The global regulatory system Csr senses glucose through the phosphoenolpyruvate: carbohydrate phosphotransferase system

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A novel connection between two regulatory systems controlling crucial biological processes in bacteria, the carbon storage regulator (Csr) system and the glucose-specific phosphotransferase system (PTS), is reported by Lenget al. in this issue. This involves the interaction of unphosphorylated EIIAGlc, a component of the glucose-specific PTS, with the CsrD protein, which accelerates the decay of the CsrB and CsrC small RNAs via RNase E inEscherichia coli. As unphosphorylated EIIAGlc is generated in the presence of glucose, the PTS thus acts as a sensor of glucose for the Csr system. Interestingly, another pathway can operate for communication between the Csr system and the glucose-specific PTS. The absence of glucose generates phosphorylated EIIAGlc, which activates the enzyme adenylate cyclase to produce cyclic adenosine monophosphate (cAMP) that, in turn, binds to the regulator cAMP receptor protein (CRP). Lenget al. show that the complex cAMP–CRP modestly reduces CsrB decay independently of CsrD. On the other hand, a previous study indicates that the complex cAMP–CRP positively regulates the transcription of CsrB and CsrC inSalmonella enterica. Therefore, EIIAGlc could work as a molecular switch that regulates the activity of the Csr system, in response to its phosphorylation state determined by the presence or absence of glucose, in order to control gene expression.

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