Template switching rates of Moloney murine leukemia virus reverse transcriptase mutants were tested using a retroviral vector-based direct-repeat deletion assay. The reverse transcriptase mutants contained alterations in residues that modeling of substrates into the catalytic core had suggested might affect interactions with primer and/or template strands. As assessed by the frequency of functional lacZ gene generation from vectors in which lacZ was disrupted by insertion of a sequence duplication, the frequency of template switching varied more than threefold among fully replication-competent mutants. Some mutants displayed deletion rates that were lower and others displayed rates that were higher than that of wild-type virus. Replication for the mutants with the most significant alterations in template switching frequencies was similar to that of the wild type. These data suggest that reverse transcriptase template switching rates can be altered significantly without destroying normal replication functions.Reverse transcriptase (RT) must perform two template switches-the first and second strong-stop template switches-to complete integration-competent cDNA synthesis (13). It has been proposed that the requirement to perform these two replicative switches confers onto RT the tendency to make additional, nonrequired template switches that can result in genetic recombination (5, 41).Most DNA polymerases do not switch templates as frequently as RT does, which suggests that this process may require some structural or biochemical properties of RT. RT has been mutagenized extensively to identify features important for polymerization, RNase H activity, fidelity, drug resistance, and processivity (39). It is thought that RT pauses before switching templates; therefore, mutants with altered pausing and/or processivity may be affected in template switching (20,27,43,44).In this report, a panel of RT mutants was constructed based on the crystal structure of a catalytic fragment of Moloney murine leukemia virus (MLV) RT (12). Although the biochemical properties of the mutants were not tested here, the basis of these mutants' design was the hypothesis that by limiting interactions with the primer-template, RT template switching rates might be altered. These mutants contained alterations in residues proximal to the primer and/or template strands in a model of nucleic acid bound in the polymerase active site. Experiments presented in this report examined the replication of Moloney MLV mutants harboring these mutations and tested these mutants for effects on template switching during viral replication.
MATERIALS AND METHODSPlasmids. RT mutations were introduced by PCR and standard cloning procedures into the infectious clone pMLV-neo (34) and pMLV ⌿Ϫ, a packagingdefective clone (30), and the entire PCR-generated region for each mutant was confirmed by dideoxynucleotide sequencing.Cells. NIH 3T3 cells and rat2 cells were grown in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% calf serum (Gibco). 293T-and ETbased...