ABSTRACT
Human
immunodeficiency virus type 1 encapsidates two copies of viral genomic
RNA in the form of a dimer. The dimerization process initiates via a
6-nucleotide palindrome that constitutes the loop of a viral RNA
stem-loop structure (i.e., stem loop 1 [SL1], also termed the
dimerization initiation site [DIS]) located within the
5′ untranslated region of the viral genome. We have now shown
that deletion of the entire DIS sequence virtually eliminated viral
replication but that this impairment was overcome by four second-site
mutations located within the matrix (MA), capsid (CA), p2, and
nucleocapsid (NC) regions of Gag. Interestingly, defective viral RNA
dimerization caused by the ΔDIS deletion was not significantly
corrected by these compensatory mutations, which did, however, allow
the mutated viruses to package wild-type levels of this DIS-deleted
viral RNA while excluding spliced viral RNA from encapsidation. Further
studies demonstrated that the compensatory mutation T12I located within
p2, termed MP2, sufficed to prevent spliced viral RNA from being
packaged into the ΔDIS virus. Consistently, theΔ
DIS-MP2 virus displayed significantly higher levels of
infectiousness than did the ΔDIS virus. The importance of
position T12 in p2 was further demonstrated by the identification of
four point mutations,T12D, T12E, T12G, and T12P, that resulted in
encapsidation of spliced viral RNA at significant levels. Taken
together, our data demonstrate that selective packaging of viral
genomic RNA is influenced by the MP2 mutation and that this represents
a major mechanism for rescue of viruses containing the ΔDIS
deletion.