61 

 three other aromatic pathway enzymes, SDH, PAT, and ADH. 

 The three enzymes are known to exist in the chloroplast of 

 higher plants, but the extent to which a fraction of each of 

 the total activities measured might be derived from the 

 cytosol is unknown. All three enzymes exhibited elevated 

 levels of activity in stationary phase and minimal levels of 

 activity in exponential -phase growth (11) . Aromatic -pathway 

 enzymes may be sorting out into two different patterns of 

 expression for reasons yet to be elucidated. Alternatively, 

 since enrichment of Ds-Co (the cytosolic isoenzyme of DAHP 

 synthase) , occurred in stationary phase, rising levels of 

 SDH, PAT and ADH in stationary phase may reflect the 

 expression of cytosolic isoenzymes. If so, the rise in 

 activity of cytosolic isoenzymes exceeds the decrease of the 

 plastidial isoenzyme counterparts. These data suggest that 

 while ADT is present in the chloroplast pathway, it may not 

 function in the cytosol. If so, perhaps prephenate 

 dehydratase of the alternative route (activity, not as yet 

 determined in plants, see Fig. 1-1) , functions in the 

 cytosol to synthesize PHE that may be used extensively for 

 secondary metabolism. 



