Genetic dissociation of the encapsidation and reverse transcription functions in the 5' R region of human immunodeficiency virus type 1.

The efficient packaging of genomic RNA into virions of human immunodeficiency virus type 1 (HIV-1) is directed by cis-acting encapsidation signals, which have been mapped to particular RNA stem-loop structures near the 5' end of the genome. Earlier studies have shown that three such stem-loops, located adjacent to the major 5' splice donor, are required for optimal packaging; more recent reports further suggest a requirement for the TAR and poly(A) hairpins of the 5' R region. In the present study, we have compared the phenotypes that result from mutating these latter elements in the HIV-1 provirus. Using a single-round infectivity assay, we find that mutations which disrupt base pairing in either the TAR or poly(A) stems cause profound defects in both packaging and viral replication. Decreased genomic packaging in a given mutant was always accompanied by increased packaging of spliced viral RNAs. Compensatory mutations that restored base pairing also restored encapsidation, indicating that the secondary structures of the TAR and poly(A) stems, rather than their primary sequences, are important for packaging activity. Despite having normal RNA contents, however, viruses with compensatory mutations at the base of the TAR stem were severely replication defective, owing to a defect in proviral DNA synthesis. Our findings thus confirm that the HIV-1 TAR stem-loop is required for at least three essential viral functions (transcriptional activation, RNA packaging, and reverse transcription) and reveal that its packaging and reverse transcription activities can be dissociated genetically by mutations at the base of the TAR stem.