A divergent simian immunodeficiency virus from sooty mangabey with an atypical Tat-TAR structure.

SIVsm, the simian immunodeficiency virus that naturally infects sooty mangabeys in West Africa, is the closest lentiviral relative of human immunodeficiency virus type 2 (HIV-2). To determine the genetic characteristics of SIVsm in its natural host, we sequenced the full-length genome of SIVsmSL92b, a primary isolate obtained from a pet sooty mangabey in Sierra Leone. SIVsmSL92b proved to be the most divergent member of the HIV-2/SIVsm lineage found thus far, having as much as 35% nucleotide divergence from other HIV-2 genomes. A phylogenetic association between SIVsmSL92b and HIV-2 PA subtype E, which had been previously revealed by the analysis of partial gag sequences, was extended to the pol gene. SIVsmSL92b showed several divergent features, including a short Tat protein of 104 residues and an atypical TAR structure. Specifically, only one of the duplicate TAR elements contained the conserved hexanucleotide loop sequence CUGGGX important for Tat-cyclin T1 binding. These features suggested that the mechanism of SIVsmSL92b Tat and TAR interaction differed from that described for HIV-2. Taken together, these findings indicated that the structural diversity within the HIV-2/SIVsm lineage was greater than previously appreciated.