There has long been uncertainty as to how plants produce the (essential) aromatic amino acid, phenylalanine (6) (Figure 7.8). Originally considered to be formed via transamination of phenylpyruvate (35) (80), work pioneered by Jensen and colleagues (81) and later by Siehl and Conn (82) provided evidence for an alternate pathway using arogenate (37); however,

the arogenate dehydratases detected in crude extracts were apparently not stable enough for purification and there were no encoding genes identified.

In Arabidopsis, using a data mining approach, we identified six putative arogenate dehy­dratases (ADTs) and/or prephenate dehydratases (PDTs) based on (relatively low) sequence homology to known bacterial PDTs (83). Subsequent expression of each of the recombinant proteins in a NUS-His-tagged form established that each was capable of more efficiently converting arogenate (37) into phenylalanine (6), rather than catalyzing the conversion of prephenate (36) into phenylpyruvate (35) (83), i. e., indicative of a six-membered aro — genate dehydratase family in Arabidopsis. It will be instructive to next establish the nature of this Phe-forming metabolic network. That is, by determining which of these genes are involved in phenylpropanoid/lignin metabolism, protein formation, and/or other biochem­ical processes, as well as to what extent this gene family is functionally redundant (due to co-expression).