THE 5'-TAR AND 3'-TAR ELEMENTS OF HUMAN-IMMUNODEFICIENCY-VIRUS EXERT EFFECTS AT SEVERAL POINTS IN THE VIRUS LIFE-CYCLE

The human immunodeficiency virus type I RNA genome contains a terminal repeat (R) sequence that encodes the TAR hairpin motif, which has bee n implicated in Tat-mediated activation of transcription. R lore recen tly, a variety of other functions have been proposed for this structur ed RNA element, To determine the replicative roles of the 5' and 3' TA R hairpins, we analyzed multiple steps in the life cycle of wild-type and mutant viruses, A structure-destabilizing mutation was introduced in either the 5', the 3', or both TAR motifs of the proviral genome. A s expected, opening of the 5' TAR hairpin caused a transcription defec t, Because the level of protein expression was not similarly reduced, the translation of this mRNA was improved. No effect of the 3' hairpin on transcription and translation was measured. Mutations of the 5' an d 3' hairpin structures reduced the efficiency of RNA packaging to sim ilar extents, and RNA packaging was further reduced in the 5' and 3' T AR double mutant. Upon infection of cells with these virions, a reduce d amount of reverse transcription products was synthesized by the TAR mutant. However, no net reverse transcription defect was observed afte r correction for the reduced level of virion RNA. This result was conf irmed in in vitro reverse transcription assays, These data indicate th at the 5' and 3' TAR motifs play important roles in several steps of t he replication cycle, but these structures have no significant effect on the mechanism of reverse transcription.