The effect of several divalent cations on the accuracy of DNA replication in vitro has been examined. Only Be2+ altered the accuracy of DNA synthesis using purified DNA polymerase (DNA nucleotidyltransferase; deoxynucleosidetriphosphate:DNA deoxynucleotidyltransferase; EC 2.7.7.7) from avian myeloblastosis virus. The Be2+-induced base substitutions occurred with all templates and with all nucleotides tested. Analysis of the product by equilibrium density centrifugation and processive hydrolysis with snake venom phosphodiesterase suggested that the noncomplementary nucleotides were present in phosphodiester linkage. Nearest (AMV), it is possible to quantitate accurately the incorporation of noncomplementary nucleotides, and to determine the reaction parameters that influence base-substitution in vtro. AMV DNA polymerase lacks detectable 3'-5' proof-reading exonuclease activity (7) and thus may not excise a mismatched nucleotide (8-10).The error frequency of AMV DNA polymerase is not influenced by most alterations in the reaction parameters (4,5,7). Accordingly, we have tested exogenous agents to determine their influence on the error frequency. We now report evidence that the treatment of AMV DNA polymerase by beryllium, a carcinogenic metal, increases base substitutions during polynucleotide replication. Modification of AMY DNA polymerase by beryllium is completely reversible, resulting in the restoration of the initial error frequency of the polymerase.MATERIALS AND METHODS Materials. Unlabeled deoxynucleotides were purchased from Calbiochem. Tritium-labeled nucleotides were purchased from New England Nuclear or from Amersham-Searle; 32P-labeled nucleotides were obtained from New England Nuclear.Poly[d(A-T)] was prepared by a de novo catalyzed reaction using Escherichia coli DNA polymerase I (11). All other synthetic polynucleotide templates were purchased from P. L.