16 

 Molecular genetics. Plant genes encoding DQT or SDH 

 have not been sequenced, but the monocistronic aroD and aroE 

 genes have been cloned and sequenced in E. coli (1, 27) . 

 Amino acid sequences are also available for discrete domains 

 encoding these activities in S. cerevisiae (28) and 

 Aspergillus nidulans (now known as Emericella nidulans) (17) 

 as part of the pentafunctional arom gene iaroM) . The E. coli 

 amino acid sequences, aroD (DQT) and aroE (SDH) exhibit only- 

 have about 25 and 21% identity, respectively, with the 

 corresponding functional domains of the yeast arom protein. 



In higher plants, aromatic -pathway genes thus far 

 cloned and sequenced are limited to DAHP synthase (29) , 

 shikimate kinase (70), EPSP synthase (73), and chorismate 

 synthase (69) of the common portion of the pathway. 

 Anthranilate synthase (63) and tryptophan synthase jS subunit 

 (3) of the tryptophan branch of the pathway have been cloned 

 and sequenced. 



The Post-Prephenate Pathway in Plants 

 The dual -pathway hypothesis of aromatic amino acid 

 biosynthesis specifies that an intact post-prephenate 

 pathway exists in the cytosol in addition to that already 

 demonstrated in the plastid compartment (41) . If a separate 

 cytosolic pathway functions: (i) separate genes encoding 

 isoenzyme counterparts of the plastid- localized enzymes may 

 exist, (ii) separate genes encoding alternative enzymes 



