A circadian rhythm in the activity of nitrate reductase (NR; EC 1.6.6.1) isolated from the marine dinoflagellate Gonyaulaxpolyedra is shown to be attributable to the daily synthesis and destruction of the protein. l h e enzyme was purified in three steps: gel filtration on S-300 Sephacryl, an Affigel-Blue column, and a diethylaminoethyl ion-exchange column. Undenatured protein shows a molecular mass of about 31 O kD; based on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the enzyme appears to be composed of six possibly identical subunits. The amino acid composition of the G. polyedra NR is very similar to that reported for the NR of barley leaves, Chlorella vulgaris, and Ankistrodesmus braunii. l h e experiments reported indicate that the cellular expression of N R is under circadian control. In extracts of cells grown under either constant dim light or a light-dark cycle, the activity of NR exhibits a daily rhythm, peaking at midday phase, as does photosynthesis. Staining with affinity-purified polyclonal antibodies, raised in rabbits against purified NR, shows that the amount of protein changes by a factor of about 10, with the maximum occurring in midday phase.

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