Glutamine synthetase mRNA releases sRNA from its 3´UTR to regulate carbon/nitrogen metabolic balance in Enterobacteriaceae .

Glutamine synthetase (GS) is the key enzyme of nitrogen assimilation induced under nitrogen limiting conditions. The carbon skeleton of glutamate and glutamine, 2-oxoglutarate, is supplied from the TCA cycle, but how this metabolic flow is controlled in response to nitrogen availability remains unknown. We show that the expression of the E1o component of 2-oxoglutarate dehydrogenase, SucA, is repressed under nitrogen limitation in Salmonella enterica and E coli . The repression is exerted at the post-transcriptional level by an Hfq-dependent sRNA GlnZ generated from the 3´UTR of the GS-encoding glnA mRNA. Enterobacterial GlnZ variants contain a conserved seed sequence and primarily regulate sucA through base-pairing far upstream of the translation initiation region. During growth on glutamine as the nitrogen source, the glnA 3´UTR deletion mutants expressed SucA at higher levels than the S. enterica and E. coli wild-type strains, respectively. In E. coli , the transcriptional regulator Nac also participates in the repression of sucA . Lastly, this study clarifies that the release of GlnZ from the glnA mRNA by RNase E is essential for the post-transcriptional regulation of sucA . Thus the mRNA coordinates the two independent functions to balance the supply and demand of the fundamental metabolites.