A systematic analysis of the evolution of aromatic amino acid biosynthesis in the Proteobacteria, previously focussed mainly upon the γ subdivision, has now been extended to the β subdivision. Five lineages were studied, represented by , rRNA Group-III pseudomonads/, and rRNA Group-II pseudomonads/ Within the phenylalanine pathway, the bifunctional P-protein (chorismate mutase/prephenate dehydratase) was present in each lineage and must have evolved in a common ancestor of the β and γ subdivisions. Each P-protein was found to be subject to activation by L-tyrosine, and to feedback inhibition by L-phenylalanine. Phenylalanine-inhibited (DS-phe) and tyrosine-inhibited (DS-tyr) isoenzymes of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase probably existed in the common β-subdivision ancestor, with DS-tyr being lost in and The participation of DS-phe in a dissociable multienzyme complex with one or more other common-pathway enzymes is known to exist in The same complex is indicated by two peaks of DS-phe seen in chromatographic profiles of Group-III pseudomonads and It is concluded that the contemporary DS-phe species present in subdivisions γ and β must have had independent origins. Tyrosine biosynthesis was found to be quite diverse within the β subdivision. possessed an arogenate dehydrogenase which was specific for NADP. In all other lineages, a broad-specificity cyclohexadienyl dehydrogenase (CDH) was present. In the CDH was specific for NAD while the remaining CDH species could utilize either NAD or NADP. Only the CDH species within the rRNA Group-II pseudomonad/ lineage was feedback-inhibited by L-tyrosine, and this correlated with an allosteric pattern where activation of the prephenate dehydratase component of the P-protein by L-tyrosine was relatively poor. However, the CDH enzyme present in and was subject to inhibition by 4-hydroxyphenylpyruvate, this being competitive with respect to the cyclohexadienyl substrate. The monofunctional species of chorismate mutase (CM-F) and cyclohexadienyl dehydratase, widely distributed among the γ-subdivision assemblage and recently shown to be periplasmic enzymes, were demonstrated in , a member of rRNA homology Group-II.


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