Cyanidioschyzon merolae is able to grow on ammonium, as well as nitrate, as the sole nitrogen source; however, this alga does not have a canonical nitrite reductase (NiR). Interestingly, a sulfite reductase-like protein (SiRB) functions as the NiR in this alga. Unlike the genomes of higher plants, the C. merolae genome encodes only one glutamine synthetase (GS), which is localized in the cytosol. Thus, the nitrogen assimilation pathway in C. merolae is complex and unique, since the intermediate metabolites of nitrogen assimilation must translocate back and forth between the chloroplast and the cytosol during the nitrogen assimilation process. Using genetic and molecular biology approaches, we found that the MYB-type transcription factor, MYB1, is the central regulator of nitrogen assimilation. Nitrogen depletion-induced transcripts of nitrogen assimilation genes were completely absent from the MYB1-knockout strain. Furthermore, direct interactions between MYB1 and the promoter regions of nitrogen assimilation genes were observed in vivo and in vitro. Although MYB1-dependent transcripts of nitrogen assimilation genes were detected in nitrate-grown cells, they disappeared upon the addition of a preferred nitrogen source such as ammonium or glutamine. These findings suggested that MYB1 mediates the nitrogen catabolite repression-sensitive transcription of nitrogen assimilation genes in response to nitrogen status in C. merolae.
KeywordsAmmonium assimilation MYB-type transcription factor Nitrate assimilation Nitrogen assimilation Nitrogen catabolite repression Sulfite reductase Target of rapamycin
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