DNA synthesis exhibited by the reverse transcriptase of mouse mammary tumor virus: processivity and fidelity of misinsertion and mispair extension.

We have recently expressed in bacteria an enzymatically active reverse transcriptase (RT) of mouse mammary tumor virus (MMTV), a mammalian retrovirus with a typical B-type morphology [Taube, R., Loya, S., Avidan, O., Perach, M. & Hizi, A. (1998) Biochemical J. 329, 579-587]. The purified recombinant protein was shown to possess the catalytic activities characteristic of retroviral reverse transcriptases. In the present study, we have analyzed two basic parameters characteristic of the DNA polymerase activity of the novel MMTV RT, namely the processivity and the fidelity of DNA synthesis. Two features related to fidelity were studied, the capacity to misinsert wrong nucleotides at the 3' end of the nascent DNA strand and the ability to extend 3' mispairs. The studied properties of MMTV RT were compared with those of the RT purified from virions of avian myeloblastosis virus (AMV), since AMV RT shows a relatively high sequence similarity to MMTV RT. MMTV RT shows a relative processivity of DNA synthesis which is as high as the reference AMV RT. Regarding fidelity of DNA synthesis, MMTV RT shows a fidelity of misinsertion lower than that of AMV RT, whereas its capacity to elongate mispaired DNA is lower than that of AMV RT indicating a somewhat higher fidelity. These fidelity properties are discussed also in the context of the RTs of lentiviruses, especially those of HIV, which were reported to exhibit an exceptionally low fidelity of DNA synthesis. It is clear that MMTV RT has a fidelity higher than that of lentiviral RTs.