A KISSING COMPLEX TOGETHER WITH A STABLE DIMER IS INVOLVED IN THE HIV-1(LAI) RNA DIMERIZATION PROCESS IN-VITRO

Retroviruses contain a dimeric RNA consisting of two identical molecul es of genomic RNA. The interaction between the two monomers is thought to occur near their 5' ends. We previously identified a region upstre am from the splice donor site, comprising an autocomplementary sequenc e, responsible for the formation of dimeric HIV-1(Lai) RNA [Muriaux, D ., Girard, P.-M., Bonnet-Mathoniere, B., & Paoletti, J. (1995) J. Biol . Chem. 270, 8209-8216]. This region appeared to be confined within a putative stem-loop structure. Here we report an in vitro model of the HIV-1 RNA dimerization process involving a two-step mechanism. We used RNA 77-402, a transcript of the HTV-1(Lai) region, which is able to d imerize spontaneously in vitro under conditions of low ionic strength. Two dimers of RNA 77-402 were identified as a function of temperature , and a significant difference was found in their thermostability. Dim er D55, formed at 55 degrees C, is more stable than dimer D37, formed at 37 degrees C, RNase probing experiments confirm the involvement of a stem-loop structure in the dimerization process. In the monomer, the free G(257)CGCGC(262) sequence forms a loop in the 240-280 region of RNA 77-402, whereas this sequence is engaged in base pairing when D55 and D37 dimers are formed. Our results show that the loop-loop interac tion of the autocomplementary G(257)CGCGC(262) sequence, through hydro gen bonding, is responsible for the formation of dimer D37 and strongl y suggest that D37 is a ''kissing'' complex. In contrast, in dimer D55 , all the nucleotides of the two hairpin stems, 243-254/264-277, are i nvolved in a complete interstrand interaction.