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Metabolism and Enzymology

Synthesis and Degradation of Nitrate Reductase during the Cell Cycle of Chlorella Sorokiniana

Department of Food Science and Technology and Plant Growth Laboratory, University of California, Davis, California 95616, Pflanzenphysiologisches Institut der Unviersität, Untere Karspüle 2, D-3400 Gottingen, Federal Republic of Germany

Studies on the diurnal variations of nitrate reductase (NR) activity during the life cycle of synchronized Chlorella sorokiniana cells grown with a 7:5 light-dark cycle showed that the NADH:NR activity, as well as the NR partial activities NADH:cytochrome c reductase and reduced methyl viologen:NR, closely paralleled the appearance and disappearance of NR protein as shown by sodium dodecyl sulfate gel electrophoresis and immunoblots. Results of pulse-labeling experiments with [³⁵S]methionine further confirmed that diurnal variations of the enzyme activities can be entirely accounted for by the concomitant synthesis and degradation of the NR protein.