CLONING AND ANALYSIS OF THE C4-PHOTOSYNTHETIC NAD-DEPENDENT MALIC ENZYME OF AMARANTH MITOCHONDRIA

In some C4 plant species, a mitochondrial NAD-dependent malic enzyme ( EC 1.1.1.39) (NAD-ME) catalyzes the decarboxylation of 4 carbon malate in the bundle sheath cells, releasing CO2 for the Calvin cycle of pho tosynthesis. In amaranth, a dicotyledonous NAD-ME-type C4 plant, the p hotosynthetic NAD-ME purified as two subunits of 65 and 60 kDa, design ated alpha and beta, respectively. Antiserum raised against the a subu nit reacted only with the 65-kDa protein in immunoblot analysis. Immun ogold electron microscopy using the alpha subunit antiserum demonstrat ed that this protein was localized specifically to the mitochondrial m atrix of bundle sheath cells. The complete nucleotide sequence of a 23 00-base pair alpha subunit cDNA clone showed that this gene encodes a protein that contains all of the motifs required for a complete and fu nctional malic enzyme. The alpha subunit has significant similarity al ong its entire length to other known NAD- and NADP-dependent malic enz ymes from plants, animals, and bacteria. The findings presented here p rovide new insights about the C4 photosynthetic NAD-ME and its evoluti onary relationship to other forms of malic enzyme present in eukaryoti c and prokaryotic organisms.