HIV-1 GENOME DIMERIZATION - KISSING-LOOP HAIRPIN DICTATES WHETHER NUCLEOTIDES DOWNSTREAM OF THE 5'-SPLICE JUNCTION CONTRIBUTE TO LOOSE AND TIGHT DIMERIZATION OF HUMAN-IMMUNODEFICIENCY-VIRUS RNA

The genome of all retroviruses consists of two identical RNAs noncoval ently linked near their 5' end. Adjacent genomic RNAs from human immun odeficiency virus type 1 (HIV-1) can form loose or tight dimers depend ing on whether their respective kissing-loop hairpins (nts 248-270 in HIV-1(Lai)) bond via their autocomplementary sequences (ACS) or via th e ACS and stem sequences [Laughrea, M., & Jette, L. (1996a) Biochemist ry 35, 1589-1598]. Loose dimers from HIV-1(Mal), but not HIV-1(Lai), a re stabilized by a sequence (3'DLS) located downstream of the 5' splic e junction [Laughrea, M., & JettC, L. (1996b) Biochemistry 35; 9366-93 74]. To understand the ACS-3'DLS interplay in the formation and stabil ity of loose and tight HIV-1 RNA dimers, we replaced the ACS of HIV-1( Lai) (GCGCGC262) by GUGCAC, GUGCGC (two alternative HIV-1 ACS), or GAG CUC (a non-HIV ACS). For each mutant, RNAs truncated immediately upstr eam or downstream of the 3'DLS were prepared; their ability to dimeriz e and their thermal stabilities were compared. The results suggest tha t the ACS determines whether the 3'DLS participates in RNA dimerizatio n: (1) GAGCUC262 led to poorly stable loose dimers due to the inabilit y of the 3'DLS to stabilize them (the 3'DLS stabilized the GUGCAC and GUGCGC RNAs); (2) GAGCUC262 led to poor formation of tight dimers, due to an inhibitory effect of the 3'DLS (the 3'DLS had little effect on the tight dimerization of the GUGCAC, GUGCGC and GCGCGC RNAs). The res ults indicate that communication exists between HIV-1 RNA sequences re spectively located upstream and downstream of the 5' splice junction; they are consistent with the idea that the 3'DLS plays two ACS-depende nt roles in the dimerization process: loose dimer stabilization in HIV -1 RNAs bearing an HIV ACS (unless the ACS already conferred a thermos tability equal or superior to that offered by the 3'DLS), and inhibiti on of Light dimer formation in an HIV-1 RNA bearing a non-HIV ACS.