Glutamine synthetase (GS), a key enzyme in biological nitrogen assimilation, is regulated in multiple ways in response to varying nitrogen sources and levels. Here we show a small regulatory RNA, NsiR4 (nitrogen stress-induced RNA 4), which plays an important role in the regulation of GS in cyanobacteria. NsiR4 expression in the unicellularSynechocystissp. PCC 6803 and in the filamentous, nitrogen-fixingAnabaenasp. PCC 7120 is stimulated through nitrogen limitation via NtcA, the global transcriptional regulator of genes involved in nitrogen metabolism. NsiR4 is widely conserved throughout the cyanobacterial phylum, suggesting a conserved function. In silico target prediction, transcriptome profiling on pulse overexpression, and site-directed mutagenesis experiments using a heterologous reporter system showed that NsiR4 interacts with the 5′UTR ofgifAmRNA, which encodes glutamine synthetase inactivating factor (IF)7. InSynechocystis, we observed an inverse relationship between the levels of NsiR4 and the accumulation of IF7 in vivo. This NsiR4-dependent modulation ofgifA(IF7) mRNA accumulation influenced the glutamine pool and thusNH4+assimilation via GS. As a second target, we identifiedssr1528, a hitherto uncharacterized nitrogen-regulated gene. Competition experiments between WT and an ΔnsiR4KO mutant showed that the lack of NsiR4 led to decreased acclimation capabilities ofSynechocystistoward oscillating nitrogen levels. These results suggest a role for NsiR4 in the regulation of nitrogen metabolism in cyanobacteria, especially for the adaptation to rapid changes in available nitrogen sources and concentrations. NsiR4 is, to our knowledge, the first identified bacterial sRNA regulating the primary assimilation of a macronutrient.
INFLUENCE OF PHOSPHORUS SUPPLY AND THE FORM OF AVAILABLE NITROGEN ON THE NITROGEN METABOLISM OF THE TOMATO PLANT
