REGULATION OF ASPARAGINASE, GLUTAMINE-SYNTHETASE, AND GLUTAMATE-DEHYDROGENASE IN RESPONSE TO MEDIUM NITROGEN CONCENTRATIONS IN A EURYHALINE CHLAMYDOMONAS SPECIES

The ammonium assimilatory enzymes glutamine synthetase (EC 6.3.1.2) and glutamate dehydrogenase (EC 1.4.1.3) were investigated for a possible role in the regulation of asparaginase (EC 3.5.1.1) in a Chlamydomonas species isolated from a marine environment. Cells grown under N limitation (0.1 mM NH4+, NO3-, or L-asparagine) possessed 6 times the asparaginase activity and approximately 1/2 the protein of cells grown at high N levels (1.5-2.5 mM). Biosynthetic glutamine synthetase activity was 1.5-1.8 times greater in N-limited cells than cells grown at high levels of the 3 N sources. Conversely, glutamate dehydrogenase (both NADH- and NADPH-dependent activities) was greatest in cells grown at high levels of asparagine or ammonium, while nitrate-grown cells possessed little activity at all concentrations employed. For all 3 N sources, glutamate dehydrogenase activity was correlated to the residual ammonium concentration of the media after growth (r = 0.88 and 0.94 for NADH- and NADPH-dependent activities, respectively). Thus, glutamate dehydrogenase may be regulated in response to ambient ammonium levels via a mechanism distinct from asparaginase or glutamine synthetase. Glutamine synthetase and asparaginase, apparently repressed by high levels of all three N sources, are perhaps regulated by a common mechanism responding to intracellular N depletion, as evidenced by low cellular protein content.