The phylogenetic origin of the bifunctional tyrosine-pathway protein in the enteric lineage of bacteria

Because bifunctional enzymes are distinctive and highly conserved productsof relatively infrequent gene-fusion events, they are particularly usefulmarkers to identify clusters of organisms at different hierarchical levelsof a phylogenetic tree. Within the subdivision of gram-negative bacteriaknown as superfamily B, there are two distinctive types of tyrosine-pathwaydehydrogenases: (1) a broad- specificity dehydrogenase (recently termedcyclohexadienyl dehydrogenase [CDH]) that can utilize either prephenate orL-arogenate as alternative substrates and (2) a bifunctional CDH that alsoposseses chorismate mutase activity. (T-proteins). The bifunctionalT-protein, thought to be encoded by fused ancestral genes for chorismatemutase and CDH, was found to be present in enteric bacteria (Escherichia,Shigella, Salmonella, Citrobacter, Klebsiella, Erwinia, Serratia,Morganella, Cedecea, Kluyvera, Hafnia, Edwardsiella, Yersinia, and Proteus)and in Aeromonas and Alteromonas. Outside of the latter "enteric lineage,"the T-protein is absent in other major superfamily-B genera, such asPseudomonas (rRNA homology group I), Xanthomonas, Acinetobacter, andOceanospirillum. Hence, the T-protein must have evolved after thedivergence of the enteric and Oceanospirillum lineages.3-Deoxy-D-arabino-heptulosonate 7-phosphate synthase-phe, an early-pathwayisozyme sensitive to feedback inhibition by L- phenylalanine, has beenfound in each member of the enteric lineage examined. The absence of boththe T-protein and DAHP synthase-phe elsewhere in superfamily B indicatesthe emergence of these character states at approximately the sameevolutionary time.