Positive Selection of Paclitaxel Biosynthetic Genes Detected at Both Nucleotide and Amino Acid Levels

Evolutionary patterns of sequence divergence were analyzed in genes from the conifer genus Taxus (yew), encoding paclitaxel biosynthetic enzymes taxadiene synthase (TS) and 10-deacetylbaccatin III-10beta-O-acetyltransferase (DBAT). Nterminal fragments of TS, full-length DBAT and internal transcribed spacer (ITS) were amplified from 15 closely related Taxus species and sequenced. While both TS and DBAT are under purifying selection, we identified a number of amino acids of TS under positive selection based on inference using maximum likelihood models. We further characterized positive selection in terms of the amino acid properties that comprise protein phenotypes that selection at the molecular level may act upon. Sites undergoing positive destabilizing selection were identified in both TS and conserved DBAT.