A heterologous, high-affinity RNA ligand for human immunodeficiency virus Gag protein has RNA packaging activity

Retroviral RNA encapsidation depends on the specific binding of Gag protein s to packaging (Jr) signals in genomic RNA. We investigated whether an in v itro-selected, high-affinity RNA ligand for the nucleocapsid (NC) portion o f the Gag protein from human immunodeficiency virus type 1 (HIV-1) could me diate packaging into HIV-1 virions. We find that this ligand can functional ly substitute for one of the Gag-binding elements (termed SL3) in the HIV-1 phi locus to support packaging and viral infectivity in cis. By contrast, this ligand, which fails to dimerize spontaneously in vitro, is unable to r eplace a different phi element (termed SL1) which is required for both Gag binding and dimerization of the HIV-1 genome. A single point mutation withi n the ligand that eliminates high-affinity in vitro Gag binding also abolis hes its packaging activity at the SL3 position. These results demonstrate t hat specific binding of Gag or NC protein is a critical determinant of geno mic RNA packaging.