Heterogeneous structures formed by conserved RNA sequences within the HIV reverse transcription initiation site

Reverse transcription is a key process in the early steps of HIV infection. This process initiates within a specific complex formed by the 5' UTR of the HIV genomic RNA (vRNA) and a host primer tRNA^Lys₃ Using nuclear magnetic resonance (NMR) spectroscopy and single-molecule fluorescence spectroscopy, we detect two distinct conformers adopted by the tRNA/vRNA initiation complex. We directly show that an interaction between the conserved 8-nucleotide viral RNA primer activation signal (PAS) and the primer tRNA occurs in one of these conformers. This intermolecular PAS interaction likely induces strain on a vRNA intramolecular helix, which must be broken for reverse transcription to initiate. We propose a mechanism by which this vRNA/tRNA conformer relieves the kinetic block formed by the vRNA intramolecular helix to initiate reverse transcription.