Studies on the Biochemical Basis of Spontaneous Mutation

The deoxyribonucleic acid polymerases induced by two mutator, two antimutator, and one neutral temperature-sensitive strain of T4D have been partially purified and compared to each other and to the wild type polymerase. Measurements of polymerase, polymerase-associated exonuclease, and deoxynucleoside triphosphate turnover indicate that the mutators and antimutators may be readily distinguished from wild type by the ratios of these activities. The enzymes prepared from mutators have a much lower exonuclease to polymerase ratio than wild type, and the antimutators have a higher ratio of exonuclease to polymerase and a higher rate of nucleotide turnover. Similar measurements with the neutral temperature-sensitive mutant indicate a close resemblance to wild type. On the basis of these experiments we propose that the spontaneous mutation rate is related to the relative rates of insertion and removal of nucleotides during synthesis of deoxyribonucleic acid. Mutations in the structural gene of the polymerase which perturb this exonuclease to polymerase ratio are expressed as mutator or antimutator genes.