PACKAGING OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 RNA REQUIRES CIS-ACTING SEQUENCES OUTSIDE THE 5' LEADER REGION

cis elements required for the encapsidation of human immunodeficiency virus type 1 (HIV-1) RNA have been investigated by using a replication -competent helper virus to package a series of HIV-1-based vectors whi ch had been stably transfected into human CD4 T-cell lines. A previous ly identified packaging signal in the 5' leader region was not suffici ent for the encapsidation of small vectors containing heterologous gen es. In contrast, vectors containing additional gag and env sequences w ere packaged with high efficiency and transduced into CD4-expressing t arget cells with titers exceeding 10(4) CFU/ml. The presence of gag se quences did not enhance vector packaging efficiency. A 1.1-kb env gene fragment encompassing the Rev-responsive element was absolutely requi red for the expression and encapsidation of vectors containing cis-act ing repressive sequences and appeared also to contain an important pac kaging signal. Vectors as small as 2.6 kb were successfully packaged i n this system. The presence of abundant, packageable vector RNA did no t appear to interfere with encapsidation of the wild-type HIV-1 genome , suggesting that HIV-1 RNA packaging capacity is not saturated during acute infection.