Deoxynucleoside 1-thiotriphosphates prevent proofreading during in vitro DNA synthesis.

The contribution of proofreading to the fidelity of catalysis by DNA polymerases has been determined with deoxyribonucleoside 1-thiotriphosphate substrates. These analogues, which contain a sulfur in place of an oxygen on the alpha phosphorus, are incorporated into DNA by DNA polymerases at rates similar to those of the corresponding unmodified deoxynucleoside triphosphates. The fidelity of DNA synthesis was measured with phi X174 am3 DNA; reversion to wild type occurs most frequently by a single base substitution, a C for a T at position 587. By using avian myeloblastosis virus DNA polymerase and DNA polymerase beta (enzymes without a proofreading 3' leads to 5' exonucleolytic activity), substitution of deoxycytidine thiotriphosphate in the reaction mixture did not alter fidelity. In contrast, with DNA polymerases from E. coli (DNA polymerase I) and bacteriophage T4 (enzymes containing a proofreading activity), fidelity was markedly reduced with deoxycytidine 1-thiotriphosphate. DNA containing phosphorothioate nucleotides is insensitive to hydrolysis by the exonuclease associated with these prokaryotic DNA polymerases. These combined results indicate that the deoxynucleoside 1-thiotriphosphates have normal base-pairing properties; however, once misinserted by a polymerase, they are not excised by proofreading. Proofreading of a C:A mismatch at position 587 is thereby found to contribute 20-fold to the fidelity of E. coli DNA polymerase I and a greater amount to the fidelity of bacteriophage T4 DNA polymerase.