Pseudodiploid Genome Organization Aids Full-Length Human Immunodeficiency Virus Type 1 DNA Synthesis

Template switching between copackaged human immunodeficiency virus type 1 (HIV-1) genomic RNAs is genetically silent when identical RNAs are copackaged but yields recombinants when virions contain two distinct RNAs. Sequencing has revealed that errors at retroviral recombination junctions are infrequent, suggesting that template switching is not intrinsically mutagenic. Here, we tested the hypothesis that template switching may instead contribute to replication fidelity. This hypothesis predicts that reverse transcription of a single-copy gene will be more error prone than replication in the presence of a second copy. To test this, HIV-1-based vectors containing bothlacZ and the puromycin resistance marker were expressed either alone or with an excess of an “empty” vector lackinglacZ andpuro . This resulted in virions with either RNA homodimers or haploid genomes with only a singlelacZ-puro RNA. In untreated cells,lacZ inactivation rates suggested that haploid vector reverse transcription was slightly more error prone than that of homodimerized pseudodiploid vectors. Haploid reverse transcription was at least threefold more error prone than pseudodiploid-templated synthesis when slowed by hydroxyurea treatment or stopped prematurely with zidovudine. Individual products of one- and two-copy genes revealed both nucleotide substitutions and deletions, with deletions more frequent than point mutations among haploid genome products. Similar spectra of defective products were observed at early reverse transcription time points and among products of haploid virions. These results indicate that faithful, full-length reverse transcription products were underrepresented in the absence of a reserve of genetic information and suggest that template switching contributes to HIV-1 genomic integrity.