Kinetics of tight dimerization of HIV-2, SIV and HIV-1 leader RNA fragments. 1–561 HIV-2 ROD isolate RNA (A), 1–550 SIV mac239 isolate RNA (B), and 1–373 HIV-1 NL4-3 isolate RNA (C) were incubated for 0.5–8 hours at 55°C in dimer buffer. After incubation, samples were subjected to electrophoresis on TBE agarose at 26°C during which only tight dimers and magnesium-independent conformers remain intact. In order to load all incubations at the same time, the incubations were initiated in reverse order, i.e. the longest incubation first. The monomer and dimer RNA species are indicated by M and D, respectively. Fast-migrating bands are indicated by asterisks. Lane 1 in each panel represents monomeric RNA that was denatured at 90°C, then quenched on ice immediately prior to loading (C for control).