8 



would co-exist as a bifunctional protein, as they do in 

 plastids. Alternative possibilities would be that they are 

 monofunctional species, or that they are domains of a large 

 pentafunctional protein like that present in fungi (21) , 



That the enzymes catalyzing the third and fourth steps 

 in the common pathway of plastids are bifunctional in higher 

 plants has been documented (14, 61) , In £. coli, DQT and 

 SDH are each found as monofunctional proteins (18,19). 

 Yeast and fungi produce the arom protein (21) , a 

 pentafunctional protein bearing functional domains 

 corresponding to steps 2 through 6 of aromatic amino acid 

 biosynthesis (Fig. 1-1) . it is interesting that when the 

 arom pentafunctional protein of Neurospora crassa is 

 subjected to limited proteolysis, the DQT/SDH domains were 

 retained as an intact and functional fragment (77) , 



Rationale for Focus upon DOT and SDH 

 The rationale for choice of the DQT/SDH protein (or S- 

 protein) for study was as follows, (i) SDH is easily 

 assayed, has very high activity and the substrates are 

 readily available. (ii) The S-protein is in the middle of 

 the common portion of the pathway and provides a link from 

 the early-pathway isoenzyme pair of DAHP synthase to the 

 second known mid-pathway pair of isoenzymes of chorismate 

 mutase. (iii) Two separable shikimate dehydrogenases had 

 been demonstrated by Rothe et al . (67) in pea seedlings, and 



