Fig. 2. Schematic diagram of the events in reverse transcription.
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Step 1. HIV reverse transcription is initiated by a cellular tRNA primer, following annealing of the 3’ 18 nt of the tRNA to the 18-nt primer binding site (PBS) near the 5’ end of the genome. Reverse transcriptase (RT) catalyzes synthesis of (-) strong-stop DNA [(-) SSDNA], which contains copies of the repeat (R) sequence and the unique 5’ genomic sequence (U5).
Step 2. As the primer is extended, the RNase H activity of RT degrades the genomic RNA sequences that have been reverse transcribed.
Step 3. (-) SSDNA is transferred to the 3’ end of viral RNA (minus-strand transfer).
Step 4. Elongation of minus-strand DNA and RNase H degradation continue. Plus-strand synthesis is initiated by the 15-nt polypurine tract (PPT) immediately upstream of the unique 3’ genomic sequence (U3). RT copies the u3, u5, and r regions in minus-strand DNA, as well as the 3’ 18 nt of the tRNA primer, thereby reconstituting the PBS. The product formed is termed (+) SSDNA.
Step 5. RNase H removal of the PPT and tRNA primers.
Step 6. Transfer of (+) SSDNA to the 3' end of minus-strand DNA (plus-strand transfer) is followed by circularization of the two DNA strands and displacement synthesis.
Step 7. Minus- and plus-strand DNAs are elongated, resulting in a linear double-stranded DNA with a long terminal repeat (LTR) at each end.
Note: Viral RNA is shown in red, minus-and plus-strand DNAs are shown in blue and green, respectively, and the tRNA primer is in orange. Minus-strand and plus-strand sequences are depicted in lower and upper case, respectively. The dashed red lines represent RNase H cleavage of genomic RNA. (Figure from Post et al. (2003) J. Virol. 77:7623-7634.)