EVIDENCE THAT THE GLUTAMINE-STIMULATED LOSS OF NITRATE REDUCTASE PROTEIN FROM THE YEAST CANDIDA-NITRATOPHILA IS NOT THE RESULT OF INDUCER EXCLUSION

Synthesis of nitrate reductase protein and increases in nitrate reduct ase activity occurred in cultures of the yeast Candida nitratophila wh en they were incubated in medium containing ammonium nitrate. Similar treatment with glutamine plus nitrate resulted in little increase in n itrate reductase activity, in cultures grown previously with reduced n itrogen compounds, and decreases in enzyme activity, in cultures adapt ed to nitrate. Labelling studies conducted in vivo revealed a rapid ce ssation of de novo nitrate reductase synthesis when glutamine was supp lied to nitrate-adapted cultures in the presence of nitrate. Intracell ular glutamine concentrations increased rapidly under these conditions and these cultures exhibited high glutamine:glutamate ratios. As nitr ate was taken up in the presence of glutamine in these experiments, it is concluded that the glutamine-stimulated inhibition of nitrate redu ctase synthesis is a consequence of repression and rapid turnover of n itrate reductase mRNA and not inducer (nitrate) exclusion.