They found that the plant sequence was most similar to a class of bacteria called Chlorobi and Bacteroidetes. “This was surprising because when people do a similar analysis for other plant genes, they usually find the closest sequence in fungi, or in cyanobacteria whose ancestor came into plants and now make plants green and photosynthetic. Our results did not fit what people expected.”
In terms of basic science, he adds, “Our study provides examples of the complex evolution of plant chemical pathways.” During evolution, the need to survive and reproduce forces organisms to continue adapting to their circumstances, he notes. “Plants have had multiple opportunities to adopt different genes (and enzymes) during evolution to meet the challenges of the environment.
“The enzyme that plants adopted from the ancient bacteria was helpful to them when they acquired it, and plants ended up maintaining it, rather than other types from fungi or cyanobacteria. This enzyme and its pathway are now seen across the plant kingdom and allow plants to make such a large variety and quantity of phenolic compounds.”
Abstract The aromatic amino acid Phe is required for protein synthesis and serves as the precursor of abundant phenylpropanoid plant natural products. While Phe is synthesized from prephenate exclusively via a phenylpyruvate intermediate in model microbes, the alternative pathway via arogenate is predominant in plant Phe biosynthesis. However, the molecular and biochemical evolution of the plant arogenate pathway is currently unknown. Here, we conducted phylogenetically informed biochemical characterization of prephenate aminotransferases (PPA-ATs) that belong to class-Ib aspartate aminotransferases (AspAT Ibs) and catalyze the first committed step of the arogenate pathway in plants. Plant PPA-ATs and succeeding arogenate dehydratases (ADTs) were found to be most closely related to homologs from Chlorobi/Bacteroidetes bacteria. The Chlorobium tepidum PPA-AT and ADT homologs indeed efficiently converted prephenate and arogenate into arogenate and Phe, respectively. A subset of AspAT Ib enzymes exhibiting PPA-AT activity was further identified from both Plantae and prokaryotes and, together with site-directed mutagenesis, showed that Thr-84 and Lys-169 play key roles in specific recognition of dicarboxylic keto (prephenate) and amino (aspartate) acid substrates. The results suggest that, along with ADT, a gene encoding prephenate-specific PPA-AT was transferred from a Chlorobi/Bacteroidetes ancestor to a eukaryotic ancestor of Plantae, allowing efficient Phe and phenylpropanoid production via arogenate in plants today. –C. Dornfeld, A. J. Weisberg, R. K C, N. Dudareva, J. G. Jelesko, H. A. Maeda. Phylobiochemical Characterization of Class-Ib Aspartate/Prephenate Aminotransferases Reveals Evolution of the Plant Arogenate Phenylalanine Pathway. The Plant Cell, 2014; DOI: 10.1105/tpc.114.127407 (paywall)