NITRITE REDUCTASE EXPRESSION IS REGULATED AT THE POSTTRANSCRIPTIONAL LEVEL BY THE NITROGEN-SOURCE IN NICOTIANA-PLUMBAGINIFOLIA AND ARABIDOPSIS-THALIANA

Higher plant nitrite reductase (NiR) is a monomeric chloroplastic prot ein catalysing the reduction of nitrite, the product of nitrate reduct ion, to ammonium. The expression of this enzyme is controlled at the t ranscriptional level by light and by the nitrogen source. In order to study the post-transcriptional regulation of NiR, Nicotiana plumbagini folia and Arabidopsis thaliana were transformed with a chimaeric NiR c onstruct containing the tobacco leaf NiR1 coding sequence driven by th e CaMV 35S RNA promoter. Transformed plants did not show any phenotypi c difference when compared with the wild-type, although they overexpre ssed NiR activity in the leaves. When these plants were grown in vitro on media containing either nitrate or ammonium as sole nitrogen sourc e, NiR mRNA derived from transgene expression was constitutively expre ssed, whereas NiR activity and protein level were strongly reduced on ammonium-containing medium. These results suggest that, together with transcriptional control, post-transcriptional regulation by the nitrog en source is operating on NiR expression. This post-transcriptional re gulation of tobacco leaf NiR1 expression was observed not only in the closely related species N. plumbaginifolia but also in the more distan t species A. thaliana.