Abstract
Nitrate reductase (NR) is an enzyme involved in nitrate assimilation. The expression of NR genes is affected by nitrate, glutamine, sugar, light and circadian rhythm. It has been reported that the activity of a tobacco NR gene promoter is high when grown on ammonium-containing medium but low when grown on nitrate-containing medium. Furthermore, NR activity in the NR mutants was only limitedly recovered by transformation with genomic fragments of NR genes in tobacco and Arabidopsis, implying the complex regulation of NR gene expression. Here, we show our analysis of the mechanism underlying NR gene expression with an NR gene of Arabidopsis, NIA1. The result of our experiment with GUS reporter gene indicated that the NIA1 promoter, 1.9-kb in length, was not activated by nitrate and that its activity was restricted to hydathodes in shoots. However, when both the 5' and 3' flanking sequences of NIA1 were fused to GUS gene, the steady level of GUS mRNA from this fusion gene was elevated in response to nitrate, and the GUS expression expanded to entire leaf blade. These results indicated that the sequence downstream of the stop codon of NIA1 is also required for proper NIA1 expression.
