REGULATION OF NITRATE REDUCTASE AND PHOSPHOENOLPYRUVATE CARBOXYLASE ACTIVITIES IN BARLEY LEAF PROTOPLASTS

Barley leaf protoplasts were incubated in light or darkness in the pre sence of various inhibitors, metabolites or weak acids/bases. Nitrate reductase (NR) and phosphoenolpyruvate carboxylase (PEPCase) were rapi dly extracted from the protoplasts and assayed under sub-optimal condi tions, i.e. in the presence of Mg2+ and malate, respectively. Under th ese conditions changes in activities are thought to reflect changes in the phosphorylation states of the enzymes. The NR was activated by il lumination to 90% of its maximal activity within 10 min. Photosyntheti c electron transport appeared necessary for light activation of NR sin ce activation was inhibited by the photosynthetic electron-transport i nhibitor 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU), and, additi onally, an electron acceptor (HCO3-) was required. The PEPCase was als o activated by light. However, this activation was not prevented by DC MU or lack of HCO3-. Loading of protoplasts in the dark with a weak ac id resulted in activation of both NR and PEPCase. For NR, full activat ion was completed within 5 min, whereas for PEPCase a slower, modest a ctivation continued for at least 40 min. Incubation of protoplasts wit h a weak base also gave activation of PEPCase, but not of NR. On the c ontrary, base loading counteracted light activation of NR. Since sever al treatments tested resulted in the modulation of either NR or PEPCas e activity, but not both, signal transduction cascades leading to chan ges in activities appear to be very different for the two enzymes.