In retroviruses, the genomic RNA is in the form of a 60S-70S complex composed of two identical genomelength RNA molecules tightly associated through numerous interactions. A major interaction, called the dimer linkage structure, has been found near the RNA 5 end and is probably involved in the control of translation, packaging, and recombination during proviral DNA synthesis. Recently, a small sequence corresponding to a stem-loop structure located in the 5 leader of human immunodeficiency

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... type 1 (HIV-1) RNA was found to be required for the initiation of HIV-1 RNA dimerization in vitro and named the dimerization initiation site (E. Skripkin, J.-C. Paillart, R. Marquet, B. Ehresmann, and C. Ehresmann, Proc. Natl. Acad. Sci. USA 91: 4945-4949, 1994). To investigate the possible role of this 5 stem-loop in HIV-1 virion formation and infectivity, four mutant viruses were generated and analyzed in vivo. Results show that deletion of the stem-loop structure reduces infectivity by a factor of 10 3 whereas loop substitutions cause a decrease of 10-to 100-fold. The level of genomic RNA packaging was found to be decreased fivefold in mutants virions containing the stem-loop deletion and only twofold in the loop-substituted virions. Surprisingly, the second DNA strand transfer during reverse transcription was found to be severely impaired upon stem-loop deletion. Taken together, these results indicate that the stem-loop structure called the dimerization initiation site is a cis element acting on both genomic RNA packaging and synthesis of proviral DNA.