Distinctiye 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 biosynthesis 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 of enzymes. This biochemical pattern of higher plants contains a collection of features which are completely different from photosynthetic prokaryotes such as the cyanobacteria. A unicellular representative of the chlorophyte algae, Chlorella sorokiniana, was found to be strikingly similar to higher-plant plastids in possessing the following distinctive enzymes: a Mn-stimulated, dithiothreitol-activated isoenzyme of 3-deoxy-D-araWno-heptulosonate 7-phosphate (DAHP) synthase, a probable bifunctional protein competent as both dehydroquinase and shikimate dehydrogenase, an allosterically controlled isoenzyme of chorismate mutase, a highly thermotolcrant species of prephenate aminotransferase, an NADP-dependent, tyrosine-inhibited arogenate dehydrogenase, and an arogenate dehydratase. In addition an isoenzyme of DAHP synthase shown in higher plants to be cytosolic, absolutely dependent upon the presence of divalent metals, and able to substitute other sugars for erythrose-4-phosphate, was also demonstrated in this alga. A broad-specificity 3-deoxy-D-ma/ino-octulosonate 8-phosphate synthase, recently discovered in higher plants, is also present in this Chlorella species.