Summary: The regulation of the NH -assimilating enzymes glutamate dehydrogenase (GDH), glutamine synthetase (GS) and glutamate synthase (GOGAT), and the cellular composition of ZV620 grown in a chemostat with methanol and NH as the supplied C- and N-sources were investigated. The influence of either C- or N-limitation (as a function of dilution rate) and of the C: N ratio (at a constant growth rate) was studied. NADP-dependent GDH was active at high NH -concentrations and was repressed at low NH -concentrations. The activity increased with increasing dilution rates under C-limited growth conditions. Derepression of NADP-dependent GDH was observed at low dilution rates under N-limited growth conditions. GS was more active at low NH -concentrations where both the total enzyme level (deadenylylated plus adenylylated forms) and the active fraction increased. C-limited growth resulted in low activities of GS, whereas activity in N-limited cells was consistently high. Dilution rate did not have a significant influence on the specific activity. The specific activity of GOGAT increased with decreasing NH concentrations. Under both C- and N-limitation the specific activity of GOGAT increased with increasing growth rates. The C-content of the cells changed very little under the various growth conditions tested. The N-content and the protein content of the cells did not alter under C-limitation. N-limited growth conditions caused the cells to accumulate poly β-hydroxybutyrate. As a consequence, both the N-content and the protein content of the cells decreased.


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