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Nitrite reductase (NiR) is the second enzyme in the nitrate assimilatory pathway reducing nitrite to ammonium. The expression of the NiR gene is induced upon the addition of nitrate. In an earlier study, a 130 bp upstream region of the spinach NiR gene promoter, located between −330 to −200, was shown to be necessary for nitrate induction of β-glucuronidase (GUS) expression in tissue-specific manner in transgenic tobacco plant [28]. To further delineate the cis-acting elements involved in nitrate regulation of NiR gene expression, transgenic tobacco plants were generated with 5′ deletions in the−330 to −200 region of the spinach NiR gene promoter fused to the GUS gene. Plants with the NiR promoter deleted to −230 showed a considerable increase in GUS activity in the presence of nitrate, indicating that the 30 bp region between −230 to −200 is crucial for nitrate-regulated expression of NiR. In vivo DMS footprinting of the −300 to −130 region of the NiR promoter in leaf tissues from two independent transgenic lines revealed several nitrate-inducible footprints. Footprinting within the −230 to −181 region revealed factor binding to two adjacent GATA elements separated by 24 bp. This arrangement of GATA elements is analogous to cis-regulatory sequences found in the promoters of nitrate-inducible genes of Neurospora crassa, regulated by the NIT2 Zn-finger protein. The −240 to −110 fragment of the NiR promoter, which contains two NIT2 consensus core elements, bound in vitro to a fusion protein comprising the zinc finger domain of the N. crassa NIT2 protein. The data presented here show that nitrate-inducible expression of the NiR gene is mediated by nitrate-specific binding of trans-acting factors to sequences preserved between fungi and higher plants. |
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