Abstract
Summary NAD(P)H-Nitrate reductase is an adaptive enzyme in Chlamydomonas reinhardii . The enzyme shows maximal activity in cells grown autotrophically with nitrate or nitrite, and its level is also significant in cells grown under mixotrophic conditions with nitrate and acetate. Ammonium, but not glutamine or glutamate, repress the synthesis of nitrate reductase in Chlamydomonas . Nitrate induces the synthesis de novo of nitrate reductase, even in the presence of cyanate which prevents nitrate assimilation. 1 μM of nitrate is effective in promoting a significant synthesis of the protein, and maximal activity expression requires 100 μM of nitrate in the medium. The nitrate reductase level is low in N-starved Chlamydomonas cells. A significant synthesis of enzyme is observed upon the addition of nitrate plus ammonium to these cells, though nitrate assimilation remains blocked in the presence of ammonium.
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