Post-transcriptional regulation of nitrate reductase by light is abolished by an N-terminal deletion

Abstract

Higher plant nitrate reductases (NRs) carry an N-terminal domain whose sequence is not conserved in NRs from other organisms. A gene composed of a full-length tobacco NR cDNA with an internal deletion of 168 bp in the 5′ end fused to the cauliflower mosaic virus 35S promoter and appropriate termination signals was constructed and designated as ΔNR. An NR-deficient mutant of Nicotians plumbaginifolia was transformed with this ΔNR gene. In transgenic plants expressing this construct, NR activity was restored and normal growth resulted. Apart from a higher thermosensitivity, no appreciable modification of the kinetic parameters of the enzyme was detectable. The post-transcriptional regulation of NR by light was abolished in ΔNR transformants. Consequently, deregulated production of glutamine and asparagine was detected in ΔNR transformants. The absence of in vitro ANR activity modulation by ATP suggests the impairment of ΔNR phosphorylation and thereby suppression of ΔNR post-translational regulation. These data imply that post-transcriptional control of NR expression is important for the flow of the nitrate assimilatory pathway.