An alternative pathway of phenylalanine and tyrosine biosynthesis from chorismate has been characterized in bacteria, fungi and higher plants (Jensen, 1985; Jensen and Fischer, 1987; Jung et al, 1986). The alternative pathway called the pretyrosine (or arogenate) pathway involves transamination of prephenate to form pretyrosine (arogenate). Tyrosine and phenylalanine can then be formed from arogenate by arogenate dehydrogenase and arogenate dehydratase, respectively (Gaies et al, 1982). Arogenate dehydrogenase is inhibited by tyrosine in sorghum (Connelly and Conn, 1986).

The major Gram-negative prokaryotes have been placed on a phylogenetic tree as Superfamily B (Whitaker et al, 1984). This group includes lineages leading to Escherichia coli, Pseudomonas aeruginosa, and Acinetobacter calcoaceticus. Xanthomonas campestris ATCC 12612 (a representative of the Group V pseudomonads) is included within this grouping. X. campestris is like P. aeruginosa (but unlike E. coli) in that it possesses dual flow routes to both L-phenylalanine and L-tyrosine from prephenate. Like all other members of Superfamily B, X. campestris possesses a bifunctional P-protein with both chorismate mutase and prephenate dehydratase activities. X. campestris also possesses an unregulated arogenate dehydratase similar to that of P. aeruginosa. Superfamily B organisms possess 3-deoxy-D-arabino-heptulosonate 7-P (DAHP) synthase as either three isozymes (e.g. E. coli), as two isozymes (e.g. P. aeruginosa), or as one enzyme (e.g. X. campestris). The two-isozyme system may correspond to the ancestral state of Superfamily B. Thus, E. coli has gained an isozyme, whereas X. campestris has lost one. The single, chorismate-sensitive DAHP synthase enzyme of X. campestris is evolutionarily related to the tryptophan-sensitive DAHP synthase present throughout the rest of Superfamily B (Whitaker et al, 1984).

Connelly JA, Conn EE 1986 Tyrosine biosynthesis in Sorghum bicolor: isolation and regulatory properties of arogenate dehydrogenase. Z. Naturforsch. 41: 69-78.

Gaies CG, Byng GS, Whitaker RJ, Jensen RA 1982 L-Tyrosine regulation and biosynthesis via arogenate dehydrogenase in suspension-cultured cells of Nicotiana sylvestris Speg. et Comes. Planta 156: 233-240.

Jensen RA 1985 The shikimate/arogenate pathway: link between carbohydrate metabolism and secondary metabolism. Physiol. Plant. 66: 164-168.

Jensen R, Fischer R 1987 The postprephenate biochemical pathways to phenylalanine and tyrosine: an overview. Methods Enzymol. 142: 472-478.

Jung E, Zamir LO, Jensen RA 1986 Chloroplasts of higher plants synthesize L-phenylalanine via L-arogenate. Proc. Natl. Acad. Sci. U.S.A. 83: 7231-7235.

Whitaker RJ, Berry A, Byng GS, Fiske MJ, Jensen RA 1984 Clues from Xanthomonas campestris about the evolution of aromatic biosynthesis and its regulation. J. Mol. Evol. 21: 139-149.