Dimerization of retrovira
l genomic RNA is essentia
l for efficient vira
l rep
lication and is mediatedby structura
l interactions between identica
l RNA motifs in the vira
l leader region. We have visua
lized, bye
lectron microscopy, RNA dimers formed from the
leader region of the prototype
lentivirus, maedi visnavirus. Characterization by in vitro assays of the domains responsib
le for this interaction has identified a20 nuc
leotide sequence that functions as the core dimerization initiation site. This region is predicted toform a GACG tetra
loop and therefore differs significant
ly from the kissing
loop pa
lindromes uti
lized toinitiate dimerization in primate
lentiviruses. The motif is strong
ly conserved across the ovine and caprine
lentiviruses, imp
lying a critica
l functiona
l ro
le. Furthermore, the proposed GACG tetra
loop exhibits markedstructura
l homo
logy with simi
lar structura
l motifs present in the
leader regions of the
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-retroviruses,and the maedi visna virus dimer
linkage region is capab
le of forming heterodimeric species with theMo
loney murine
leukemia virus
domain. This may be indicative of commona
lity of origin of the twoviruses or convergent evo
lution.