5 

 (32), CM (the mid-pathway enzyme) (23) and ANS (the initial 

 TRP-branch enzyme) (15) are present as pairs of separately 

 compartmented isoenzymes (Fig. 1-2) . It is also known that 

 starting substrates for aromatic biosynthesis (E4P and PEP) 

 are synthesized by duplicate pathways of carbohydrate 

 metabolism located in both the plastid and cytosolic 

 compartments (Fig. 1-2) . Since most secondary metabolites 

 are synthesized in the cytosol of plant cells (39) (Fig. 1- 

 2) , it seems likely that an aromatic pathway should be 

 available in the cytosol to produce aromatic amino acids not 

 only for protein synthesis but as starting substrates for 

 secondary metabolism as well. 



Cytosolic isoenzymes of DAHP synthase and chorismate 

 mutase (Ds-Co and CM- II, respectively) are markedly 

 different from Ds-Mn and CM- I (the corresponding plastid- 

 localized isoenzymes) in both catalytic and regulatory 

 properties (32, 23) . Such differences and indeed a lack of 

 homology would be consistent with the endosymbiotic 

 hypothesis of organelle evolution (55) . If cytosolic 

 species of other pathway steps are present in plant cells, 

 it would not be surprising to find them to have equally 

 divergent properties that reflect their xenologous origin at 

 the time of endosymbiosis . Thus, the alternative post- 

 prephenate steps shown in Fig. 1 (solid arrows) might be 

 used in the cytosol. From this perspective it seems 

 unlikely that dehydroquinase and shikimate dehydrogenase 



