The calf 5'- to 3'-exonuclease is also an endonuclease with both activities dependent on primers annealed upstream of the point of cleavage

Abstract

The catalytic activity of the calf thymus 5'- to 3'-exonuclease was measured on substrates consisting of two primers annealed adjacent to each other on a template. Exonucleolytic degradation of the downstream primer is very slow if the primers are separated by a gap of one nucleotide or if no upstream primer is present. When only a nick separates the primers, degradation is rapid. This suggests that the nuclease is designed to work with calf DNA polymerases such that synthesis from an upstream primer creates the favored nuclease substrate. Nuclease action then destroys the substrate, but it is regenerated by further polymerization. This process, termed nick translation, is necessary for both DNA replication and repair. If the downstream primer has an unannealed 5'-region, that region is removed by an endonuclease activity residing in the same enzyme. Efficient endonuclease action also requires an upstream primer that is annealed such that its 3'-end is directly adjacent to the annealed region of the downstream primer. This reaction is likely to be important for removal of DNA segments that are damaged such that exonuclease cleavage of the damaged site is not possible.