Glutamine synthetase mRNA releases sRNA from its 3'UTR to regulate carbon/nitrogen metabolic balance in <i>Enterobacteriaceae</i>.

Abstract

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 <i>Salmonella enterica</i> and <i>Escherichia coli</i>. The repression is exerted at the post-transcriptional level by an Hfq-dependent sRNA GlnZ generated from the 3'UTR of the GS-encoding <i>glnA</i> mRNA. Enterobacterial GlnZ variants contain a conserved seed sequence and primarily regulate <i>sucA</i> through base-pairing far upstream of the translation initiation region. During growth on glutamine as the nitrogen source, the <i>glnA</i> 3'UTR deletion mutants expressed SucA at higher levels than the <i>S. enterica</i> and <i>E. coli</i> wild-type strains, respectively. In <i>E. coli</i>, the transcriptional regulator Nac also participates in the repression of <i>sucA</i>. Lastly, this study clarifies that the release of GlnZ from the <i>glnA</i> mRNA by RNase E is essential for the post-transcriptional regulation of <i>sucA</i>. Thus, the mRNA coordinates the two independent functions to balance the supply and demand of the fundamental metabolites.