HIV-2 Nef, Rev and Tat RNAs form more tight dimers than the unspliced genomic leader RNA. (A) Genomic, Nef, Rev or Tat HIV-2 1–561 RNAs were incubated in dimer buffer at 55°C. After incubation for 30 min, RNAs were subjected to electrophoresis on a TBE agarose gel run at 28°C. (B) Kinetics of tight dimerization of 1–561 Tat RNA at 55°C. The time-dependent yield of dimerization of 1–561 Tat RNA at 55°C was monitored on a TBE/28°C agarose gel. Lanes 1–11 represent aliquots loaded after incubation for 0, 2, 4, 6, 8, 10, 12, 14, 16, 18 and 20 min, respectively. (C) Plots of the kinetic data for genomic, Nef, Rev or Tat HIV-2 1–561 RNAs (represented by open circles, closed circles, open squares and open diamonds, respectively). [M]t is the concentration of monomer at time t and [M]0 is the initial concentration of dimerization-competent monomer. Kinetic constants, kdim, were obtained by fitting the data to a second-order reaction model (20).