NITROGEN EFFECTS ON THE INDUCTION OF FERREDOXIN-DEPENDENT GLUTAMATE SYNTHASE AND ITS MESSENGER-RNA IN MAIZE LEAVES UNDER THE LIGHT

Changes in the levels of ferredoxin (Fd)-dependent glutamate synthase (EC 1.4.7.1) and its mRNA were investigated in leaves of maize (Zea ma ys L. cv DEA), In etiolated leaves, detached from nitrogen-starved dar k-grown seedlings, Fd-glutamate synthase was present at a low level. T he enzyme protein and the activity were induced from 3- to 5-fold duri ng the 35 h after transfer of etiolated leaves to a medium containing either 10 mM KNO3, 10 mM NH4Cl or 10 mM NH4NO3 under the continuous ph otosynthetic photon flux density of 300 mu mol quanta/m(2) per s. A sl ight increase in the activity occurred in the leaves grown in a nitrog en-free medium under continuous light. The increase in the enzyme acti vity was paralleled by the incorporation of L-[S-35]methionine in the medium into the Fd-glutamate synthase polypeptide under the same condi tions. The production of the enzyme protein and the uptake of labeled amino acid into the enzyme protein were blocked by adding 71 mu M cycl oheximide to the medium supplemented with KNO3, NH4Cl or NH4NO3. The a ddition of the different nitrogenous compounds under continuous darkne ss did not significantly alter the Fd-glutamate synthase activity and L-[S-35]methionine incorporation into the enzyme protein. A partial cD NA of 2479 base pairs long encoding maize Fd-glutamate synthase was cl oned and characterized. Using this cDNA as hybridization probe, Fd-glu tamate synthase mRNA was observed to be present at a low level in nitr ogen-starved etiolated leaves, The mRNA increased about 5-fold when et iolated leaves were incubated under continuous light in a medium conta ining either 10 mM KNO3, 10 mM NH4Cl or LD mM NH4NO3. The level of mRN A was also slightly enhanced in the leaves incubated in a nitrogen-fre e medium under the light. The exogenous nitrogen compounds did not inc rease the level of the mRNA under continuous darkness. The presence of 71 mu M cycloheximide in either of the media did not significantly ch ange the level of the transcript during the initial 6 h. The induction of mRNA in the presence of exogenous nitrogen under the light was con sistent with a protein synthesis-independent process in maize leaves.