Primer unblocking and rescue of DNA synthesis by azidothymidine (AZT)-resistant HIV-1 reverse transcriptase: comparison between initiation and elongation of reverse transcription and between (-) and (+) strand DNA synthesis.

Azidothymidine (AZT) is a widely used inhibitor of type 1 human immunodeficiency virus reverse transcriptase (RT) that acts as chain terminator. Upon treatment, mutations conferring AZT resistance to RT are gradually selected. It has been shown that resistant RT is able to unblock the AZT-terminated primer by an ATP-dependent mechanism. However, this resistance mechanism has only been demonstrated for DNA-dependent DNA elongation. Here, we compared the AZT resistance of mutant RT during DNA elongation on DNA and RNA templates. We showed that, during DNA elongation, primer unblocking and rescue of DNA synthesis take place with similar rate constants on DNA and RNA templates. However, the fraction of a primer eventually repaired during RNA-dependent DNA synthesis is 2x lower compared with that of DNA-dependent synthesis, leading to reduced resistance. We also compared the initiation of reverse transcription, which uses tRNA(3)(Lys) as a primer and displays characteristic kinetic features, and the subsequent RNA-dependent elongation. Unlike during elongation, resistant RT was unable to unblock the AZT-terminated primer during initiation of (-) DNA strand synthesis. Our results demonstrate that the efficiency of primer unblocking conferred by the AZT resistance mutations greatly vary during the different steps of the provirus synthesis. These results also suggest that inhibitors specifically targeting the initiation of reverse transcription might prove to be advantageous, as compared with elongation inhibitors.