DISTINCTIVE ENZYMES OF AROMATIC AMINO-ACID BIOSYNTHESIS THAT ARE HIGHLY CONSERVED IN LAND PLANTS ARE ALSO PRESENT IN THE CHLOROPHYTE ALGA CHLORELLA-SOROKINIANA

Considerable enzymological diversity underlies the capacity for biosyn thesis of aromatic amino acids in nature. For this biochemical pathway , higher plants as a group exhibit a uniform pattern of pathway steps, compartmentation, and catalytic, physical and allosteric properties o f enzymes. This biochemical pattern of higher plants contains a collec tion of features which are completely different from photosynthetic pr okaryotes such as the cyanobacteria. A unicellular representative of t he chlorophyte algae, Chlorella sorokiniana, was found to be strikingl y similar to higher-plant plastids in possessing the following distinc tive enzymes: a Mn2+-stimulated, dithiothreitol-activated isoenzyme of 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase, a probab le bifunctional protein competent as both dehydroquinase and shikimate dehydrogenase, an allosterically controlled isoenzyme of chorismate m utase, a highly thermotolerant species of prephenate aminotransferase, an NADP(+)-dependent, tyrosine-inhibited arogenate dehydrogenase, and an arogenate dehydratase. In addition an isoenzyme of DAHP synthase s hown in higher plants to be cytosolic, absolutely dependent upon the p resence of divalent metals, and able to substitute other sugars for er ythrose-4-phosphate, was also demonstrated in this alga. A broad-speci ficity 3-deoxy-D-manno-octulosonate 8-phosphate synthase, recently dis covered in higher plants, is also present in this Chlorella species.