139 

 are completely conserved in the catalytic proteins, but not 

 in the two regulatory proteins. This implies that some or 

 all of these residues may be important for catalysis. Six 

 residues are absolutely conserved for all nine proteins. 

 Homolocrv relationships of the SDH domain 



A multiple alignment is shown in Fig. 7-8 of the AroE 

 domain of the S-protein with its homologues. The 

 corresponding dendrogram is shown in Fig. 7-9. Again two 

 evolutionary clusters are seen in the dendrogram, and the 

 AroE domain clusters with the monofunctional AroE protein of 

 E. coli, rather than with the AroE domains of the 

 pentafunctional proteins (Arol and AroM) or with the 

 repressor proteins (Qa-lS and QutR) . The Nicotiana AroD 

 domain is also homologous with the catabolic quinate 

 dehydrogenase of yeast and fungi, which are monofunctional 

 and specific for NAD*. However, the latter (QutB and Qa-3) 

 are evolutionarily closer to the repressor proteins (Qa-lS 

 and QutR) than to the shikimate dehydrogenase domain of the 

 plant bifunctional S-protein. It is surprising that the 

 catabolic QDH exhibits homology with biosynthetic SDH while 

 the catabolic dehydroquinases display no obvious homology 

 with biosynthetic dehydroquinases (32, 35) . This is all the 

 more striking in that the former differ in substrate 

 specificity, whereas the latter do not. One might also have 

 expected the catabolic dehydroshikimate dehydratase to be 

 homologous with the biosynthetic dehydroquinases or 



