Abstract
We previously cloned two nitrate reductase (NR) genes and two nitrite reductase (NiR) genes from Physcomitrella patens, whose expression was induced by nitrate and repressed by ammonium. In Arabidopsis thaliana, expression of the NR gene was recently reported to be repressed by nitrite. NR gene expression was repressed by nitrite in P. patens as well, but the negative effect of nitrite might have arisen from feedback inhibition by the ammonium generated by nitrite reduction. To address this question, we constructed a mutant lacking both NiR genes by double homologous recombination and found that nitrite by itself does not repress NR gene expression in P. patens. The negative effect of ammonium on expression of the NR and NiR genes was abolished by an inhibiter of glutamine synthetase but not by an inhibiter of glutamate synthase , suggesting that expression of these genes was repressed by glutamine.
