Binding and kinetic properties of HIV-1 reverse transcriptase markedly differ during initiation and elongation of reverse transcription

We recently showed that primer tRNA(3)(Lys), human immunodeficiency virus type 1 (HIV-1) RNA and HIV-1 reverse transcriptase (RT) form a specific complex of initiation of reverse transcription that can be functionally distinguished from the elongation complex, which can be obtained by substituting an 18mer oligodeoxy-ribonucleotide (ODN) for the natural primer (Isel et al., 1996). Here, we compared the binding properties and the single and multiple turnover kinetics of HIV-1 RT in the initiation and elongation complexes. Even though the equilibrium dissociation constants of HIV-1 RT are not very different for the two complexes, RT dissociates similar to 200-fold faster from the initiation complex. Furthermore, nucleotide incorporation by the preformed primer-template-RT complexes is reduced by a similar to 50-fold factor during initiation of reverse transcription, compared with elongation. As a consequence, processivity of HIV-1 RT in the initiation complex is close to unity while it increases by four orders of magnitude during elongation, as expected for a replication enzyme. This processivity change is reminiscent of the transition from initiation to elongation of transcription, Furthermore, our results indicate that the post-transcriptional modifications of tRNA(3)(Lys) play a role similar to that of the sigma factor in transcription by the Escherichia coli RNA polymerase: they favour the formation of the specific initiation complex but do not affect the polymerization rate of the bound enzyme.