A SHORT AUTOCOMPLEMENTARY SEQUENCE PLAYS AN ESSENTIAL ROLE IN AVIAN SARCOMA-LEUKOSIS VIRUS-RNA DIMERIZATION

Retroviral genomes consist of two identical RNA molecules joined nonco valently near their 5'-ends. Recently, two models have been proposed f or RNA dimer formation on the basis of results obtained in vitro with human immunodeficiency virus type 1 RNA and Moloney murine leukemia vi rus RNA. It was first proposed that viral RNA dimerizes by forming an interstrand quadruple helix with purine tetrads. The second model post ulates that RNA dimerization is initiated by a loop-loop interaction b etween the two RNA molecules. In order to better characterize the dime rization process of retroviral genomic RNA, we analyzed the in vitro d imerization of avian sarcoma-leukosis virus (ASLV) RNA using different transcripts. We determined the requirements for heterodimer formation , the thermal dissociation of RNA dimers, and the influence of antisen se DNA oligonucleotides on dimer formation. Our results strongly sugge st that purine tetrads are not involved in dimer formation. Data show that an autocomplementary sequence located upstream from the splice do nor site and within a major packaging signal plays a crucial role in A SLV RNA dimer formation in vitro. This sequence is able to form a stem -loop structure, and phylogenetic analysis reveals that it is conserve d in 28 different avian sarcoma and leukosis viruses. These results su ggest that dimerization of ASLV RNA is initiated by a loop-loop intera ction between two RNA molecules and provide an additional argument for the ubiquity of the dimerization process via loop-loop interaction.