A nitrogen stress-inducible small RNA regulates CO2 fixation in Nostoc

In the absence of fixed nitrogen, some filamentous cyanobacteria differentiate heterocysts, specialized cells devoted to fixing atmospheric nitrogen (N2). This differentiation process is controlled by the global nitrogen regulator NtcA and involves extensive metabolic reprogramming, including shutdown of photosynthetic CO2 fixation in heterocysts, to provide a microaerobic environment suitable for N2 fixation. Small regulatory RNAs (sRNAs) are major post-transcriptional regulators of gene expression in bacteria. In cyanobacteria, responding to nitrogen deficiency involves transcribing several nitrogen-regulated sRNAs. Here, we describe the participation of nitrogen stress-inducible RNA 4 (NsiR4) in post-transcriptionally regulating the expression of two genes involved in CO2 fixation via the Calvin cycle: glpX, which encodes bifunctional sedoheptulose-1,7-bisphosphatase/fructose-1,6-bisphosphatase (SBPase), and pgk, which encodes phosphoglycerate kinase (PGK). Using a heterologous reporter assay in Escherichia coli, we show that NsiR4 interacts with the 5'-untranslated region (5'-UTR) of glpX and pgk mRNAs. Overexpressing NsiR4 in Nostoc sp. PCC 7120 resulted in a reduced amount of SBPase protein and reduced PGK activity, as well as reduced levels of both glpX and pgk mRNAs, further supporting that NsiR4 negatively regulates these two enzymes. In addition, using a gfp fusion to the nsiR4 promoter, we show stronger expression of NsiR4 in heterocysts than in vegetative cells, which could contribute to the heterocyst-specific shutdown of Calvin cycle flux. Post-transcriptional regulation of two Calvin cycle enzymes by NsiR4, a nitrogen-regulated sRNA, represents an additional link between nitrogen control and CO2 assimilation.