Regulation of nitrate assimilation by amino acids in Chlorella

The assimilation of nitrate and its control by the exogenous supply of amino acids to the medium was investigated in the green alga Chlorella fusca. Of the thirteen amino acids tested, methionine, proline, valine, threonine, histidine and glutamic acid inhibited nitrate assimilation, whereas alanine, tryptophan, arginine, lysine, leucine and aspartic acid did not. This inhibition of nitrate assimilation was due to the repression of nitrate reductase. The repression by methionine, proline and valine was also accompanied by complete inactivation of the enzyme. The two classes of amino acids interacted but none of the non-repressor amino acids behaved as universal derepressors. Leucine derepressed in the presence of all the repressors, except glutamic acid. Similarly, aspartic acid also derepressed, but not in the presence of threonine. Certain amino acid pairs made up of two repressors were non-repressive, whereas pairs made up of methionine, proline and valine were always repressive.