Retroviral genomes consist of two identical RNA molecules joined noncovalently near their 5' ends, at domains called dimerization linked sequences (DLS). This physical linkage of the genomic RNAs is considered important for the control of several steps in the viral life cycle, such as recombination, translation, and encapsidation. The putative DLS of human immunodeficiency virus-1 (HIV-1), a 111-nucleotide, purine-rich stretch of RNA, has been found necessary and sufficient for a salt-induced dimerization of the genome in vitro. Our investigation into the mechanism of this dimerization reveals sharply varying influences of the different alkali cations on both the formation and the stabilization of the dimer, a pattern closely related to that of telomeric G-DNA complexes. To probe this phenomenon, we have carried out experiments using short antisense DNA oligomers to define the segments of the DLS that are required for dimerization and methylation protection to implicate sets of guanines in forming Hoogsteen hydrogen bonds within the dimer. Cumulatively, these data provide further evidence for the existence of guanine quartets within the dimerized HIV-1 DLS. We propose models in which guanine quartets not only allow the homodimerization of HIV-1 and other retroviral genomic RNAs but also permit the two RNA strands in a dimer to exist in an overall parallel orientation, as has been observed by electron microscopy.