Heteromultimeric structure of the nitrate reductase complex of Chlamydomonas reinhardii

The NAD(P)H-nitrate reductase complex (overall-NR) of Chlamydomonas reinhardii exhibits two partial activities: NAD(P)H-cytochrome c reductase (diaphorase) and reduced benzyl viologen-NR (terminal-NR). Mild tryptic digestion of the enzyme complex resulted in the loss of both overall and terminal-NR activities, whereas diaphorase activity remained unaltered. The diaphorase activity of mutant 104 and the terminal-NR activity of mutant 305 of C. reinhardii, which are the sole activities related to NR present in these mutants, responded to tryptic treatment to the same extent as the corresponding activities of the wild enzyme complex. Trypsin disassembled the 220-kd NR native complex by destroying the aggregation capability of the diaphorase subunits without affecting their activity nor molecular size (45 kd). A 67-kd thermostable protein, containing molybdenum co-factor, was also released from trypsin-treated NR. This protein lacked diaphorase and NR activities but was able to reconstitute the overall-NR complex by complementation with untreated diaphorase subunit of mutant 104. Our results support a tetrameric structure for the C. reinhardii NR complex, containing two kinds of subunits.