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The ammonium-inducible NADP-specific glutamate dehydrogenase of Chlorella sorokiniana was shown to require light for both its induction by ammonia in uninduced cels, and its continuous accumulation in fully induced cells. Addition of ammonia to uninduced cells in the light resulted in a 35-minute induction lag followed by linear and coincident increases in enzyme activity and antigen. Enzyme activity was not induced in the dark; however, transfer of these cells to the light resulted in an immediate increase in enzyme activity and antigen. The absence of an induction lag suggested that mRNA sequences and/or an enzyme precursor with different antigenic properties than the active holoenzyme accumulated in cells in the dark in ammonium medium. When fully induced cells were transferred to the dark, the activity of the enzyme quickly ceased to accumulate. In contrast to the NADP-speclfic isozyme, the cells also contain a constitutive NAD-specific isozyme which was shown to accumulate in cells in the dark in either ammonium or nitrate medium.

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