3 NRT2 GENES ARE DIFFERENTIALLY REGULATED IN CHLAMYDOMONAS-REINHARDTII

Two new nitrate assimilation-related genes, Nrt-2;3 and Nar5, have bee n identified in Chlamydomonas reinhardtii. The Nrt2;3 gene is a new me mber of the Nrt2 family, encoding high-affinity nitrate (nitrite) tran sporters. Like that of the nitrate assimilation genes, expression of t he Nrt2;3 gene is down-regulated by ammonium and positively controlled by Nit2, a regulatory locus specific for the pathway. The three Nrt2 genes of C. reinhardtii are differentially regulated by the nitrogen s ource. Expression of Nrt2;3 and of Nrt2;1, a nitrate/nitrite-bispecifi c transporter gene, was induced by nitrate and more efficiently by nit rite. Accumulation of mRNA of Nrt2;2. the nitrate-specific transporter gene, was only induced efficiently by nitrate. The Nar5 gene is locat ed upstream of the Nrt2:3 genomic region and expression of its mRNA is down-regulated by ammonium. The Nrt2;3 and Nar5 genes are overexpress ed in a deletion mutant that lacks nitrate assimilation loci.