CLUSTERING OF THE NITRITE REDUCTASE GENE AND A LIGHT-REGULATED GENE WITH NITRATE ASSIMILATION LOCI IN CHLAMYDOMONAS-REINHARDTII

Two new loci have been found to be clustered with five other genes for the nitrate assimilation pathway in the Chlamydomonas reinhardtii gen ome. One gene, located close to the 3'-end of the high-affinity nitrat e transporter (HANT) gene Nrt2;2, corresponds to the nitrite reductase (NiR) structural gene Nii1. This is supported by a number of experime ntal findings: (i) NiR-deficient mutants have lost Nii1 gene expressio n; (ii) Nii1 mRNA accumulation is co-regulated with the expression of other structural genes of the nitrate assimilation pathway; (iii) nitr ite (nitrate) utilization ability is recovered in the NIR mutants by f unctional complementation with a wild-type Nii1 gene; (iv) the elucida ted NII1 amino acid sequence is highly similar to that of the cyanobac terial and higher-plant enzyme, and contains the predicted domains for plastidic ferredoxin-NiRs. Thus: the mutant phenotype and the mRNA se quence and expression of the Nii1 gene have been unequivocally related . Accumulation of mRNA for the second locus identified, Lde1 (light-de pendent expression), was not regulated by nitrogen, but like nitrate-a ssimilation clustered genes, its expression was downregulated in the d ark.