Role of a dipeptide insertion between codons 69 and 70 of HIV-1 reverse transcriptase in the mechanism of AZT resistance

The 3'-azido-3'-deoxythymidine (AZT)-resistant phenotype of a heavily mutat ed human immunodeficiency virus type 1 (HPV-1) reverse transcriptase (RT) c arrying a dipeptide (Ser-Ser) insertion between codons 69 and 70 as well as other mutations related to resistance to RT inhibitors has been studied. R ecombinant virus carrying this variant RT (termed SS RT) showed reduced sus ceptibility to all nucleoside RT inhibitors in clinical use, particularly t o AZT, In the presence of ATP, recombinant SS RT had an increased ability t o remove the 3'-terminal nucleotide from AZT-terminated primers and extend the unblocked printer, compared with wild-type HIV-1 RT (BH10 isolate), Ins ertion of two serines in the sequence context of BH10 RT did not affect the ATP-dependent phosphorolytic activity of the enzyme, and had no influence in resistance to RT inhibitors. However, SS RT mutants lacking the dipeptid e insertion or bearing a four-serine insertion showed reduced ATP-dependent phosphorolytic activity that correlated with increased AZT sensitivity, as determined using a recombinant virus assay. Therefore, the insertion appea rs to be critical to enhance AZT resistance in the sequence context of mult idrug-resistant HIV-1 RT.