Synthesis of integration-competent, double-stranded DNA from the (؉)-RNA strand genome of retroviruses and long terminal repeat-containing retrotransposons reflects a multistep process catalyzed by the virus-encoded reverse transcriptase (RT). In conjunction with RNA-and DNA-templated DNA synthesis, a hydrolytic activity of the same enzyme (RNase H) is required to remove genomic RNA of the RNA/DNA replication intermediate. Together, these combined synthetic and degradative functions ensure correct selection, extension, and removal of the RNA primers of (؊)-and (؉)-strand DNA synthesis (tRNA and the polypurine tract, respectively). For HIV-1 RT, a quarter century of research has not only illuminated the biochemical properties, structure, and conformational dynamics of this highly versatile enzyme but has also witnessed drug discovery advances from the first Food and Drug Administration-approved anti-RT drug to recent use of RT inhibitors as potential colorectal microbicides. Salient features of HIV-1 RT and extension of these findings into programs of drug discovery are reviewed here.
HIV-1 DNA SynthesisThe individual steps of HIV-1 DNA synthesis, catalyzed by the multifunctional reverse transcriptase (RT), 2 are summarized schematically in Fig. 1. (Ϫ)-Strand DNA synthesis, initiated from a cellular tRNA (tRNA 3 Lys ) hybridized to the genomeencoded primer-binding site, continues to the 5Ј terminus, creating (Ϫ)-strong-stop DNA. RNase H-mediated degradation of the resulting RNA/DNA hybrid promotes relocation of nascent (Ϫ)-DNA to the genome 3Ј terminus by a strand transfer event that exploits sequence homology between the 5Ј and 3Ј termini. RNA-templated DNA synthesis continues, accompanied by RNase H-mediated degradation of the RNA genome, the exception to which are two short purine-rich segments (the 3Ј-and central polypurine tracts (PPTs)) from which (ϩ)-strand DNA-dependent DNA synthesis is initiated. Newly synthesized (Ϫ)-strand DNA and 18 nucleotides of the covalently attached tRNA 3 Lys primer provide the template for 3Ј-PPTprimed (ϩ)-strand DNA synthesis until the replication complex stalls at a position corresponding to the first modified tRNA base (A57). As a consequence, the C-terminal RNase H domain is positioned at the (Ϫ)-DNA/tRNA junction, and degradation of the tRNA "template" promotes a second or (ϩ)-strand transfer event supported by homology between (Ϫ)-and (ϩ)-strand DNA primer-binding sites. Although bidirectional DNA synthesis would be sufficient to complete DNA synthesis, HIV utilizes a second, central PPT primer, thereby producing a (ϩ)-strand discontinuity (1). Following (ϩ)-strand transfer, 3Ј-PPT-mediated DNA synthesis continues, displacing ϳ100 nucleotides of central PPT-primed (ϩ)-DNA, but abruptly ceases at the central termination sequence, a prominent feature of which is phased A-tracts that induce minor groove compression (1-3), creating the "central flap" (Fig. 1) (4 -7). Although central flap function remains controversial (8, 9), its mutation or deletion has been shown to impair...