REGULATION OF MAIZE LEAF NITRATE REDUCTASE-ACTIVITY INVOLVES BOTH GENE-EXPRESSION AND PROTEIN-PHOSPHORYLATION

Nitrate reductase (NR; EC 1.6.6.1) activity increased at the beginning of the photoperiod in mature green maize (Zea mays L.) leaves as a re sult of increased enzyme protein level and protein dephosphorylation. In vitro experiments suggested that phosphorylation of maize leaf NR a ffected sensitivity to Mg2+ inhibition, as shown previously in spinach . When excised leaves were fed P-32-labeled inorganic phosphate, NR wa s phosphorylated on seryl residues in both the light and dark. Tryptic peptide mapping of NR labeled in vivo indicated three major P-32-phos phopeptide fragments, and labeling of all three was reduced when leave s were illuminated. Maize leaf NR mRNA levels that were low at the end of the dark period peaked within 2 h in the light and decreased there after, and NR activity generally remained high. It appears that light signals, rather than an endogenous rhythm, account primarily for diurn al variations in NR mRNA levels. Overall, regulation of NR activity in mature maize leaves in response to light signals appears to involve c ontrol of gene expression, enzyme protein synthesis, and reversible pr otein phosphorylation.