MARKED INFIDELITY OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 REVERSE-TRANSCRIPTASE AT RNA AND DNA-TEMPLATE ENDS

Human immunodeficiency virus type 1 (HIV-1) is genetically highly vari able. This is attributed to the error-prone nature of HIV-1 replicatio n and its proclivity for recombination. During replication and recombi nation, reverse transcriptase (RT) must polymerize DNA to the 5' ends of multiple RNA and DNA template termini while converting HIV-1 RNA to double-stranded DNA. We have determined the fidelity of HIV-1 RT in v itro during polymerization to the 5' ends of HIV-1 long terminal repea t DNA template sequences and to the end of a partial HIV-1 genomic RNA template that mimics a recombination intermediate. HIV-1 RT readily e xtended recessed DNA primers to form full-length blunt-end DNA.DNA and DNA.RNA duplexes. In addition, HIV-1 RT catalyzed high yields of prod ucts with one to four extra nucleotides at the 3' ends of the nascent DNAs. These products were formed processively via a nontemplated mecha nism that is highly specific for the addition of purine nucleotides (A > G >> T greater than or equal to C). Thus, HIV-1 RT is extremely unf aithful at both DNA and RNA template ends, introducing errors (extra n ucleotides) in one out of every two or three nascent strands processiv ely polymerized. This error rate is 1000 times higher than for HIV-1 R T-catalyzed errors at internal template positions. Blunt-end additions were also catalyzed by other retroviral RTs at relative rates of HIV- 1 approximate to Moloney murine leukemia virus > avian myeloblastosis virus. These data suggest a potentially important mechanism for retrov iral mutation mediated by nontemplated blunt-end addition of purines p rior to forced copy-choice recombination.