Initiation complex dynamics direct the transitions between distinct phases of early HIV reverse transcription.

Human immunodeficiency virus (HIV) initiates reverse transcription of its viral RNA (vRNA) genome from a cellular tRNA(3)(Lys) primer. This process is characterized by a slow initiation phase with specific pauses, followed by a fast elongation phase. We report a single-molecule study that monitors the dynamics of individual initiation complexes, comprised of vRNA, tRNA and HIV reverse transcriptase (RT). RT transitions between two opposite binding orientations on tRNA-vRNA complexes, and the prominent pausing events are related to RT binding in a flipped orientation opposite to the polymerization-competent configuration. A stem-loop structure within the vRNA is responsible for maintaining the enzyme predominantly in this flipped orientation. Disruption of the stem-loop structure triggers the initiation-to-elongation transition. These results highlight the important role of the structural dynamics of the initiation complex in directing transitions between early reverse transcription phases.