Evidence for NADH- and NADPH-specific isozymes of glutamate dehydrogenase and the continuous inducibility of the NADPH-specific isozyme throughout the cell cycle of the eucaryote Chlorella.

Two isozymes of glutamate dehydrogenase were shown to exist in a thermophilic strain of Chlorella pyrenoidosa. The NADPH:NADH activity ratios of the NADHand NADPHspecific isozymes were 1: 5 and 33: 1, and the molecular weights were estimated to be 179,000 and 269,000, respectively. Only the NADH-specific isozyme was detectable in nitrate-cultured cells; the synthesis of the NADPH-specific isozyme was inducible by ammonium. In light-dark-synchronized cells in nitrate medium, the activity of the NADH-specific isozyme remained constant for 4 hours and then increased continuously for the remainder of the first and into the second synchronous cell cycle. The induction of the NADPH-specific isozyme was dependent on both RNA and protein synthesis and this isozyme was inducible at all times during the cell cycle. The potential, i.e. maximum rate of induction, increased in a single step during the period of DNA replication and the fold increases in potential and in DNA were essentially equal. These data indicate that the structural gene for this isozyme is continuously available for transcription during the cell cycle of this eucaryotic microorganism, and are consistent with the hypothesis that, under fully induced conditions, the gene dosage of the cell governs the potential.