Decreased processivity of human immunodeficiency virus type 1 reverse transcriptase (RT) containing didanosine-selected mutation Leu74Val: a comparative analysis of RT variants Leu74Val and lamivudine-selected Met184Val

we previously showed that a didanosine-selected mutation in pNL43 backgroun d conferred a replication disadvantage on human immunodeficiency virus type 1, resulting in a loss of replication fitness. This work has been extended by showing that a recombinant virus with the HXBc2 backbone and reverse tr anscriptase (RT) fragments from pNL4-3 containing the Leu74Val mutation pro duce decreasing amounts of p24 antigen over a 3-week period. The HXBc2 reco mbinant containing the wild-type RT from pNL4-3 replicated efficiently. Whe n the virion-associated RT containing the Leu74Val mutation was used in an RT processivity assay with homopolymer RNA template-primer, poly(A), and ol igo(dT), the RT with altered Leu74Val mutation was less processive, generat ing fewer cDNA products in comparison to wild-type pNL4-3 RT. The replicati on kinetics and RT processivity of the mutant with the Leu74Val mutation we re compared to those of a lamivudine-selected mutant Met184Val. In replicat ion kinetics assays, mutant Leu74Val replicated slower than the mutant Met1 84Val. In a processivity assay, the mutant RTs from both viruses show compa rable decreases in processivity. These observations provide biochemical evi dence of decreased processivity to support the decrease in replication fitn ess observed with the Leu74Val or Met184Val mutations.