FIG. 1.

E. coli central carbon metabolism is regulated by CsrA and Cra. (A) The transcriptional regulator Cra inhibits expression of glycolytic genes while activating expression of genes involved in gluconeogenesis. CsrA regulates stability of mRNAs for genes involved in glycolysis and gluconeogenesis. The noncoding RNAs CsrB and CsrC sequester the CsrA protein from its targets. Expression of csrB and csrC is induced by the BarA-UvrY two-component system, which is activated by CsrA. CsrD decreases the stability of both CsrB and CsrC. Based on their genetic relatedness and the data gathered in this study, CsrA and Cra likely regulate metabolism similarly in S. flexneri and E. coli. (Illustration modified from reference 2 with permission of the publisher). (B) Irreversible steps in the metabolic pathways allow for independent regulation of glycolysis and gluconeogenesis. The opposing effects of CsrA and Cra are due in part to their reciprocal regulation of the glycolytic gene pfkA and the gluconeogenic gene ppsA.