Infidelity of DNA synthesis by themperature-sensitive DNA polymerases from RNA tumor viruses

Fidelity of two retroviral reverse transcriptases during DNA-dependent DNA synthesis in vitro

We determined the fidelity of avian myeloblastosis virus and Moloney murine leukemia virus reverse transcriptases (RTs) during DNA synthesis in vitro using the M13mp2 lacZ alpha gene as a mutational target. Both RTs commit an error approximately once for every 30,000 nucleotides polymerized. DNA sequence analysis of mutants generated in a forward mutation assay capable of detecting many types of errors demonstrated that avian myeloblastosis virus RT produced a variety of different mutations. The majority (58%) were single-base substitutions; all of which resulted from the misincorporation of either dAMP or dGMP. Minus-one frameshifts were also common, composing about 30% of the mutations. In addition to single-base events, eight mutants contained sequence changes involving from 2 to 59 bases. The frequency of these mutants suggests that, at least during DNA synthesis in vitro, RTs also commit errors by mechanisms other than classical base miscoding and misalignment. We examined the ability of RTs to synthesize DNA from a mismatched primer terminus at a sequence where the mismatched base was complementary to the next base in the template. Unlike cellular DNA polymerases which polymerize from the mismatched template-primer, RTs preferred to polymerize from a rearranged template-primer containing a matched terminal base pair and an unpaired base in the template strand. The unusual preference for this substrate suggests that the interactions between RTs and the template-primer are different from those of cellular DNA polymerases. The overall error rate of RT in vitro is sufficient to account for the estimated mutation rate of these viruses.

Fidelity of two retroviral reverse transcriptases during DNA-dependent DNA synthesis in vitro

We determined the fidelity of avian myeloblastosis virus and Moloney murine leukemia virus reverse transcriptases (RTs) during DNA synthesis in vitro using the M13mp2 lacZ alpha gene as a mutational target. Both RTs commit an error approximately once for every 30,000 nucleotides polymerized. DNA sequence analysis of mutants generated in a forward mutation assay capable of detecting many types of errors demonstrated that avian myeloblastosis virus RT produced a variety of different mutations. The majority (58%) were single-base substitutions; all of which resulted from the misincorporation of either dAMP or dGMP. Minus-one frameshifts were also common, composing about 30% of the mutations. In addition to single-base events, eight mutants contained sequence changes involving from 2 to 59 bases. The frequency of these mutants suggests that, at least during DNA synthesis in vitro, RTs also commit errors by mechanisms other than classical base miscoding and misalignment. We examined the ability of RTs to synthesize DNA from a mismatched primer terminus at a sequence where the mismatched base was complementary to the next base in the template. Unlike cellular DNA polymerases which polymerize from the mismatched template-primer, RTs preferred to polymerize from a rearranged template-primer containing a matched terminal base pair and an unpaired base in the template strand. The unusual preference for this substrate suggests that the interactions between RTs and the template-primer are different from those of cellular DNA polymerases. The overall error rate of RT in vitro is sufficient to account for the estimated mutation rate of these viruses.

Utilization of mismatched initiator termini by avian myeloblastosis virus DNA polymerase

DNA synthesis by avian myeloblastosis virus was studied using poly(C) as template and modified oligo(dG) as primer. The addition of one noncomplementary base to the 3'-end of the primer has no important effect on synthesis. The mispaired base is incorporated into the product and the apparent Km (for primer) and the V of the reaction remain unchanged. This confirms the absence of a 3' leads to 5'-exodeoxynuclease activity using a template that is transcribed faithfully rather than one that can undergo a slippage reaction.

DNA replication in human lymphocytes during aging

An analysis of the accuracy of protein and DNA synthesis in human lymphocytes with respect to aging has been carried out. The response of human peripheral lymphocytes, from young and old adults, to phytohemagglutinin was measured at varying temperatures. This should provide a sensitive test for the accumulation of altered thermolabile proteins that are rate limiting in the response to phytohemagglutinin. At 37 degrees C the rate of thymidine incorporation as well as the induction of DNA polymerase in phytohemagglutinin-stimulated lymphocytes from old and young adults were similar. Also at elevated temperatures, the thermosensitivity of DNA replication in lymphocytes from young and old adults was the same. DNA polymerase was purified from PHA-stimulated lymphocytes from young and old adults. The fidelity of DNA synthesis using poly (dC) as a template was similar with both enzymes. However, DNA polymerase-alpha purified from old adults was thermolabile compared to the enzyme from young adults. Thus, while the lymphocytes from old individuals may have heat labile proteins, they do not limit their proliferative capacity.