoxyribonuclease activity, which is specific for double-stranded DNA
• RNase activity
• 3’ phosphate activity
• AP endonuclease activity (later found to be called endonuclease II).

Exonuclease IV adds a water molecule, so it can break the bond of an oligonucleotide to nucleoside 5’ monophosphate. This exonuclease requires Mg 2+ in order to function and works at higher temperatures than exonuclease I.

Exonuclease V is a 3’ to 5’ hydrolyzing enzyme that catalyzes linear double-stranded DNA and single-stranded DNA, which requires Ca2+. This enzyme is extremely important in the process of homologous recombination.

Exonuclease VIII is 5’ to 3’ dimeric protein that does not require ATP or any gaps or nicks in the strand, but requires a free 5’ OH group to carry out its function.