No abstract

Classical genetic selection was combined with site-directed mutagenesis to study bacteriophage T4 DNA polymerase 3' -S 5' exonuclease activity. A mutant DNA polymerase with very little (<1%) 3' -*5' exonuclease activity was generated. In vivo, the 3' -*5' exonuclease-deficient DNA polymerase produced the highest level ofspontaneous mutation observed in T4, 500-to 1800-fold above that of wild type. The large reduction in 3' -* 5' exonuclease activity appears to be due to two amino acid substitutions: clease motif. Therefore, extrapolation from E. coli DNA polymerase I sequence and structure to other DNA polymerases for which there is no structural information may not be valid.Bacteriophage T4 DNA polymerase is one of the best experimental systems for studying the role of DNA polymerase exonucleolytic proofreading in enhancing DNA replication fidelity (1-10). Wild-type T4 DNA polymerase has a potent 3' 5' exonuclease activity (11), which is important for accurate DNA replication. Mutant DNA polymerases with reduced 3' -* 5' exonuclease activity produce more DNA replication errors (mutator phenotype), whereas mutants with elevated 3' 5' exonuclease activity, relative to polymerizing activity, increase DNA replication accuracy (antimutator phenotype) (3). The mutator phenotype was used to select mutant DNA polymerases with reduced 3' -* 5' exonuclease activity; amino acid substitutions in the mutants were clustered'between T4 DNA polymerase residues 255 and 363 (8)(9)(10)12). Although amino acid changes within this region decreased 3' -* 5' exonuclease activity, the' mutant DNA polymerases still retained significant residual proofreading activity, which suggests that these particular residues do not function catalytically.In the case of Escherichia coli DNA polymerase I (pol I), residues essential for 3' 5' exonuclease activity have been identified. They include four metal ion binding residues; The hypothesis drawn from these sequence comparisons was that many eukaryotic, viral, and bacteriophage DNA polymerases have a conserved 3' -* 5' exonuclease domain similar to that of E. coli pol 1. T4 genetic studies are consistent with this proposal because mutations that reduce 3' -* 5' exonuclease activity are located near the proposed conserved metal ion binding residues. The hypothesis was tested directly in phage 429'DNA polymerase by substituting alanine residues for proposed conserved metal ion binding residues, which resulted in a 1000-fold reduction in 3' -* 5' exonuclease activity without affecting polymerization activity (23). The 429 DNA polymerase result compares favorably with the 105-fold reduction observed when alanine residues were substituted for E. coli pol I residues .We present here in vitro mutagenesis and genetic studies that were designed to test if proposed T4 DNA' polymerase metal ion binding residues, identified on the basis of sequence similarities to those of E. coli pol l (8, 23), are required for 3' 5' exonuclease activity. In 'contrast to the 429 DNA polymerase'studies, alanine subs...

Mutations that conferred sensitivity to the pyrophosphate analog phosphonoacetic acid in bacteriophage T4 DNA polymerase were identified. The mutations were loosely clustered in four regions of the gene. As found for herpes simplex virus DNA polymerase, T4 mutations that altered sensitivity to phosphonoacetic acid also altered sensitivity to nucleotide analogs. Some of the T4 DNA polymerase mutations also altered the ability of the enzyme to translocate from one template position to the next and affected DNA replication fidelity.