Oxygen Inhibition of Nitrate Reductase Biosynthesis in Detached Corn Leaves via Inhibition of Total Soluble Protein Synthesis

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Abstract
Detached first leaves of 3-day-old corn seedlings (Zea mays L. W64AxW183E) were incubated with nitrate in air or 100% O2 in the light. Nitrate accumulation in the leaves was not depressed by O2. NADH:nitrate reductase activity and enzyme protein, as measured with an enzyme-linked immunosorbent assay, increased in parallel during the 8 h nitrate treatment in air, but in O2 the levels of enzyme activity and protein were depressed. NADH:nitrate reductase mRNA levels were the same in the air- and O2-treated leaves. Total soluble protein levels in leaves were slightly depressed by O12 and shifting from O2 to an air environment increased the protein level. Incorporation of [35S]methionine during nitrate treatment revealed tht total soluble protein and nitrate reductase protein synthesis were both depressed by the O2 environment relative to air, but both recovered when leaves were shifted from O2 to air. Although O2 accelerated inactivation of nitrate reductase in vitro, the in vivo inactivation rate appeared to be be too low to account for the depressed level of nitrate reductase activity in O2-treated leaves. We concluded that O2 inhibition of nitrate reductase biosynthesis in detached corn leaves was largely due to inhibition of total soluble protein synthesis at the level of translation.