Cr. Hipkin et al., EVIDENCE THAT THE GLUTAMINE-STIMULATED LOSS OF NITRATE REDUCTASE PROTEIN FROM THE YEAST CANDIDA-NITRATOPHILA IS NOT THE RESULT OF INDUCER EXCLUSION, Biochemical journal, 295, 1993, pp. 611-615
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.