| Abstract: | Nitrate-grown cells of Stichococcus bacillaris Naeg. (UTEX 314) contained much higher activities of glutamine synthetase (GS) and NADPH-glutamate dehydrogenase (GDH) than ammonium-grown cells. Methylamine, a non-metabolizable ammonium analog, caused a decrease in GS activity in nitrate-grown cells suggesting that GS is regulated by the size of the endogenous ammonium pool. The decrease in GS observed in methylammonium-loaded nitrate-grown cells was accompanied by an increase in NADPH-GDH activity. Stichococcus bacillaris can be grown in the presence of methionine sulfoximine (MSX), a potent inhibitor of GS. However, only a fraction of a control cell population showed a requirement for glutamine or arginine for growth following MSX addition. Fully adapted MSX-grown cells were indistinguishable from control cells in their ability to photosynthesize and utilize amino acids as nitrogen sources. Alanine, arginine, asparagine, glutamine, glycine and proline were good nitrogen sources, and maximum capacity for amino acid transport was developed in cells grown on these amino acids. Compared to nitrate-grown cells the activity of GS in ammo acid-grown cells was low, whereas NADPH-GDH was very active. The activity of NADH-GDH in amino acid-grown cells was highest under heterotrophic conditions. |
