Nuclease activity in a fragment of bacteriophage T4 deoxyribonucleic acid polymerase induced by the amber mutant am B22.

T4 bacteriophage mutant am B22 (gene 43), an amber mutant in the gene for DNA polymerase, produces a protein which lacks the polymerase but retains the nuclease function of the wild type enzyme. The am B22 nuclease is shown to be a fragment of the wild type polymerase by comparing the tryptic peptides of the two proteins. The wild type and mutant enzymes each consist of a single polypeptide chain. The molecular weight of the mutant enzyme is consistent with the position of the am B22 mutation within the gene. The only other gene 43 amber mutants producing a similar nuclease map at or very close to the site of the am B22 mutation. Both the mutant and wild type enzymes are exonucleases which hydrolyze denatured DNA from the 3’ terminus in a random rather than processive fashion. The mutant enzyme differs from the wild type in its markedly decreased affinity for DNA and oligonucleotides. A higher concentration of deoxynucleoside triphosphates is required to inhibit the nuclease activity of the mutant enzyme. The am B22 protein will not polymerize deoxynucleotides under any conditions which have been tested.