Retroviruses (family Retroviridae, notably Human Immunodeficiency Virus Type 1, HIV-1) occupy a unique paradigm in molecular biology: they permanently overwrite the central dogma by converting single-stranded RNA genomes into double-stranded DNA and stably stitching themselves into the host eukaryotic chromosome, establishing lifelong, irreversible latent reservoirs.
1. The Retroviral Virion and Reverse Transcriptase Complex
The retroviral particle encloses a conical fullerene-cone capsid (composed of CA / p24 hexamers and pentamers) encapsidating two identical copies of positive-sense single-stranded genomic RNA ((+ ext{ssRNA})). Base-paired near the (5′)-terminus is a host cellular transfer RNA (( ext{tRNA}^{ ext{Lys,3}}) in HIV-1) serving as the indispensable primer for reverse transcription.
The catalytic engine is Reverse Transcriptase (RT), an asymmetric heterodimer composed of a p66 subunit (containing polymerase and RNase H domains) and a cleaved p51 scaffolding subunit. RT exhibits three distinct enzymatic activities:
- RNA-dependent DNA polymerase (RDDP)
- Ribonuclease H (RNase H): Degrades the RNA strand of RNA:DNA heteroduplexes.
- DNA-dependent DNA polymerase (DDDP)
2. The Stepwise Mechanics of Reverse Transcription
Reverse transcription proceeds through an intricate choreography featuring two obligatory strand transfers (“template jumps”):
- Minus-Strand Strong-Stop DNA Synthesis: RT initiates DNA synthesis from the (3′)-OH of the host tRNA primer annealed to the primer binding site (PBS), copying the (5′)-end of the viral RNA (U5 and R regions) to yield ((-) ext{ss-strong-stop DNA}).
- First Strand Transfer: RNase H degrades the (5′)-RNA template (R and U5). The exposed ((-))ss-strong-stop DNA hybridizes to the complementary identical repetitive (R) sequence at the (3′)-end of either the same or the second viral RNA molecule.
- Elongation of Minus-Strand DNA: RT extends the minus strand toward the (5′)-end of the template, while RNase H progressively degrades the genomic RNA.
- Polypurine Tract (PPT) Resistance: Two purine-rich RNA segments—the central PPT (cPPT) and (3′) PPT—resist RNase H degradation, serving as primers for plus-strand DNA synthesis.
- Plus-Strand Strong-Stop DNA Synthesis: RT synthesizes ((+))DNA from the PPT primers, copying the U3, R, U5, and the first 18 nucleotides of the tRNA primer. RNase H then specifically removes the tRNA primer.
- Second Strand Transfer: The exposed (3′)-PBS on the plus-strand DNA anneals to the complementary PBS at the (3′)-end of the minus-strand DNA.
- Dual Extension: RT completes bidirectional synthesis, producing a full double-stranded linear DNA duplex flanked by complete Long Terminal Repeats (LTRs: U3-R-U5) at both ends.
3. Nuclear Import: The Pre-Integration Complex (PIC)
Simple retroviruses (e.g., Moloney murine leukemia virus) require mitotic nuclear envelope breakdown to access host chromatin. In stark contrast, lentiviruses like HIV-1 actively penetrate intact interphase nuclear pores. The viral reverse transcription complex inside an intact or remodeled capsid navigates through the nuclear pore complex (NPC) via direct interactions between the capsid lattice and nucleoporins (Nup153, Nup358) and transport receptors (Transportin-1), releasing the Pre-Integration Complex (PIC) into the nucleoplasm.
4. The Catalytic Mechanism of Integrase (IN)
Integration of the linear viral cDNA into host chromosomal DNA is catalyzed by the tetrameric or octameric Integrase (IN) intasome through two consecutive transesterification reactions:
Step 1: (3′)-Processing
Integrase cleaves a conserved GT dinucleotide from the (3′)-ends of both LTRs, exposing an invariant CA-3′-OH terminus:
[
ext{Viral DNA-CA-GT-}3′ xrightarrow{ ext{IN}} ext{Viral DNA-CA-}3′ ext{-OH} + ext{pGT}
]
Step 2: Strand Transfer
Integrase introduces a staggered cut (5 base pairs apart in HIV-1) in the target host chromosomal DNA. The exposed viral (3′)-OH groups perform a concerted nucleophilic attack on the phosphodiester bonds on opposite strands of the host DNA:
[
ext{Host-5′-P-O-Host} + ext{Viral-}3′ ext{-OH} longrightarrow ext{Host-5′-P-O-Viral} + ext{Host-}3′ ext{-OH}
]
Host DNA repair machinery (DNA polymerase (beta), ligase) subsequently fills the 5-nucleotide single-stranded gaps and seals the nicks, creating the characteristic 5-base-pair flanking target site duplications. The viral genome is now a permanent provirus, indomitable against all classical antiviral therapies unless eliminated at the chromosomal level.
