Abstract
Nitrate reductase (EC 1.6.6.1) catalyzes the pyridine nucleotide-linked reduction of nitrate to nitrite in higher plants. We have shown that in squash ( Cucurbita maxima Duchesne var. Buttercup), exogenous nitrate supplied to plants enhances in vitro nitrate reductase activity in cotyledons, while glutamine represses this activity. Maximal activity was observed 20 h following the addition of exogenous nitrate. Two dimensional polyacrylamide gel electrophoresis revealed no significant changes in the amounts of other major cellular proteins from plants supplied nitrate compared with those supplied glutamine. Dot blots of poly A + mRNA screened with a 1.2 kb cDNA clone of the nitrate reductase gene have shown nitrate reductase specific mRNA to be most abundant in squash plants given 50 mM nitrate. Little detectable hybridization was seen with mRNA isolated from plants given either no nitrogen or 10 mM glutamine. The combination of 50 mM nitrate and 10 mM glutamine resulted in intermediate levels of hybridization. Thus, changes in nitrate reductase activity in squash cotyledons correlate with changes in the steady state levels of nitrate reductase specific mRNA.
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