Abstract
Tat-specific cytotoxic T cells have previously been shown to exert positive Darwinian selection favoring amino acid replacements of an epitope of simian immunodeficiency virus (SIV). The region of the
tat
gene encoding this epitope falls within a region of overlap between the
tat
and
vpr
reading frames, and nonsynonymous nucleotide substitutions in the
tat
reading frame were found to occur disproportionately in such a way as to cause synonymous changes in the
vpr
reading frame. Comparison of published complete SIV genomes showed Tat to be the least conserved at the amino acid level of nine proteins encoded by the virus, while Vpr was one of the most conserved. Numerous parallel amino acid changes occurred within the Tat epitope independently in different monkeys, and purifying selection on the
vpr
reading frame, by limiting acceptable nonsynonymous substitutions in the
tat
reading frame, evidently has enhanced the probability of parallel evolution.
