4 



important. Figure 1 shows a composite schematic of the 

 biosynthetic steps of the common portion of the pathway and 

 of the three divergent amino acid branches. 



It has been established that a complete aromatic 

 biosynthetic pathway is present in the chloroplasts of plant 

 cells (2, 41) . The common portion of the pathway begins 

 with the condensation of erythrose 4 -phosphate and 

 phosphoenol pyruvate to form 3-deoxy-D-arajbino-heptulosonate 

 7-phosphate by DAHP synthase and continues with six more 

 enzymatic steps to form CHA, the major branchpoint 

 intermediate. From CHA, the pathway branches to form L- 

 tryptophan via anthranilate synthase (ANS, enzyme 8) and to 

 L-phenylalanine or L-tyrosine via chorismate mutase (CM, 

 enzyme 13) . From prephenate, the product of the CM 

 reaction, different pathway variations have been shown in 

 nature to complete the synthesis of PHE and TYR (16) . In 

 plant chloroplasts, the post -prephenate pathway begins with 

 the transamination of PPA to yield L-arogenate by prephenate 

 aminotransferase (PAT, enzyme 14) . Finally, AGN is either 

 converted to PHE by arogenate dehydratase (ADT, enzyme 15) 

 or to TYR by arogenate dehydrogenase (ADH, enzyme 16) . This 

 pathway is illustrated with dashed arrows in Fig. 1. 



The basic hypothesis underlying my studies is that a 

 second pathway for aromatic amino acid biosynthesis exists 

 in the cytosol compartment of higher plants. The supportive 

 evidence is that DAHP synthase (the early-pathway enzyme) 



