EXPRESSION OF NITRATE ASSIMILATION RELATED GENES IN CHLAMYDOMONAS-REINHARDTII

The mRNA accumulation pattern of the Chlamydomonas reinhardtii nitrate assimilation-related gene cluster has been elucidated. In ammonium-gr own wild-type cells, nit-1 (nitrate reductase, NR), nar-1, nar-2 and n ar-3 (nitrate transporter) genes showed very similar kinetics of expre ssion when transferred to nitrate medium. Transcripts of all these gen es accumulated transiently in ammonium-grown wild-type cells after a o ne-hour incubation in nitrogen-free medium, and practically disappeare d at about 2 hours. Mutant strains lacking functional nitrate reductas e showed similar accumulation kinetics of these transcripts during bot h nitrate induction and derepression in nitrogen-free media. In contra st to the other nar transcripts, that nar-4, a gene sharing similar se quences with nar-3, accumulated in small amounts in wild-type cells, a nd only increased after a long nitrate induction period. Nitrate and l ight showed a strong positive effect on the accumulation of nit-1 gene transcripts. Acetate as a carbon source allowed accumulation of nit-1 mRNA in the dark, indicating the existence of interactions between li ght and carbon metabolism in nit-1 gene expression. Our data strongly suggest that NR negatively autoregulates its own expression and that o f nar genes.