17 

 Steps may exist, or (iii) the unprocessed preprotein 

 precursor of the plastid enzymes may function in the 

 cytosol . 



Prephenate aminotransferase. PAT of higher plants was 

 first characterized in partially purified extracts, 

 separated from aromatic aminotransferases, and shown to be 

 unusually specific for its substrate, prephenate. Activity 

 was optimal at VO^C for PAT, a temperature which inactivated 

 other interfering aminotransferases (5) . PAT was purified 

 about 1000-fold (7) , and localization studies showed the 

 enzyme to be located in the chloroplast (Bonner and Jensen, 

 unpublished data, 1985; 41, 76). A simple, inexpensive 

 spectrophotometric assay was developed for PAT by following 

 the increase in oxaloacetate when the L-aspartate/a- 

 ketoglutarate couple was used (6) . In the same study, the 

 ASP/aKG couple was used with native PAGE activity stained 

 gels to relate the position of PAT with respect to other 

 aminotransferases and to show the disappearance of heat- 

 inactivated aminotransferases. 



Aroqenate dehydrogenase . Partially purified ADH was 

 studied in N. silvestris with respect to allosteric 

 regulation by TYR by Gaines et al . (31) . ADH was partially 

 purified and regulatory properties were determined in 

 Sorghum bicolor (22) . The enzyme was purified about 1000- 

 fold (8) . PAT, ADH and SDH activities were followed 

 throughout a growth cycle in N. silvestris suspension cells 



