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 capacityfor biosynthesis of aromatic amino acids in nature. For thisbiochemical pathway, higher plants as a group exhibit a uniformpattern of pathway steps, compartmentation, and catalytic, physicaland allosteric properties of enzymes. This biochemical patternof higher plants contains a collection of features which arecompletely different from photosynthetic prokaryotes such asthe cyanobacteria. A unicellular representative of the chlorophytealgae, Chlorella sorokiniana, was found to be strikingly similarto higher-plant plastids in possessing the following distinctiveenzymes: a Mn²⁺-stimulated, dithiothreitol-activated isoenzymeof 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase,a probable bifunctional protein competent as both dehydroquinaseand shikimate dehydrogenase, an allosterically controlled isoenzymeof chorismate mutase, a highly thermotolerant species of prephenateaminotransferase, an NADP⁺-dependent, tyrosine-inhibited arogenatedehydrogenase, and an arogenate dehydratase. In addition anisoenzyme of DAHP synthase shown in higher plants to be cytosolic,absolutely dependent upon the presence of divalent metals, andable to substitute other sugars for erythrose-4-phosphate, wasalso demonstrated in this alga. A broad-specificity 3-deoxy-D-manno-octulosonate8-phosphate synthase, recently discovered in higher plants,is also present in this Chlorella species. (Received March 25, 1995; Accepted June 14, 1995)