Final Stages in Tyrosine Biosynthesis

Some microorganisms may resemble the higher organisms in being able to convert phenylalanine directly to tyrosine thus it can occur in Vibrio (167) and Pseudomonas (605) and has been claimed for E. coli (48 but cf. 807). However in Lactobacillus arabinosus tyrosine is formed by a route not involving phenylalanine (20), as is apparently also the case in Aero-bacter aerogenes (605). The direct conversion of phenylalanine to tyrosine is claimed by advocates of the straight-chain pathway of aromatic biosynthesis described later. 

Otherwise no immediate precursors of tyrosine appear to have been reported. Transamination of p-hydroxyphenylpyruvic acid has been suggested to be the final stage in yeasts (474), and may occur in E. coli (809), and isotopic evidence, discussed later, suggests that even if tyrosine is not formed from phenylalanine, the method of introduction of the tyrosine side chain is very similar to that postulated above for phenylalanine. Formation of p-hydroxyphenylpyruvic acid from prephenic acid can be readily visualized. 

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