Efficient encapsidation of human immunodeficiency virus type 1 vectors and further characterization of cis elements required for encapsidation.

To determine whether there is a cis-acting effect of translational expression of gag on RNA encapsidation, we compared the encapsidation of wild-type RNA with that of a mutant in which the translation of gag was ablated. This comparison indicated that there is not such a cis effect. To determine what is necessary and sufficient for encapsidation, we measured the relative encapsidation efficiencies of human immunodeficiency virus type 1 vector RNAs containing mutations in domains proximal to the canonical encapsidation signal or containing large deletions in the remainder of the genome. These data indicate that TAR and two additional regions are required for encapsidation and that the 5' end of the genome is sufficient for encapsidation. The Rev-responsive element is required mainly for efficient RNA transport from the nucleus to the cytoplasm. A foreign sequence was found to have a negative effect on encapsidation upon placement within the parental vector. Interestingly, this negative effect was compounded by multiple copies of the sequence.