Light Requirement forInduction andContinuous Accumulation of anAmmonium-Inducible NADP-Specific Glutamate

Theammonium-inducible NADP-specific glutamate dehydrogenase of Chlorella sorokiniana wasshown torequire light forbothits induction by ammonia inuninduced cels, anditscontinuous accumulation infully induced cells. Addition ofammonia touninduced cells inthelight resulted ina35-minute induction lagfollowed bylinear andcoincident increases in enzyme activity andantigen. Enzyme activity wasnotinduced inthedark; however, transfer ofthese cells tothelight resulted inanimmediate increase inenzyme activity andantigen. Theabsence ofaninduction lag suggested that mRNAsequences and/or anenzyme precursor with differentantigenic properties thantheactive holoenzyme accumulated incells inthedarkinammonium medium. Whenfully induced cells weretransferred tothedark, theactivity oftheenzyme quickly ceased toaccumulate. Incontrast totheNADP-speclfic isozyme, thecells also contain aconstitutive NAD-specific isozyme which wasshown toaccumulate incells inthe darkineither ammonium ornitrate medium. synchronous cells growing inthecontinuous presence ofinducer foranentire cell cycle (10, 11). Thislatter experimental approach revealed theoperation ofaregulatory system whichalters the timing between genereplication andtheexpression ofnewly. replicated genes incells growing atdifferent rates inammonium medium. Underthese different cell cycle conditions, theactivity oftheNAD-GDHincreased inatypical steppattern during the last 0.5hofthecell cycle. Instudies onthecell cycle inducibility oftheNADP-GDH,it wasobserved that theinitial rate ofenzymeinduction (i.e., enzyme potential) wasproportional totherateofaccumulation oftotal cellular protein (21, 22). Since theaccumulation rateofcellular protein hasbeenobserved tobeproportional totheeffective light intensity percell (17, 19), itseemed possible that theinduction or accumulation oftheNADP-GDHmight belight-dependent. The experimental evidence described inthepresent paperisconsistent withalight requirement forboththeinduction andcontinuous accumulation oftheNADP-GDHinChlorella cells inammonium medium.